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  • Common network performance issues related to routing include the following:Excessive routing updates—Excessive routing updates can decrease network performance, because if a Layer 3 switch or a router receives a large number of updates, it must process all of those updates. The size of the CPU utilization spike during this processing depends on the following factors:The size of the routing update: This depends on the routing protocol used, the number of changes being reported, and the number of devices in the autonomous system (AS).The frequency of the updates: This depends on the routing protocol used.The design: The IP addressing plan and use of summarization can affect the number of routing updates sent.The presence of any route maps or filters—Incorrectly configured route maps or filters can cause too much or the wrong data to be sent.The number of routing protocols running in the same autonomous system—This affects the number of routing protocol processes receiving and processing the updates. Routes may also be redistributed between protocols, which can add to the number of updates that a specific protocol must process.
  • Cisco routers allow internetworks using different routing protocols (referred to as routing domains or autonomous systems) to exchange routing information through a feature called route redistribution. Route redistribution is the capability of boundary routers connecting different routing domains to exchange and advertise routing information between those routing domains.
  • ACLs are usually associated with interfaces and are usually used to control user traffic (data plane traffic) rather than routing protocol traffic (or other control plane traffic).Using route maps, distribute lists, or prefix lists instead of access lists for route filtering gives the administrator greater flexibility in determining just which routes will be permitted and which will be denied.
  • The many reasons why a change in routing protocols or running multiple protocols might be required, resulting in a more complex routing environment, include the following:You are migrating from an older IGP to a new IGP. Multiple redistribution boundaries may exist until the new protocol has displaced the old protocol completely. Running multiple routing protocols during a migration is effectively the same as a network that has multiple routing protocols running as part of its design.You want to use another protocol but need to keep the old routing protocol because of the host system’s needs. For example, UNIX host-based routers might run only RIP.Different departments might not want to upgrade their routers to support a new routing protocol.If you have a mixed-router vendor environment, you can use a Cisco-specific routing protocol, such as EIGRP, in the Cisco portion of the network and then use a common standards-based routing protocol, such as OSPF, to communicate with non-Cisco devices.Your network may become larger and more complex because of political and geographic border issues, and as you merge with or are acquired by other organizations.Your organization uses a Layer 3 Multiprotocol Label Switching (MPLS) virtual private network (VPN) service from a service provider that does not use the same IGP as the rest of your network.
  • This network initially used RIP Version 1 (RIPv1) and is migrating to OSPF, necessitating the following changes:Conversion of the old fixed-length subnet mask (FLSM) addressing scheme to a variable-length subnet mask (VLSM) configurationUse of a hierarchical addressing scheme to facilitate route summarization and make the network more scalableDivision of the network from one large area into a transit backbone area and two other areas
  • Administrative DistanceLower administrative distances are considered more believable (better).Static routes are configured with the ip route prefixmask {address| interface-type interface-number [ip-address]} [dhcp] [distance] [name next-hop-name] [permanent | track number] [tag tag] global configuration command. If the address parameter is used in this command, specifying the address of the next-hop router to use to reach the destination network, the default administrative distance is 1. If the interface parameter is used instead, specifying the local router outbound interface to use to reach the destination network, the router considers this a directly connected route; however, the default administrative distance in this case appears to be somewhere between 0 and 1.
  • A metric of infinity tells the router that the route is unreachable, and therefore, it should not be advertised. When redistributing routes into RIP, IGRP, and EIGRP, you must specify a default metric. If you do not configure a default metric, the default will be infinity which means that all routes redistributed from these three protocols will be unreachable. For OSPF, the redistributed routes have a default type 2 metric of 20, except for redistributed BGP routes, which have a default type 2 metric of 1. For IS-IS, the redistributed routes have a default metric of 0. But unlike RIP, IGRP, or EIGRP, a seed metric of 0 will not be treated as unreachable by IS-IS. Configuring a seed metric for redistribution into IS-IS is recommended. For BGP, the redistributed routes maintain the IGP routing metrics.
  • As we know, each routing protocol defines a metric for each route. When a router redistributes routes from one routing domain to another this information cannot be translated from one routing protocol to another. A RIP hop cannot be dynamically recalculated to an OSPF cost by the router doing redistribution. Therefore, a seed metric is used to artificially set the distance, cost, etc., to each external (redistributed) network from the redistribution point.For example, OSPF has been defined to make the seed metric for redistributed RIP routes to be 30. If the link cost of the Serial link to R4 is 100. The routes are redistributed as E2 routes, so the cost for networks 172.16.0.0, 172.17.0.0, and 172.18.0.0 in R4 is only the seed metric (30). Notice that the metrics of the three networks in the RIP cloud are irrelevant in the OSPF cloud because the router in the OSPF network (R4) forwards any traffic for these three networks to the border (redistributing) router, R3. R3 then forwards the traffic within the RIP network appropriately.
  • In the one-point, one-way redistribution example, the OSPF domain would have knowledge of the RIP routes. However the RIP domain would require a static route to reach the OSPF domain.In the one-point, two-way redistribution example, the RIP domain would know the OSPF networks, and the OSPF domain would know the RIP routes.
  • The following example displays how one-point one-way redistribution can result in suboptimal routing .Router R1 running EIGRP is announcing an external route to routers R2 and R3. Both of these routers are running the two routing protocols OSPF and EIGRP, but the redistribution between EIGRP and OSPF only occurs on router R2. Router R3 receives routing update information for the external route directly from router R1 via EIGRP, and via OSPF from router R2. The administrative distance of OSPF (110) is lower than administrative distance of external EIGRP routes (170), so router R3 selects the OSPF route. Instead of sending packets directly from router R3 to router R1, router R3 prefers the path via router R2, resulting in suboptimal routing.
  • Technique #1 helps prevent route feedback, suboptimal routing, and routing loops.Technique #2 works if there is only one redistribution point; multiple redistribution points might cause route feedback.Technique #3 works when there are multiple boundary routers. Routes redistributed from the edge AS at one boundary router should not be redistributed back into the edge AS from the core at another redistribution point.For technique #4, recall that if two routing protocols advertise routes to the same destination, information from the routing protocol with the lowest administrative distance is placed in the routing table. A route redistributed into a routing protocol by default inherits the default administrative distance of that routing protocol.
  • The following steps for configuring redistribution are generic enough to apply to all routing protocol combinations. However, the commands used to implement the steps vary, as identified in the following sections. It is important that you review the Cisco IOS documentation for the configuration commands that apply to the specific routing protocols you want to redistribute.
  • On R1, routes from OSPF process 1 are redistributed into RIP and are given a seed metric of 3. Because no route type is specified, both internal and external OSPF routes are redistributed into RIP. R2 learns about the 172.16.0.0 network from R1 via RIP; R2’s routing table has 172.16.0.0 installed as a RIP route. Notice that the route is automatically summarized by R1.
  • Note that the subnets keyword is used so subnets are redistributed. If this keyword were omitted, no subnets would be redistributed into the OSPF domain (including the 172.16.1.0 subnet). Omitting the subnets keyword is a common configuration error.Also notice that a metric value is optional since OSPF will automatically assign a cost of 20 and identify the route as an E2 type route.
  • A metric is specified to ensure that routes are redistributed.The metric configured in this example is interpreted as follows:Bandwidth in kbps = 10,000Delay in tens of microseconds = 100Reliability = 255 (maximum)Load = 1 (minimum)MTU = 1500 bytesThe redistributed routes appear in Router B’s table as external EIGRP (D EX) routes with an administrative distance of 170 instead of the EIGRP default of 90. This makes internal EIGRP routes preferred over external EIGRP routes.
  • RIP, with an administrative distance of 120, chooses the R1 to R4 to R6 path based on hop count (two hops versus four hops the other way).OSPF, with an administrative distance of 110, by default calculates the default metric as 100 megabits per second (Mbps) divided by the interface bandwidth, where the interface bandwidth is the speed of each link in Mbps.OSPF chooses the R1 to R2 to R3 to R5 to R6 path.EIGRP chooses the R1 to R2 to R3 to R5 to R6 path. Although EIGRP and OSPF routing protocols both converge quickly and consider bandwidth, EIGRP is considered more trustworthy than OSPF because EIGRP takes more information into account in its calculation.Because EIGRP has the lowest administrative distance of the three protocols, only the EIGRP path to 10.0.0.0/8 is put into the routing table.
  • The administrative distance affects only the choice of path for identical IP routes—in other words, for routes with the same prefix and mask. Routers use the longest prefix match in the routing table if more than one entry in the routing table matches a particular destination.
  • You should change the default administrative distance carefully and by considering the network’s specific requirements.
  • A passive interface prevents routing updates for the specified protocol from being sent through an interface.Default routes—A default route instructs the router that if it does not have a route for a given destination, it should send the packet to the default route. Therefore, no dynamic routing updates about the remote destinations are necessary.Static routes—A static route allows routes to remote destinations to be manually configured on the router. Therefore, no dynamic routing updates about the remote destinations are necessary.Route maps—Route maps are complex access lists that allow conditions to be tested against a packet or route, and then actions taken to modify attributes of the packet or route.Distribute lists—A distribute list allows an access list to be applied to routingupdates.Prefix lists—A prefix list is a specialized access list designed to filter routes.
  • Route filtering during redistribution—Redistribution nearly always requires some amount of route filtering. Although distribute lists can be used for this purpose, route maps offer the added benefit of manipulating routing metrics through the use of set commands.Policy-based routing (PBR)—Route maps can be used to match source and destination addresses, protocol types, and end-user applications. When a match occurs, a set command can be used to define the interface or next-hop address to which the packet should be sent. PBR allows the operator to define routing policy other than basic destination-based routing using the routing table. NAT—Route maps can better control which private addresses are translated to public addresses. Using a route map with NAT also provides more detailed show commands that describe the address-translation process. BGP—Route maps are the primary tools for implementing BGP policy. Network administrators assign route maps to specific BGP sessions (neighbors) to control which routes are allowed to flow in and out of the BGP process. In addition to filtering, route maps provide sophisticated manipulation of BGP path attributes. Route maps for BGP are discussed in Chapter 6, “Implementing a BGP Solution for ISP Connectivity.”
  • Note: When route maps are used for redistribution, the route maps are applied to the redistribute router configuration command.PBR is covered in more detail in chapter 5.
  • Note: When route maps are used for redistribution, the route maps are applied to the redistribute router configuration command.PBR is covered in more detail in chapter 5.
  • Sequence number 10 of the route map is looking for an IP address match in access list 23 or access list 29. Routes 10.1.0.0/16 and 172.16.1.0/24 match these lists. If a match is found, the router redistributes the route into OSPF with a cost metric of 500 and sets the new OSPF route to external type 1.If there is no match to sequence number 10, sequence number 20 is checked. If there is a match in access list 37 (10.0.0.0/8), that route is not redistributed into OSPF, because sequence number 20 specifies deny.If there is no match to sequence number 20, sequence number 30 is checked. Because sequence number 30 is a permit and there is no match criterion, all remaining routes are redistributed into OSPF with a cost metric of 5000 and an external metric of type 2.Note that the decision of whether to redistribute a route or not is based on the deny or permit in the route-map command, and not the deny or permit in the ACLs referenced by the route-map command. The ACLs are used only to match routes in the route-map statements.
  • To prevent the routing feedback loop, a route map called OSPF-into-RIP has been applied to Routers A and B when redistributing OSPF routes into RIP.The route-map statement with sequence number 10 refers to access list 1, which matches the original RIPv2 network 192.168.1.0/16. This route-map statement is a deny, so the 192.168.1.0 route is denied from being redistributed back into RIPv2. If the route does not match sequence number 10, the router then checks sequence number 20, which is an empty permit statement. This statement matches all routes, so all other routes are redistributed into RIP.
  • Access lists are usually associated with interfaces and are usually used to control user traffic (data plane traffic) rather than routing protocol traffic (or other control plane traffic). However, routers can have many interfaces, and route information can also be obtained through route redistribution, which does not involve a specific interface.In addition, access lists do not affect traffic originated by the router, so applying one on an interface has no effect on outgoing routing advertisements. However, when you configure an access list and use it with a distribute list, routing updates can be controlled, no matter what their source is.
  • To prevent the routing feedback loop, a route map called OSPF-into-RIP has been applied to Routers A and B when redistributing OSPF routes into RIPThe route-map statement with sequence number 10 refers to access list 1, which matches the original RIPv2 network 192.168.1.0/16. This route-map statement is a deny, so the 192.168.1.0 route is denied from being redistributed back into RIPv2. If the route does not match sequence number 10, the router then checks sequence number 20, which is an empty permit statement. This statement matches all routes, so all other routes are redistributed into RIP.
  • To prevent the routing feedback loop, a route map called OSPF-into-RIP has been applied to Routers A and B when redistributing OSPF routes into RIPThe route-map statement with sequence number 10 refers to access list 1, which matches the original RIPv2 network 192.168.1.0/16. This route-map statement is a deny, so the 192.168.1.0 route is denied from being redistributed back into RIPv2. If the route does not match sequence number 10, the router then checks sequence number 20, which is an empty permit statement. This statement matches all routes, so all other routes are redistributed into RIP.
  • With mutual redistribution, using a distribute list helps prevent route feedback and routing loops. Route feedback occurs when routes originally learned from one routing protocol are redistributed back into that protocol.The configuration on R2 is shown. (R4’s configuration would be similar to R2’s configuration.)R2 redistributes the networks it learns via RIPv2 (in this case, 10.1.0.0 to 10.3.0.0) into OSPF. Without filtering, route feedback could occur if OSPF on R4, the other redistribution point, redistributes those same networks back into RIP.Therefore, the configuration includes a distribute list configuration that prevents route feedback. Access list 2 denies the routes originating in the OSPF network (10.8.0.0 through 10.11.0.0) and permits all others; the distribute list configured under OSPF refers to this access list. The result is that networks 10.8.0.0 to 10.11.0.0, originated by OSPF, are not redistributed back into OSPF from RIPv2. All other routes are redistributed into OSPF. Redistribution from OSPF into RIPv2 is filtered with access list 3; it permits routes 10.8.0.0 through 10.11.0.0 to be redistributed into RIPv2.
  • A significant performance improvement over ACLs in loading and route lookup of large lists. The router transforms the prefix list into a tree structure, with each branch of the tree representing a test, allowing the Cisco IOS software to determine whether to permit or deny much faster. Compared to a normal access list in which one no command erases the whole access list, prefix list entries can be modified incrementally. You can assign a sequence number to each line of a prefix-list; the router uses this number to sort the entries in the list. If you initially sequence the lines with some gaps between the sequence numbers, you can easily insert lines later. You can also remove individual lines without removing the entire list.Greater flexibility . For example, routers match network numbers in a routing update against the prefix-list using as many bits as indicated. For example, you can specify a prefix list to match 10.0.0.0/16, which will match 10.0.0.0 routes but not 10.1.0.0 routes. Optionally, the prefix list can also specify the size of the subnet mask, or that the subnet mask must be in a specified range.
  • R1 initially has two /24 routes and is aggregating (summarizing) those routes to 172.16.0.0/16 so that the summary /16 route and the two /24 routes are available to be advertised. The prefix list determines which routes neighbor 10.1.1.1 learns from R1.
  • Transcript

    • 1. © 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public ROUTE v6 Chapter 4 1 Chapter 4: Manipulating Routing Updates CCNP ROUTE: Implementing IP Routing
    • 2. Chapter 4 2© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Chapter 4 Objectives  Describe network performance issues and ways to control routing updates and traffic.  Describe the purpose of and considerations for using multiple routing protocols in a network.  Configure and verify route redistribution of multiple protocols.  Describe, configure and verify various methods for controlling routing update traffic.
    • 3. Chapter 4 3© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Assessing Network Routing Performance Issues
    • 4. Chapter 4 4© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Common Routing Performance Issues  Excessive routing updates • CPU utilization can easily spike during this processing depending on: • The size of the routing update • The frequency of the updates • The Layer 3 network design  The presence of any incorrectly configured route maps or filters  The number of routing protocols running in the same autonomous system
    • 5. Chapter 4 5© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Running Multiple Protocols  Different routing protocols were not designed to interoperate with one another. • Each protocol collects different types of information and reacts to topology changes in its own way.  Running muliple routing protocols increases CPU utilization and requires more memory resources to maintain all the topology, database and routing tables.
    • 6. Chapter 4 6© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Routing Protocol Performance Solutions  Design changes, such as limiting the number of routing protocols used.  Using passive interfaces to prevent routing protocol updates from being advertised out an interface.  Route filtering techniques to block specific routes from being advertised: • Access control lists (ACLs) • Route maps • Distribute lists • Prefix lists
    • 7. Chapter 4 7© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Route Filtering  Using route maps, distribute lists, or prefix lists instead of access lists provides greater route filtering flexibility.  Filters can be configured to: • Prevent updates through router interfaces. • Control the advertising of routes in routing updates. • Control the processing of routing updates.  If filters are not configured correctly or if filters are applied to wrong interfaces, network performance issues may occur.
    • 8. Chapter 4 8© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Route Filtering Process 1. A router stores the incoming routing update in the buffer and triggers a decision. 2. Is there an incoming filter applied to this interface? • If no, then the routing update packet is processed normally. 3. Otherwise, is there an entry in the filter matching the routing update packet? • If no, then the routing update packet is dropped. 4. Otherwise, the router processes the routing update according to the filter.
    • 9. Chapter 4 9© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Using Multiple Routing Protocols on a Network
    • 10. Chapter 4 10© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Simple to Complex Networks  Simple routing protocols work well for simple networks. • Typically only require one routing protocol.  Running a single routing protocol throughout your entire IP internetwork is desirable.  However, as networks grow they become more complex and large internetworks may have to support several routing protocols. • Proper inter-routing protocol exchange is vital.
    • 11. Chapter 4 11© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Why have multiple routing protocols?  Interim during conversion • Migrating from an older IGP to a new IGP.  Application-specific protocols • One size does not always fit all.  Political boundaries • Multiple departments managed by different network administrators • Groups that do not work well with others  Mismatch between devices • Multivendor interoperability • Host-based routers  Company mergers
    • 12. Chapter 4 12© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Complex Networks  Complex networks require careful routing protocol design and traffic optimization solutions, including the following: • Redistribution between routing protocols • Route filtering (covered in the next chapter) • Summarization (covered in EIGRP and OSPF)
    • 13. Chapter 4 13© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Redistribution  Cisco routers allow different routing protocols to exchange routing information through a feature called route redistribution. • Route redistribution is defined as the capability of boundary routers connecting different routing domains to exchange and advertise routing information between those routing domains (autonomous systems).
    • 14. Chapter 4 14© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Route Redistribution Example
    • 15. Chapter 4 15© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Redistributed Routes  Redistribution is always performed outbound; the router doing redistribution does not change its routing table.  The boundary router’s neighbors see the redistributed routes as external routes.  Routes must be in the routing table for them to be redistributed.
    • 16. Chapter 4 16© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Redistribution Considerations  The key issues that arise when using redistribution: • Routing feedback (loops) • If more than one boundary router is performing route redistribution, then the routers might send routing information received from one autonomous system back into that same autonomous system. • Incompatible routing information • Each routing protocol uses different metrics to determine the best path therefore path selection using the redistributed route information might not be optimal. • Inconsistent convergence times • Different routing protocols converge at different rates.  Good planning should solve the majority of issues but additional configuration might be required. • Some issues might be solved by changing the administrative distance, manipulating the metrics, and filtering using route maps, distribute lists, and prefix lists.
    • 17. Chapter 4 17© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Selecting the Best Route Routers use the following two parameters to select the best path:  Administrative distance: • Used to rate a routing protocol’s believability (also called its trustworthiness). • This criterion is the first thing a router uses to determine which routing protocol to believe if more than one protocol provides route information for the same destination.  Routing metric: • The routing metric is a value representing the path between the local router and the destination network, according to the routing protocol being used. • The metric is used to determine the routing protocol’s “best” path to the destination.
    • 18. Chapter 4 18© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Cisco IOS Administrative Distance Routing Protocol Default Administrative Distance Value Connected interface 0 Static route out an interface 1 Static route to a next-hop address 1 EIGRP summary route 5 External BGP 20 Internal EIGRP 90 IGRP 100 OSPF 110 IS-IS 115 RIPv1 and RIP v2 120 Exterior Gateway Protocol (EGP) 140 On-Demand Routing (ODR) 160 External EIGRP 170 Internal BGP 200 Unknown 255 Trustworthiness More Less
    • 19. Chapter 4 19© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Routing Metric  A boundary router must be capable of translating the metric of the received route into the receiving routing protocol. • Redistributed route must have a metric appropriate for the receiving protocol.  The Cisco IOS assigns the following default metrics when a protocol is redistributed into the specified routing protocol: Protocol That Route Is Redistributed Into … Default Seed Metric RIP 0 (interpreted as infinity) IGRP / EIGRP 0 (interpreted as infinity) OSPF 20 for all except BGP routes (BGP routes have a default seed metric of 1) IS-IS 0 BGP BGP metric is set to IGP metric value
    • 20. Chapter 4 20© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Defining a Seed Metric  A seed metric, different than the default metric, can be defined during the redistribution configuration. • After the seed metric for a redistributed route is established, the metric increments normally within the autonomous system. • The exception to this rule is OSPF E2 routes.  Seed metrics can be defined in two ways: • The default-metric router configuration command establishes the seed metric for all redistributed routes. • The redistribute can also be used to define the seed metric for a specific protocol.
    • 21. Chapter 4 21© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public OSPF Seed Metric Example #1 R3(config)# router rip R3(config-router)# network 172.18.0.0 R3(config-router)# network 172.19.0.0 R3(config-router)# router ospf 1 R3(config-router)# network 192.168.2.0 0.0.0.255 area 0 R3(config-router)# redistribute rip subnets metric 30 R3(config-router)# 172.16.0.0 172.17.0.0 172.20.0.0 172.19.0.0 172.18.0.0 192.168.2.0 Cost = 10 Cost = 100 RIP AS OSPF R1 R2 R3 R4 C 172.16.0.0 C 172.20.0.0 R [120/1] 172.17.0.0 R [120/1] 172.19.0.0 R [120/2] 172.18.0.0 Table R1 C 172.17.0.0 C 172.19.0.0 C 172.20.0.0 R [120/1] 172.16.0.0 R [120/1] 172.18.0.0 Table R2 C 172.18.0.0 C 172.19.0.0 R [120/1] 172.17.0.0 R [120/1] 172.20.0.0 R [120/2] 172.16.0.0 C 192.168.2.0 O [110/110] 192.168.1.0 Table R3 C 192.168.1.0 C 192.168.2.0 O E2 [110/30] 172.16.0.0 O E2 [110/30] 172.17.0.0 O E2 [110/30] 172.18.0.0 O E2 [110/30] 172.19.0.0 O E2 [110/30] 172.20.0.0 Table R4 192.168.1.0
    • 22. Chapter 4 22© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public OSPF Seed Metric Example #2 R3(config)# router rip R3(config-router)# network 172.18.0.0 R3(config-router)# network 172.19.0.0 R3(config-router)# router ospf 1 R3(config-router)# network 192.168.2.0 0.0.0.255 area 0 R3(config-router)# redistribute rip subnets R3(config-router)# default-metric 30 172.16.0.0 172.17.0.0 172.20.0.0 172.19.0.0 172.18.0.0 192.168.2.0 Cost = 10 Cost = 100 RIP AS OSPF R1 R2 R3 R4 C 172.16.0.0 C 172.20.0.0 R [120/1] 172.17.0.0 R [120/1] 172.19.0.0 R [120/2] 172.18.0.0 Table R1 C 172.17.0.0 C 172.19.0.0 C 172.20.0.0 R [120/1] 172.16.0.0 R [120/1] 172.18.0.0 Table R2 C 172.18.0.0 C 172.19.0.0 R [120/1] 172.17.0.0 R [120/1] 172.20.0.0 R [120/2] 172.16.0.0 C 192.168.2.0 O [110/110] 192.168.1.0 Table R3 C 192.168.1.0 C 192.168.2.0 O E2 [110/30] 172.16.0.0 O E2 [110/30] 172.17.0.0 O E2 [110/30] 172.18.0.0 O E2 [110/30] 172.19.0.0 O E2 [110/30] 172.20.0.0 Table R4 192.168.4.0
    • 23. Chapter 4 23© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Redistribution Methods  Redistribution can be done through: • One-point redistribution • Only one router is redistributing one-way or two-way (both ways). • There could still be other boundary routers but they are not configured to redistribute. • Multipoint redistribution • Multiple routers are used to redistribute either one-way or two- way (both ways). • More prone to routing loop problems. RIP OSPF Multipoint Redistribution RIP OSPF One-Point Redistribution
    • 24. Chapter 4 24© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public One-Point Redistribution  One-point redistribution can be configured in either: • One-point One-way • Redistributes networks from one routing protocol into the other routing protocol. • Typically uses a default or static route so that devices in that other part of the network can reach the first part of the network. • One-point Two-way • Redistributes routes between the two routing processes, in both directions. RIP OSPF One-Point One-Way Redistribution Redistributing from RIP to OSPF RIP OSPF One-Point Two-Way Redistribution Redistributing from RIP to OSPF and from OSPF to RIP Default route to the OSPF network
    • 25. Chapter 4 25© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public OSPF One-Point One-Way Redistribution Issue  Although one-point one-way or two-way redistribution is usually safe from routing loops, issues can still occur if multiple boundary routers exist and only one router is performing one-point one-way redistribution. • In this example, R2 is redistributing an external EIGRP route into the OSPF domain. R2 R3 R1 EIGRP R1 announces the external EIGRP route 10.0.0.0 with an administrative distance of 170 to both R2 and R3. Only R2 is configured to redistribute the EIGRP routes into the OSPF domain. Therefore the external 10.0.0.0 network is redistributed into the OSPF domain with an administrative distance of 110. O E2 10.0.0.0/8 [110/20] Although R3 has a direct connection to R1, R3 will use the OSPF route via R2 to get to the 10.0.0.0 network due to the lower administrative distance of OSPF (110). This creates a suboptimal routing issue. 1 2 3
    • 26. Chapter 4 26© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Multipoint Redistribution  Multipoint redistribution has two (or more) separate routers running both routing protocols.  Redistribution can be configured as: • Multipoint one-way redistribution • Multipoint two-way redistribution  Although multipoint two-way redistribution is especially problematic, either method is likely to introduce potential routing feedback loops. RIP OSPF Multipoint One-Way Redistribution Redistributing RIP into OSPF RIP OSPF Multipoint Two-Way Redistribution Redistributing RIP into OSPF and OSPF into RIP Redistributing RIP into OSPF Redistributing RIP into OSPF and OSPF into RIP
    • 27. Chapter 4 27© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Routing Protocol A Multipoint Redistribution  Multipoint one-way redistribution only works well if: • The receiving routing protocol is either EIGRP, BGP and OSPF because they support different administrative distances for internal and external routes. • The administrative distance of protocol B’s external routes is higher than the administrative distance of protocol A’s routes, so that R2 and R3 will use the appropriate routes to destinations in the protocol A side of the network. R2 R3 R1 Routing Protocol B R1 announces protocol B routes to both R2 and R3. R2 is configured to redistribute routing protocol B routes. R3 is configured to redistribute routing protocol B routes. 1 2 3 Redistributed protocol B routes
    • 28. Chapter 4 28© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Core and Edge Routing Protocols  Two terms are often used to distinguish redistribution roles between IGPs: • Core routing protocol • Edge routing protocol  In a network that run multiple IGPs: • The core routing protocol is the main and more advanced routing protocol running in the network (e.g.; EIGRP, OSPF). • The edge routing protocol is the simpler IGP (e.g., RIP).  If this is an IGP migration from an older IGP to a newer IGP: • The core routing protocol is the new routing protocol. • The edge routing protocol is the old routing protocol.
    • 29. Chapter 4 29© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Then modify the administrative distance associated with redistributed routes so that they are not the selected routes when multiple routes exist for the same destination. Redistribution Techniques Redistribute routes from the edge into the core. Core Routing Protocol Edge Routing Protocol When using multiple boundary routers, redistribute routes from the core into the edge and filter inappropriate routes. 172.16.0.0 10.0.0.0 Redistribute a default route from the core into the edge. Technique #1 Redistribute routes from the edge into the core. Redistribute static routes about the core into the edge. Technique #2 Technique #3 Technique #4 Redistribute all routes from the edge into the core. Redistribute all routes from the core into the edge.
    • 30. Chapter 4 30© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Preventing Routing Loops  The safest way to perform redistribution is to redistribute routes in only one direction, on only one boundary router within the network. • However, that this results in a single point of failure in the network.  If redistribution must be done in both directions or on multiple boundary routers, the redistribution should be tuned to avoid problems such as suboptimal routing and routing loops.
    • 31. Chapter 4 31© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Redistribution Guidelines  Do not overlap routing protocols. • Do not run two different protocols in the same Internetwork. • Instead, have distinct boundaries between networks that use different routing protocols.  Be familiar with your network. • Knowing the network will result in the best decision being made.
    • 32. Chapter 4 32© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Implementing Route Redistribution
    • 33. Chapter 4 33© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Redistribution Supports All Protocols R1(config)# router rip R1(config-router)# redistribute ? bgp Border Gateway Protocol (BGP) connected Connected eigrp Enhanced Interior Gateway Routing Protocol (EIGRP) isis ISO IS-IS iso-igrp IGRP for OSI networks metric Metric for redistributed routes mobile Mobile routes odr On Demand stub Routes ospf Open Shortest Path First (OSPF) rip Routing Information Protocol (RIP) route-map Route map reference static Static routes R1(config-router)# redistribute
    • 34. Chapter 4 34© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Key Route Redistribution Points  Routes are redistributed into a routing protocol. • Therefore, the redistribute command is configured under the routing process that is receiving the redistributed routes.  Routes can only be redistributed between routing protocols that support the same protocol stack. • For example IPv4 to IPv4 and IPv6 to IPv6. • However, IPv4 routes cannot be redistributed into IPv6.  The method used to configure redistribution varies among combinations of routing protocols. • For example, some routing protocols require a metric to be configured during redistribution, but others do not.
    • 35. Chapter 4 35© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Generic Redistribution Steps 1. Identify the boundary router(s) that will perform redistribution. 2. Determine which routing protocol is the core protocol. 3. Determine which routing protocol is the edge protocol. • Determine whether all routes from the edge protocol need to be propagated into the core and consider methods that reduce the number of routes. 4. Select a method for injecting the required routes into the core. • Summarized routes at network boundaries minimizes the number of new entries in the routing table of the core routers. 5. Consider how to inject the core routing information into the edge protocol.
    • 36. Chapter 4 36© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Redistributing into RIP  Redistribute routes into RIP. Router(config-router)# redistribute protocol [process-id] [match route-type] [metric metric-value] [route-map map-tag] Parameter Description protocol The source protocol from which routes are redistributed. process-id For OSPF, this value is an OSPF process ID. For EIGRP or BGP, this value is an AS number. This parameter is not required for IS-IS. route-type (Optional) A parameter used when redistributing OSPF routes into another routing protocol. metric-value (Optional) A parameter used to specify the RIP hop count seed metric for the redistributed route. If this value is not specified and no value is specified using the default- metric router configuration command, then the default metric is 0 and interpreted as infinity which means that routes will not be redistributed. map-tag (Optional) Specifies the identifier of a configured route map to be interrogated to filter the importation of routes from the source routing protocol to the current RIP routing protocol.
    • 37. Chapter 4 37© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Redistributing into RIP Example R1(config)# router rip R1(config-router)# redistribute ospf 1 metric 3 R1(config-router)# C 10.1.1.0 R 192.168.1.0 [120/1] 0 172.16.1.0 [110/50] Table R1 C 10.1.1.0 C 192.168.1.0 R 172.16.0.0 [120/3] Table R2 R1 .2 Fa0/0 10.1.1.0 /24 OSPF RIP .1 Fa0/0 O 172.16.1.0/24 [110/50] R 172.16.0.0 [120/3] 192.168.1.0 /24 R2
    • 38. Chapter 4 38© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Redistributing into OSPF  Redistribute routes into OSPF. Router(config-router)# redistribute protocol [process-id] [metric metric-value] [metric-type type-value] [route-map map-tag] [subnets] [tag tag-value] Parameter Description protocol The source protocol from which routes are redistributed. process-id For EIGRP or BGP, this value is an AS number. This parameter is not required for RIP or IS-IS. metric-value (Optional) A parameter that specifies the OSPF seed metric used for the redistributed route. The default metric is a cost of 20 (except for BGP routes, which have a default metric of 1). map-tag (Optional) Specifies the identifier of a configured route map to be interrogated to filter the importation of routes from the source routing protocol to the current OSPF routing protocol. subnets (Optional) OSPF parameter that specifies that subnetted routes should be redistributed. Otherwise, only classful routes are redistributed. tag-value (Optional) A 32-bit decimal value attached to each external route to be used by ASBRs.
    • 39. Chapter 4 39© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Redistributing into OSPF Example R1(config)# router ospf 1 R1(config-router)# redistribute eigrp 100 subnets metric-type 1 R1(config-router)# C 10.1.1.0 0 192.168.1.0 [110/20] D 172.16.1.0 [90/409600] Table R1 C 10.1.1.0 C 192.168.1.0 O E1 172.16.1.0 [110/20] Table R2 R1 .2 Fa0/0 10.1.1.0 /24 EIGRP AS 100 OSPF .1 Fa0/0 D 172.16.1.0/24 [90/409600] O E1 172.16.1.0 [110/20] 192.168.1.0 /24 R2
    • 40. Chapter 4 40© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Default Metric for RIP, OSPF, BGP  Apply default metric values for RIP, OSPF, and BGP. Router(config-router)# default-metric number  The number parameter is the value of the metric.  For RIP this is the number of hops.  For OSPF this is the assigned cost.
    • 41. Chapter 4 41© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public OSPF Default-Metric Example R1(config)# router ospf 1 R1(config-router)# default-metric 30 R1(config-router)# redistribute eigrp 100 subnets metric-type 1 R1(config-router)# C 10.1.1.0 0 192.168.1.0 [110/20] D 172.16.1.0 [90/409600] Table R1 C 10.1.1.0 C 192.168.1.0 O E1 172.16.1.0 [110/30] Table R2 R1 .2 Fa0/0 10.1.1.0 /24 EIGRP AS 100 OSPF .1 Fa0/0 D 172.16.1.0/24 [90/409600] O E1 172.16.1.0 [110/30] 192.168.1.0 /24 R2
    • 42. Chapter 4 42© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Redistributing into EIGRP  Redistribute routes into EIGRP. Router(config-router)# redistribute protocol [process-id] [match route-type] [metric metric-value] [route-map map-tag] Parameter Description protocol The source protocol from which routes are redistributed. process-id For OSPF, this value is an OSPF process ID. For BGP, this value is an AS number. This parameter is not required for RIP or IS-IS. route-type (Optional) A parameter used when redistributing OSPF routes into another routing protocol. metric-value Required if the default-metric command is not configured otherwise it is optional . A parameter that specifies the EIGRP seed metric, in the order of bandwidth, delay, reliability, load, and maximum transmission unit (MTU), for the redistributed route. If this value is not specified when redistributing from another protocol and no default metric has been configured, then no routes will not be redistributed. map-tag (Optional) Specifies the identifier of a configured route map to be interrogated to filter the importation of routes from the source routing protocol to the current EIGRP routing protocol.
    • 43. Chapter 4 43© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Redistributing into EIGRP Example R1(config)# router eigrp 100 R1(config-router)# redistribute ospf 1 metric 10000 100 255 1 1500 R1(config-router)# C 10.1.1.0 0 192.168.1.0 [90/307200] O 172.16.1.0 [110/50] Table R1 C 10.1.1.0 C 192.168.1.0 D EX 172.16.1.0 [170/307200] Table R2 R1 .2 Fa0/0 10.1.1.0 /24 OSPF EIGRP AS 100 .1 Fa0/0 O 172.16.1.0/24 [110/50] D EX 172.16.1.0/24 [170/281600] 192.168.1.0 /24 R2
    • 44. Chapter 4 44© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Default Metric for EIGRP  Apply metric values for EIGRP. Router(config-router)# default-metric bandwidth delay reliability loading mtu Parameter Description bandwidth The route’s minimum bandwidth in kilobits per second (kbps). It can be 0 or any positive integer. delay Route delay in tens of microseconds. It can be 0 or any positive integer that is a multiple of 39.1 nanoseconds. reliability The likelihood of successful packet transmission, expressed as a number from 0 to 255, where 255 means that the route is 100 percent reliable, and 0 means unreliable. loading The route’s effective loading, expressed as a number from 1 to 255, where 255 means that the route is 100 percent loaded. mtu Maximum transmission unit. The maximum packet size in bytes along the route; an integer greater than or equal to 1.
    • 45. Chapter 4 45© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public EIGRP Default-Metric Example R1(config)# router eigrp 100 R1(config-router)# default-metric 10000 100 255 1 1500 R1(config-router)# redistribute ospf 1 R1(config-router)# C 10.1.1.0 0 192.168.1.0 [90/307200] O 172.16.1.0 [110/50] Table R1 C 10.1.1.0 C 192.168.1.0 D EX 172.16.1.0 [170/307200] Table R2 R1 .2 Fa0/0 10.1.1.0 /24 OSPF EIGRP AS 100 .1 Fa0/0 O 172.16.1.0/24 [110/50] D EX 172.16.1.0/24 [170/281600] 192.168.1.0 /24 R2
    • 46. Chapter 4 46© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Which Path From R1 to 10.0.0.0 /8?  RIP, OSPF, and EIGRP are all configured on the routers.  Which path would R1 choose if: • RIP made the decision? • OSPF made the decision? • EIGRP made the decision?  Because EIGRP has the lowest administrative distance of the three protocols, only the EIGRP path to 10.0.0.0/8 is put into the routing table. R1  R4  R6 R1  R2  R3  R5  R6 R1  R2  R3  R5  R6
    • 47. Chapter 4 47© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Quiz Question  Assume that a router has three routing processes running simultaneously on it, and each process has received these routes: • EIGRP (internal): 192.168.32.0/26 • RIP: 192.168.32.0/24 • OSPF: 192.168.32.0/19  Which of these routes will be installed in the routing table? • All of them!  Although EIGRP has the best administrative distance, each of these routes has a different prefix length (subnet mask).  They are therefore considered different destinations and are all installed in the routing table.
    • 48. Chapter 4 48© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Modifying the Administrative Distance  When routes are redistributed between two different routing protocols, some information may be lost making route selection more confusing.  One approach to correct this is to control the administrative distance to indicate route selection preference and ensure that route selection is unambiguous. • Although, this approach does not always guarantee the best route is selected, only that route selection will be consistent.  For all protocols use the distance administrative- distance router configuration command. • Alternatively for OSPF, use the distance ospf command. • Alternatively for EIGRP, use the distance eigrp command.
    • 49. Chapter 4 49© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Modifying the Administrative Distance  Change the default administrative distances. Router(config-router)# distance administrative-distance [address wildcard-mask [ip-standard-list] [ip-extended-list]] Parameter Description administrative-distance Sets the administrative distance, an integer from 10 to 255. address (Optional) Specifies the IP address; this allows filtering of networks according to the IP address of the router supplying the routing information. wildcard-mask (Optional) Specifies the wildcard mask used to interpret the IP address. ip-standard-list ip-extended-list (Optional) The number or name of a standard or extended access list to be applied to the incoming routing updates. Allows filtering of the networks being advertised.
    • 50. Chapter 4 50© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Modifying OSPF Administrative Distance  Change the default administrative distances of OSPF. Router(config-router)# distance ospf {[intra-area dist1] [inter-area dist2] [external dist3] Parameter Description dist1 (Optional) Specifies the administrative distance for all OSPF routes within an area. Acceptable values are from 1 to 255 while the default is 110. dist2 (Optional) Specifies the administrative distance for all OSPF routes from one area to another area. Acceptable values are from 1 to 255 while the default is 110. dist3 (Optional) Specifies the administrative distance for all routes from other routing domains, learned by redistribution. Acceptable values are from 1 to 255 while the default is 110.
    • 51. Chapter 4 51© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Modifying EIGRP Administrative Distance  Change the default administrative distance of EIGRP. Router(config-router)# distance eigrp internal-distance external-distance Parameter Description internal-distance Specifies the administrative distance for EIGRP internal routes. The distance can be a value from 1 to 255 while the default is 90. external-distance Specifies the administrative distance for EIGRP external routes. The distance can be a value from 1 to 255 while the default is 170.
    • 52. Chapter 4 52© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Verifying Redistribution Operation  Know the network topology. • Pay particularly attention to where redundant routes exist.  Study the routing tables on a variety of routers in the network. • For example, check the routing table on the boundary router and on some of the internal routers in each autonomous system.  Examine the topology table of each configured routing protocol to ensure that all appropriate prefixes are being learned.  Use the traceroute EXEC command on some of the routes to verify that the shortest path is being used for routing. • Be sure to run traces to networks for which redundant routes exist.  When troubleshooting, use the traceroute and debug commands to observe the routing update traffic on the boundary routers and on the internal routers.
    • 53. Chapter 4 53© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Controlling Routing Update Traffic
    • 54. Chapter 4 54© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Controlling Routing Updates  Propagating routing information can be controlled by using: • Passive interface • Static routes • Default route • Route maps • Distribute lists • Prefix lists  NOTE: • There is not one type of route filter that is appropriate for every situation. • A variety of techniques may be used to make the network run smoothly.
    • 55. Chapter 4 55© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Passive Interfaces  Passive interfaces prevent routing updates from being sent and/or received for a specified protocol. • RIP interfaces listen but will not send updates. • OSPF and EIGRP interfaces do not listen for or send updates and therefore no neighbor adjacencies can be established.
    • 56. Chapter 4 56© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public passive-interface default Command  Large enterprise may need to set multiple interfaces as passive. • In some networks, this could mean coding 200 or more passive- interface statements.  The passive-interface default command sets all interfaces as passive by default. • Interfaces on which adjacencies updates are desired can be set as active with the no passive-interface command.
    • 57. Chapter 4 57© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Static and Default Routes  Static routes are manually configured routes that are used to: • Define specific routes to use when two autonomous systems must exchange routing information. • Define routes to destinations over a WAN link to eliminate the need for a dynamic routing protocol.  Static route configuration considerations: • If you want a router to advertise a static route in a routing protocol, it might need to be redistributed. • To reduce the number of static route entries, define a default static route.
    • 58. Chapter 4 58© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Understanding Route Maps  Route maps are similar in function to ACLs, but provide far more control.  Route maps are more similar to a scripting language. • They can be named rather than numbered for easier documentation. • Lines are sequence-numbered for easier editing. • Match and set criteria can be used, similar to the “if, then” logic. • They allow conditions to be tested using match commands and if the conditions match, actions specified by set commands can be taken to modify attributes of the packet or routes.  Just as ACLs are used by a variety of Cisco IOS features, route maps can also be used for various applications. • The actual route map implementation will vary based on how its applied.
    • 59. Chapter 4 59© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Route Map Applications  Route filtering during redistribution • All IP routing protocols can use route maps for redistribution filtering. • Applied using the redistribute protocol route-map router configuration command.  Policy-based routing (PBR) • PBR allows the operator to define routing policy other than basic destination-based routing using the routing table. • Applied using the ip policy route-map interface configuration command.  NAT • Route maps provide more control over which private addresses are translated to public addresses.  BGP • Route maps are the primary tools for implementing BGP policy.
    • 60. Chapter 4 60© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Defining a Route Map  Define a route map and enter route map configuration mode. Router(config)# route-map map-tag [permit | deny] [sequence-number]  Each route map statement is numbered by a sequence number and for this reason can be edited.  The default for the route-map command is permit, with a sequence-number of 10. Parameter Description map-tag Name of the route map. permit | deny (Optional) A parameter that specifies the action to be taken if the route map match conditions are met; the meaning of permit or deny is dependent on how the route map is used. sequence-number (Optional) A sequence number that indicates the position that a new route map statement will have in the list of route map statements already configured with the same name.
    • 61. Chapter 4 61© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Route Map Operation Logic  A route map consists of a list of statements. • The list is processed top-down like an access list. • Sequence numbers are used for inserting or deleting specific statements.  Route map permit or deny determines if the candidate will be redistributed. • At least one reference must permit the route for it to be a candidate for redistribution.  The first match found for a route is applied. • The match statement may contain multiple references. • Multiple match criteria in the same line use a logical OR. • Multiple match criteria in multiple separate lines use a logical AND. • Once there is a match, set the action (if defined) and leave the route map. • Other route-map statements are not processed.
    • 62. Chapter 4 62© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Route Map Operation Example • Match criteria on the same line mean a logical OR condition (If this or this or …). • Multiple match and set criteria on separate lines indicates an AND condition (and if this …). • A route-map statement without any match statements will be considered matched. • Like an access list, an implicit deny any appears at the end of a route map. • The consequences of this deny depend on how the route map is being used. route-map DEMO permit 10 match X Y Z match A set B set C route-map DEMO permit 20 match Q set R route-map DEMO permit 30 If {(X OR Y OR Z) AND A match} Then {Set B AND C} (and exit route-map) Else If Q matches Then set R (and exit route-map) Else Set nothing (and exit route-map) OR AND AND
    • 63. Chapter 4 63© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public match Statements  Specify criteria to be matched. Router(config-route-map)# match condition  The match condition route map configuration commands are used to define the conditions to be checked.  Some of these conditions are used for BGP policy, some for PBR, and some for redistribution filtering.
    • 64. Chapter 4 64© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public The match Commands Command Description match community Matches a BGP community match interface Matches any routes that have the next hop out of one of the interfaces specified match ip address Matches any routes that have a destination network number address that is permitted by a standard or extended ACL match ip next-hop Matches any routes that have a next-hop router address that is passed by one of the ACLs specified match ip route- source Matches routes that have been advertised by routers and access servers at the address that is specified by the ACLs match length Matches based on the layer 3 length of a packet match metric Matches routes with the metric specified match route-type Matches routes of the specified type match tag Matches tag of a route
    • 65. Chapter 4 65© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public set Statements  Modify matching conditions. Router(config-route-map)# set action  The command modifies parameters in redistributed routes.  The specific action changes or add characteristics, such as metrics, to any routes that have met a match condition.
    • 66. Chapter 4 66© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public The set Commands Command Description set as-path Modifies an AS path for BGP routes set automatic-tag Computes automatically the tag value set community Sets the BGP communities attribute set default interface Indicates where to output packets that pass a match clause of a route map for policy routing and have no explicit route to the destination set interface Indicates where to output packets that pass a match clause of a route map for policy routing set ip default next- hop Indicates where to output packets that pass a match clause of a route map for policy routing and for which the Cisco IOS software has no explicit route to a destination set ip next-hop Indicates where to output packets that pass a match clause of a route map for policy routing set level Indicates where to import routes for IS-IS and OSPF set local-preference Specifies a BGP local preference value set metric Sets the metric value for a routing protocol set metric-type Sets the metric type for the destination routing protocol set tag Sets tag value for destination routing protocol set weight Specifies the BGP weight value
    • 67. Chapter 4 67© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Configuring Route Maps for PBR  PBR allows the operator to define a routing policy other than basic destination-based routing using the routing table. • For example to make packets to take a route other than the obvious shortest path.  Sample implementation plan: • Define and name the route map with the route-map command. • Define the conditions to match (the match statements). • Define the action to be taken when there is a match (the set statements). • Define which interface the route map will be attached to using the ip policy route-map interface configuration command. • PBR is applied to incoming packets.
    • 68. Chapter 4 68© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public ip policy route-map map-tag Router(config-if)#  Apply the route-map to the incoming interface. route-map Commands for PBR route-map map-tag [permit | deny] [sequence-number] Router(config)#  Defines the route map conditions. match {conditions} Router(config-route-map)#  Defines the conditions to match. set {actions} Router(config-route-map)#  Defines the action to be taken on a match.
    • 69. Chapter 4 69© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public match Commands Used in PBR Command Description match community Matches a BGP community match interface Matches any routes that have the next hop out of one of the interfaces specified match ip address Matches any routes that have a destination network number address that is permitted by a standard or extended ACL match ip next-hop Matches any routes that have a next-hop router address that is passed by one of the ACLs specified match ip route-source Matches routes that have been advertised by routers and access servers at the address that is specified by the ACLs match length Matches based on the layer 3 length of a packet match metric Matches routes with the metric specified match route-type Matches routes of the specified type match tag Matches tag of a route
    • 70. Chapter 4 70© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public set Commands Used in PBR Command Description set as-path Modifies an AS path for BGP routes set automatic-tag Computes automatically the tag value set community Sets the BGP communities attribute set default interface Indicates where to output packets that pass a match clause of a route map for policy routing and have no explicit route to the destination set interface Indicates where to output packets that pass a match clause of a route map for policy routing set ip default next-hop Indicates where to output packets that pass a match clause of a route map for policy routing and for which the Cisco IOS software has no explicit route to a destination set ip next-hop Indicates where to output packets that pass a match clause of a route map for policy routing set level Indicates where to import routes for IS-IS and OSPF set local-preference Specifies a BGP local preference value set metric Sets the metric value for a routing protocol set metric-type Sets the metric type for the destination routing protocol set tag Sets tag value for destination routing protocol set weight Specifies the BGP weight value
    • 71. Chapter 4 71© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Configuring Route Maps for PBR Example  The route map has only one permit statement. • Any packets that match the IP address specified by ACL 1 (172.21.16.18) should be sent to the next hop IP address 172.30.3.20.  This route map applies to incoming packets on the S0/0/0 interface. R1(config)# access-list 1 permit 172.21.16.18 0.0.0.0 R1(config)# R1(config)# route-map MY-ROUTE-MAP permit 10 R1(config-route-map)# match ip address 1 R1(config-route-map)# set ip next-hop 172.30.3.20 R1(config-route-map)# R1(config-route-map)# interface S0/0/0 R1(config-if)# ip policy route-map MY-ROUTE-MAP
    • 72. Chapter 4 72© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public LECCION OSPF 
    • 73. Chapter 4 73© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Configuring Route Maps for Redistribution  Use route maps when you want detailed control over how routes are redistributed between routing protocols.  Sample implementation plan: • Define and name the route map with the route-map command. • Define the conditions to match (the match statements). • Define the action to be taken when there is a match (the set statements). • Specify the route map to use when redistributing. • Use the redistribute protocol route-map map-tag router configuration command.
    • 74. Chapter 4 74© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public redistribute protocol [process-id] route-map map-tag Router(config-router)#  Allows for detailed control of routes being redistributed into a routing protocol. route-map Commands for Redistribution route-map map-tag [permit | deny] [sequence-number] Router(config)#  Defines the route map conditions. match {conditions} Router(config-route-map)#  Defines the conditions to match. set {actions} Router(config-route-map)#  Defines the action to be taken on a match.
    • 75. Chapter 4 75© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public match Commands Used in Redistribution Command Description match community Matches a BGP community match interface Matches any routes that have the next hop out of one of the interfaces specified match ip address Matches any routes that have a destination network number address that is permitted by a standard or extended ACL match ip next-hop Matches any routes that have a next-hop router address that is passed by one of the ACLs specified match ip route-source Matches routes that have been advertised by routers and access servers at the address that is specified by the ACLs match length Matches based on the layer 3 length of a packet match metric Matches routes with the metric specified match route-type Matches routes of the specified type match tag Matches tag of a route
    • 76. Chapter 4 76© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public set Commands Used in Redistribution Command Description set as-path Modifies an AS path for BGP routes set automatic-tag Computes automatically the tag value set community Sets the BGP communities attribute set default interface Indicates where to output packets that pass a match clause of a route map for policy routing and have no explicit route to the destination set interface Indicates where to output packets that pass a match clause of a route map for policy routing set ip default next-hop Indicates where to output packets that pass a match clause of a route map for policy routing and for which the Cisco IOS software has no explicit route to a destination set ip next-hop Indicates where to output packets that pass a match clause of a route map for policy routing set level Indicates where to import routes for IS-IS and OSPF set local-preference Specifies a BGP local preference value set metric Sets the metric value for a routing protocol set metric-type Sets the metric type for the destination routing protocol set tag Sets tag value for destination routing protocol set weight Specifies the BGP weight value
    • 77. Chapter 4 77© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Configuring Route Maps for Redistribution  The route map REDIS-RIP tests the following; • In sequence 10, any routes matching ACLs 23 or 29 will have their metric changed accordingly. • In sequence 20, any routes matching ACLs 37 will not be redistributed. • In sequence 30, all other routes will have their metric changed accordingly.  Finally, all RIP routes and subnets will be redistributed into OSPF according to the REDIS-RIP route map statements. R1(config)# access-list 23 permit 10.1.0.0 0.0.255.255 R1(config)# access-list 29 permit 172.16.1.0 0.0.0.255 R1(config)# access-list 37 permit 10.0.0.0 0.255.255.255 R1(config)# R1(config)# route-map REDIS-RIP permit 10 R1(config-route-map)# match ip address 23 29 R1(config-route-map)# set metric 500 R1(config-route-map)# set metric-type type-1 R1(config-route-map)# R1(config-route-map)# route-map REDIS-RIP deny 20 R1(config-route-map)# match ip address 37 R1(config-route-map)# R1(config-route-map)# route-map REDIS-RIP permit 30 R1(config-route-map)# set metric 5000 R1(config-route-map)# set metric-type type-2 R1(config-route-map)# R1(config-route-map)# router ospf 10 R1(config-router)# redistribute rip route-map REDIS-RIP subnets R1(config-router)#
    • 78. Chapter 4 78© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Route Feedback  There is a possibility that routing feedback might cause suboptimal routing when routes are redistributed by more than one router such as in the two-way multipoint redistribution configuration on R1 and R2.  The following explains the routing feedback loop for this scenario: • RIPv2 on R3 advertises network 192.168.1.0. • R1 redistributes the 192.168.1.0 network into OSPF. • OSPF then propagates this route through the OSPF domain. • An OSPF router eventually advertises the 192.168.1.0 network to R2. • R2 then redistributes 192.168.1.0 from OSPF back into the original RIPv2 network creating a routing feedback loop. RIPv2 OSPF Area 0 O E2 192.168.1.0 [110/20] O E2 192.168.1.0 [110/20] R1 R2 R 192.168.1.0 [120/1] R3
    • 79. Chapter 4 79© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Route Maps to Avoid Route Feedback R1(config)# access-list 1 permit 192.168.1.0 0.0.0.255 R1(config)# route-map OSPF-into-RIP deny 10 R1(config-route-map)# match ip address 1 R1(config-route-map)# route-map OSPF-into-RIP permit 20 R1(config-route-map)# router rip R1(config-router)# redistribute ospf 10 metric 5 route-map OSPF-into-RIP R1(config-router)# router ospf 10 R1(config-router)# redistribute rip subnets R1(config-router)#  To prevent the routing feedback loop, a route map called OSPF-into-RIP has been applied to R1 and R2.  In sequence 10, any routes matching ACL 1 is denied and will not be redistributed back into RIP.  In sequence 20, all other routes are permitted to be redistributed and will be assigned a RIP metric of 5. RIPv2 OSPF Area 0 O E2 192.168.1.0 [110/20] O E2 192.168.1.0 [110/20] R1 R2 R 192.168.1.0 [120/1] R3
    • 80. Chapter 4 80© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Using Distribute Lists  Another way to control routing updates is to use a distribute list which allows an ACL to be applied to routing updates for filtering purposes. • Administrators control which routes get distributed. • This control is for security, overhead, and management reasons.  It’s important to understanding that the distribution lists are used to control (filter) routing updates while ACLs filter user traffic.  Sample implementation plan: • Identify network traffic to be filtered using an ACL or route map. • Associate the distribute list with the ACL or route-map using the distribute-list router configuration command.
    • 81. Chapter 4 81© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Filter Incoming Routing Updates  Define a filter for incoming routing updates. Router(config-router)# distribute-list {access-list-number | name} [route-map map- tag] in [interface-type interface-number] Parameter Description access-list-number | name Specifies the standard access list number or name. map-tag (Optional) Specifies the name of the route map that defines which networks are to be installed in the routing table and which are to be filtered from the routing table. This argument is supported by OSPF only *** in Applies the access list to incoming routing updates. interface-type interface-number (Optional) Specifies the interface type and number from which updates are filtered.
    • 82. Chapter 4 82© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Filter Outgoing Routing Updates  Define a filter for outgoing routing updates. Router(config-router)# distribute-list {access-list-number | name} out [interface- name | routing-process [routing-process parameter]] Parameter Description access-list-number | name Specifies the standard access list number or name. out Applies the access list to outgoing routing updates. interface-name (Optional) Specifies the name of the interface out of which updates are filtered. routing-process (Optional) Specifies the name of the routing process, or the keyword static or connected, that is being redistributed and from which updates are filtered. routing-process parameter (Optional) Specifies a routing process parameter, such as the AS number of the routing process.
    • 83. Chapter 4 83© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public distribute-list out Or in  It is important to understand the differences between: • The distribute-list out command filters updates going out of the interface into the routing process under which it is configured. • The distribute-list in command filters updates going into the interface specified in the command, into the routing process under which it is configured. Filter outgoing routing updates R1(config-router)# distribute-list out Filter incoming routing updates R1 R1 R1(config-router)# distribute-list in
    • 84. Chapter 4 84© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Filter Outgoing Routing Updates Example 1a R2(config)# access-list 7 permit 172.16.0.0 0.0.255.255 R2(config)# R2(config)# router eigrp 1 R2(config-router)# network 172.16.0.0 R2(config-router)# network 192.168.5.0 R2(config-router)# distribute-list 7 out Serial0/0/0 R2(config-router)#  In this example, the network 10.0.0.0 must be hidden from the devices in network 192.168.5.0.  The distribute-list out command on R2 applies ACL 7 to packets going out S0/0/0 which only permits 172.16.0.0 routing information to be distributed out.  The implicit deny any at the end of the ACL prevents updates about any other networks from being advertised and as a result, network 10.0.0.0 is hidden. 10.0.0.0 172.16.0.0 192.168.5.0 EIGRP AS 1 R1 R2 R3 D 10.0.0.0/8 [90/...] S0/0/0 D 172.16.0.0/16 [90/...] D 10.0.0.0/8 [90/...]
    • 85. Chapter 4 85© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Filter Outgoing Routing Updates Example 1b R2(config)# access-list 7 deny 10.0.0.0 0.255.255.255 R2(config)# access-list 7 permit any R2(config)# R2(config)# router eigrp 1 R2(config-router)# network 172.16.0.0 R2(config-router)# network 192.168.5.0 R2(config-router)# distribute-list 7 out Serial0/0/0 R2(config-router)#  As an alternative, network 10.0.0.0 can be explicitly denied and all other routes are valid.  The distribute-list out command on R2 applies ACL 7 to packets going out S0/0/0 which denies the 10.0.0.0/8 network but permits all other routes. 10.0.0.0 172.16.0.0 192.168.5.0 EIGRP AS 1 R1 R2 R3 D 10.0.0.0/8 [90/...] S0/0/0 D 172.16.0.0/16 [90/...] D 10.0.0.0/8 [90/...]
    • 86. Chapter 4 86© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Distribute Lists to Avoid Route Feedback 10.0.0.8/30 RIPv2 R1 R2 R3S0/0/3 R4 10.8.0.0/16 10.9.0.0/16 10.10.0.0/16 10.11.0.0/16 OSPF 10.0.0.0/3010.1.0.0/16 10.2.0.0/16 10.3.0.0/16 R2(config)# access-list 2 deny 10.8.0.0 0.3.255.255 R2(config)# access-list 2 permit any R2(config)# access-list 3 permit 10.8.0.0 0.3.255.255 R2(config)# router ospf 1 R2(config-router)# network 10.0.0.8 0.0.0.3 area 0 R2(config-router)# redistribute rip subnets R2(config-router)# distribute-list 2 out rip R2(config-router)# router rip R2(config-router)# network 10.0.0.0 R2(config-router)# version 2 R2(config-router)# passive-interface Serial0/0/3 R2(config-router)# redistribute ospf 1 metric 5 R2(config-router)# distribute-list 3 out ospf 1 R2(config-router)#
    • 87. Chapter 4 87© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Drawback of Distribute Lists  Using distribute lists as route filters has several drawbacks, including: • A subnet mask cannot be easily matched. • ACLs are evaluated sequentially for every IP prefix in the routing update. • An extended ACL can be cumbersome to configure. • A distribute list hides network information, which could be considered a drawback in some circumstances. • For example, in a network with redundant paths, a distribute list might permit routing updates for only specific paths, to avoid routing loops. • In this case, if the primary path goes down, the backup paths are not used because the rest of the network does not know they exist. • When redundant paths exist, use other techniques.
    • 88. Chapter 4 88© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Using Prefix Lists  Prefix lists can be used as an alternative to access lists in many route filtering commands.  Prefix list characteristics include: • A significant performance improvement over ACLs in loading and route lookup of large lists. • Support for incremental modifications. • An improved user-friendly command-line interface. • Greater flexibility in specifying subnet mask ranges.
    • 89. Chapter 4 89© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Similarities Between Prefix Lists and ACLs  A prefix list can consist of any number of lines, each of which indicates a test and a result.  When a router evaluates a route against the prefix list, the first line that matches results in either a permit or deny.  If none of the lines in the list match, the result is “implicitly deny,” just as it is in an access list.
    • 90. Chapter 4 90© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Prefix List Filtering rules  An empty prefix list permits all prefixes.  If a prefix is permitted, the route is used. If a prefix is denied, the route is not used.  Prefix lists consist of statements with sequence numbers. The router begins the search for a match at the top of the prefix list, which is the statement with the lowest sequence number.  When a match occurs, the router does not need to go through the rest of the prefix list. For efficiency, you might want to put the most common matches (permits or denies) near the top of the list by specifying a lower sequence number.  An implicit deny is assumed if a given prefix does not match any entries in a prefix list.
    • 91. Chapter 4 91© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Configure a Prefix List  Define a prefix list. Router(config)# ip prefix-list {list-name | list-number} [seq seq-value] {deny | permit} network/length [ge ge-value] [le le-value] Parameter Description list-name The name of the prefix list that will be created (it is case sensitive). list-number The number of the prefix list that will be created. seq seq-value A 32-bit sequence number of the prefix-list statement. Default sequence numbers are in increments of 5 (5, 10, 15, and so on). deny | permit The action taken when a match is found. network / length The prefix to be matched and the length of the prefix. The network is a 32-bit address; the length is a decimal number. ge ge-value (Optional) The range of the prefix length to be matched. The range is assumed to be from ge-value to 32 if only the ge attribute is specified. le le-value (Optional) The range of the prefix length to be matched. The range is assumed to be from length to le-value if only the le attribute is specified.
    • 92. Chapter 4 92© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Configure a Prefix List  Use the no ip prefix-list list-name global configuration command to delete a prefix list.  The ip prefix-list list-name description text global configuration command can be used to add or delete a text description for a prefix list.  Tip: • For best performance, the most frequently processed prefix list statements should be configured with the lowest sequence numbers. • The seq seq-value keyword can be used for re-sequencing.
    • 93. Chapter 4 93© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Prefix-list Scenario #1 R1(config)# ip prefix-list TEN-ONLY permit 172.16.10.0/8 le 24 R1(config)# router bgp 65000 R1(config-router)# aggregate-address 172.16.0.0 255.255.0.0 R1(config-router)# neighbor 10.1.1.1 remote-as 65001 R1(config-router)# neighbor 10.1.1.1 prefix-list TEN-ONLY out R1(config-router)# exit R1(config)# do show running-config | include ip prefix-list ip prefix-list TEN-ONLY seq 5 permit 172.0.0.0/8 le 24 R1(config)# 172.16.11.0 172.16.10.0 AS 65000 R2 R3 R1 10.1.1.1 AS 65001  Notice that the last line of this configuration changed to ip prefix-list TEN-ONLY permit 172.0.0.0/8 le 24  This is because only the first 8 bits in the address are considered significant when a prefix length of /8 is used.  In this case, neighbor R3 learns about 172.16.0.0/16, 172.16.10.0/24, and 172.16.11.0/24.  These are the routes that match the first 8 bits of 172.0.0.0 and have a prefix length between 8 and 24.
    • 94. Chapter 4 94© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Prefix-list Scenario #2 R1(config)# ip prefix-list TEN-ONLY permit 172.16.10.0/8 le 16 R1(config)# router bgp 65000 R1(config-router)# aggregate-address 172.16.0.0 255.255.0.0 R1(config-router)# neighbor 10.1.1.1 remote-as 65001 R1(config-router)# neighbor 10.1.1.1 prefix-list TEN-ONLY out R1(config-router)# exit R1(config)# 172.16.11.0 172.16.10.0 AS 65000 R2 R3 R1 10.1.1.1 AS 65001  Now neighbor R3 learns only about 172.16.0.0/16.  This is the only route that matches the first 8 bits of 172.0.0.0 and has a prefix length between 8 and 16.
    • 95. Chapter 4 95© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Prefix-list Scenario #3 R1(config)# ip prefix-list TEN-ONLY permit 172.16.10.0/8 ge 17 R1(config)# router bgp 65000 R1(config-router)# aggregate-address 172.16.0.0 255.255.0.0 R1(config-router)# neighbor 10.1.1.1 remote-as 65001 R1(config-router)# neighbor 10.1.1.1 prefix-list TEN-ONLY out R1(config-router)# exit R1(config)# 172.16.11.0 172.16.10.0 AS 65000 R2 R3 R1 10.1.1.1 AS 65001  Now neighbor R3 learns only about 172.16.10.0/24 and 172.16.11.0/24.  R1 ignores the /8 parameter and treats the command as if it had the parameters ge 17 le 32.
    • 96. Chapter 4 96© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Prefix-list Scenario #4 R1(config)# ip prefix-list TEN-ONLY permit 172.16.10.0/8 ge 16 le 24 R1(config)# router bgp 65000 R1(config-router)# aggregate-address 172.16.0.0 255.255.0.0 R1(config-router)# neighbor 10.1.1.1 remote-as 65001 R1(config-router)# neighbor 10.1.1.1 prefix-list TEN-ONLY out R1(config-router)# exit R1(config)# 172.16.11.0 172.16.10.0 AS 65000 R2 R3 R1 10.1.1.1 AS 65001  Now neighbor 10.1.1.1 learns about 172.16.0.0/16, 172.16.10.0/24, and 172.16.11.0/24.  R1 ignores the /8 parameter and treats the command as if it had the parameters ge 16 le 24.
    • 97. Chapter 4 97© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Prefix-list Scenario #5 R1(config)# ip prefix-list TEN-ONLY permit 172.16.10.0/8 ge 17 le 24 R1(config)# router bgp 65000 R1(config-router)# aggregate-address 172.16.0.0 255.255.0.0 R1(config-router)# neighbor 10.1.1.1 remote-as 65001 R1(config-router)# neighbor 10.1.1.1 prefix-list TEN-ONLY out R1(config-router)# exit R1(config)# 172.16.11.0 172.16.10.0 AS 65000 R2 R3 R1 10.1.1.1 AS 65001  Now neighbor 10.1.1.1 learns about 172.16.10.0/24 and 172.16.11.0/24.  R1 ignores the /8 parameter and treats the command as if it had the parameters ge 17 le 24.
    • 98. Chapter 4 98© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Verifying Prefix Lists Command Description show ip prefix-list [detail | summary] Displays information on all prefix lists. Specifying the detail keyword includes the description and the hit count in the display. show ip prefix-list [detail | summary] prefix-list-name Displays a table showing the entries in a specific prefix list. show ip prefix-list prefix-list- name [network/length] Displays the policy associated with a specific network/length in a prefix list. show ip prefix-list prefix-list- name [seq sequence-number] Displays the prefix list entry with a given sequence number. show ip prefix-list prefix-list- name [network/length] longer Displays all entries of a prefix list that are more specific than the given network and length. show ip prefix-list prefix-list- name [network/length] first-match Displays the entry of a prefix list that matches the network and length of the given prefix. clear ip prefix-list prefix-list- name [network/length] Resets the hit count shown on prefix list entries.
    • 99. Chapter 4 99© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Multiple Methods to Control Routing Updates  The example displays how a combination of prefix lists, distribute lists, and route maps can be applied to incoming or outgoing information. • All must permit the routes that are received from a neighbor before they will be accepted into the IP routing table. • Outgoing routes must pass the outgoing distribute list, the outgoing prefix list, and the outgoing route map before being forwarded to the neighbor.
    • 100. Chapter 4 100© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public Resources  Commonly Used IP ACLs http://cisco.com/en/US/tech/tk648/tk361/technologies_configuration_exa mple09186a0080100548.shtml  Default Passive Interface Feature http://cisco.com/en/US/products/sw/iosswrel/ps1830/products_feature_g uide09186a008008784e.html  Route-Maps for IP Routing Protocol Redistribution Configuration http://cisco.com/en/US/tech/tk365/technologies_tech_note09186a00804 7915d.shtml
    • 101. Chapter 4 101© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco Public  Lab 4-1 Redistribution Between RIP and OSPF  Lab 4-2 Redistribution Between EIGRP and OSPF  Lab 4-3 Manipulating Administrative Distance  Lab 4-4 EIGRP and OSPF Case Study Chapter 4 Labs

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