Implementing Routing


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  • Once you create an internetwork by connecting your WANs and LANs to a router, you then need to configure logical network addresses, such as IP addresses, to all hosts on the internetwork so that they can communicate across that internetwork. The term routing is used for taking a packet from one device and sending it through the network to another device on a different network. Routers don’t care about hosts—they only care about networks and the best path to each network. The logical network address of the destination host is used to get packets to a network through a routed network, then the hardware address of the host is used to deliver the packet from a router to the correct destination host.
  • The router will packet switch the packet to the FastEthernet 0/0 interface and then frame it and send it out the LAN
  • The IP routing process is fairly simple and doesn’t change, regardless of the size of network you have. For an example, we’ll describe step by step what happens when Host A wants to communicate with Host B on a different network. In this example, a user on Host A pings Host B’s IP address.
  • Here’s the command you use to add a static route to a routing table: ip route [ destination_network ] [ mask ] [ next-hop_address or exitinterface ] [ administrative_distance ] [permanent] This list describes each command in the string: Ip route: The command used to create the static route. Destination network: The network you’re placing in the routing table. Mask: The subnet mask being used on the network. Next-hop address: The address of the next-hop router that will receive the packet and forward it to the remote network. This is a router interface that’s on a directly connected network. You must be able to ping the router interface before you add the route. Exit interface: You can use it in place of the next-hop address if you want, but it’s got to be on a point-to-point link, like a WAN. This command won’t work on a LAN like Ethernet. Administrative distance: By default, static routes have an administrative distance of 1. You can change the default value by adding an administrative weight at the end of the command. Permanent: If the interface is shut down, or the router can’t communicate to the next-hop router, the route will automatically be discarded from the routing table. Choosing the permanent option keeps the entry in the routing table no matter what happens.
  • We use default routing to send packets with a remote destination network not in the routing table to the next-hop router. You can only use default routing on stub networks—those with only one exit port out of the network.
  • A routing protocol is used by routers to dynamically find all the networks in the internetwork and to ensure that all routers have the same routing table. Basically, a routing protocol determines the path of a packet through an internetwork. Examples of routing protocols are RIP, IGRP, EIGRP and OSPF. Okay—once all routers know about all networks, a routed protocol can be used to send user data (packets) through the established enterprise. Routed protocols are assigned to an interface and determine the method of packet delivery. Examples of routed protocols are IP and IPX.
  • There are two types of routing protocols used in internetworks: interior gateway protocols (IGPs) and exterior gateway protocols (EGPs). IGPs are used to exchange routing information with routers in the same autonomous system (AS). An AS is a collection of networks under a common administrative domain, which basically means that all routers sharing the same routing table information are in the same AS. EGPs are used to communicate between ASs. An example of an EGP is Border Gateway Protocol (BGP), which is discussed in the GlobalNet CCNP course.
  • Classful routing means that all devices in the network must use the same subnet mask.
  • 5 The administrative distance (AD) is used to rate the trustworthiness of routing information received on a router from a neighbor router. An administrative distance is an integer from 0 to 255, where 0 is the most trusted and 255 means no traffic will be passed via this route. If a router receives two updates listing the same remote network, the first thing the router checks is the AD. If one of the advertised routes has a lower AD than the other, then the route with the lowest AD will be placed in the routing table. If both advertised routes to the same network have the same AD, then routing protocol metrics (such as hop count or bandwidth of the lines) will be used to find the best path to the remote network. The advertised route with the lowest metric will be placed in the routing table. But if both advertised routes have the same AD as well as the same metrics, then the routing protocol will load-balance to the remote network.
  • The distance-vector protocols find the best path to a remote network by judging distance. Each time a packet goes through a router, that’s called a hop . The route with the least number of hops to the network is determined to be the best route. The vector indicates the direction to the remote network. Both RIP and IGRP are distance-vector routing protocols.
  • Routers, when powered up and the interfaces are enabled, have only their directly connected networks in the routing table
  • 5 Routing Information Protocol (RIP) is a true distance-vector routing protocol. It sends the complete routing table out to all active interfaces every 30 seconds. RIP only uses hop count to determine the best way to a remote network, but it has a maximum allowable hop count of 15 by default, meaning that 16 is deemed unreachable. RIP works well in small networks, but it’s inefficient on large networks with slow WAN links or on networks with a large number of routers installed. RIP version 1 uses only classful routing , which means that all devices in the network must use the same subnet mask.
  • To configure RIP routing, just turn on the protocol with the router rip command and tell the RIP routing protocol which networks to advertise. That’s it. Understand that RIP is configured with classful routing network addresses!
  • Easy configuration, just add the command “version 2” under the router rip configuration. RIPv2 is the preferred choice over RIPv1 because it supports VLSM and discontiguous networks.
  • If you create VLSM network, sometimes you may find that the backbone connecting buildings together is a different class of network. This is called discontiguous addressing. By default routing protocols will not work across discontiguous networks. By using the “no auto-summary” command on the network boundaries, routing protocols will be able do work across a discontiguous addressed network.
  • You probably don’t want your RIP network advertised everywhere on your LAN and WAN—there’s not a whole lot to be gained by advertising your RIP network to the Internet, now is there? No worries—there are a few different ways to stop unwanted RIP updates from propagating across your LANs and WANs. The easiest one is through the passive-interface command. This command prevents RIP update broadcasts from being sent out a defined interface, but that same interface can still receive RIP updates.
  • Show ip protocols: show routing protocols information and timers Show protocols: show routed protocol information Show ip route: displays the routing table Debug ip rip: show rip updates being sent and received on your router Undebug all or no debug ip rip: turns off debugging
  • An autonomous system is a collection of networks under a common administrative domain. Interior Gateway Routing Protocol (IGRP) is a Cisco-proprietary distance-vector routing protocol. This means that all your routers must be Cisco routers to use IGRP in your network. Cisco created this routing protocol to overcome the problems associated with RIP. IGRP: Allows up to 6 unequal paths to a remote network (4 by default) Supports up to 255 hops Full route table broadcast every 90 seconds Hold down timer is 280 seconds Flush timer is 630 seconds Suitable for use in large Internetworks (compared to RIP)
  • IGRP uses bandwidth and delay of the line by default as a metric for determining the best route to an internetwork. This is called a composite metric . Reliability, load, and maximum transmission unit (MTU) can also be used, although they are not used by default. IGRP can load-balance up to six unequal links. RIP networks must have the same hop count to load-balance, whereas IGRP uses bandwidth to determine how to load-balance. To load-balance over unequal-cost links, the variance command controls the load balancing between the best metric and the worst acceptable metric.
  • The command used to configure IGRP is the same as the one used to configure RIP routing with one important difference: you use an autonomous system (AS) number. All routers within an autonomous system must use the same AS number, or they won’t communicate with routing information. You absolutely must remember that you type a classful network number in when configuring IGRP!
  • Show ip protocols: show routing protocols information and timers Show ip route: displays the routing table debug ip igrp transactions command shows message requests from neighbor routers asking for an update and the broadcasts sent from your router towards that neighbor router. debug ip igrp events command is a summary of the IGRP routing information that is running on the network. Undebug all: turns off debugging
  • Implementing Routing

    1. 1. Routing Design, Configuration, and Troubleshooting
    2. 2. Topics <ul><li>Designing a LAN </li></ul><ul><li>Designing a simple internetwork </li></ul><ul><li>Designing IP addressing schemes </li></ul><ul><li>Configuring routers and hosts </li></ul><ul><li>Troubleshooting routing protocols and addressing schemes </li></ul><ul><li>Describing the communications process </li></ul>
    3. 3. <ul><li>Packet destination address </li></ul><ul><li>Neighbor routers to learn about remote networks </li></ul><ul><li>Possible routes to all remote networks </li></ul><ul><li>The best route to each remote network </li></ul><ul><li>How to maintain and verify the routing information </li></ul>What Routers Need to Know to Route Packets C B A D
    4. 4. Basic Path Selection <ul><li>Routers consult tables stored in memory to determine next hop </li></ul><ul><li>The network portion of the IP address stored in the IP header is compared to network addresses stored in router tables </li></ul><ul><li>If no match is found a default gateway is used to route the packet </li></ul>
    5. 5. Simple IP Routing B A e0 e0 >ping B s0 s0 Host A Host B
    6. 6. The IP Routing Process <ul><li>IP packets are created and sent out a default gateway </li></ul><ul><li>Each intermediate router strips framing headers and trailers from packet and checks IP address </li></ul><ul><li>After consulting routing tables the packet is reframed and sent to the next hop </li></ul><ul><li>Once the destination network is reached, an ARP request identifies the MAC address fo the destination computer </li></ul>
    7. 7. Route Types <ul><li>Static Routes </li></ul><ul><ul><li>Route tables are manually configured by an administrator </li></ul></ul><ul><li>Dynamic Routes </li></ul><ul><ul><li>A routing protocol is configured on the router </li></ul></ul><ul><ul><li>Routers then communicate with neighboring routers to update table information </li></ul></ul>
    8. 8. Advantages of Static Routing <ul><li>No overhead on router CPU </li></ul><ul><ul><li>No dynamic processing of route table info required </li></ul></ul><ul><li>No bandwidth usage between routers </li></ul><ul><ul><li>No communication needed to update route tables </li></ul></ul><ul><li>Adds security </li></ul><ul><ul><li>Administrator chooses network access </li></ul></ul>
    9. 9. Disadvantages of Static Routing <ul><li>Administrator must have a thorough understanding of networks and routing </li></ul><ul><li>Administrator must add new routes and maintain all existing routes by hand </li></ul><ul><li>Scaling is an issue – maintaining a large network becomes a full-time job for the administrator </li></ul>
    10. 10. Default Routes <ul><li>Used to send packets with a remote destination not in the routing table to the next hop router </li></ul><ul><li>Used on in stub networks </li></ul><ul><ul><li>networks with only one exit path out of the network </li></ul></ul>
    11. 11. Routing Protocol Basics <ul><li>Routing protocols provide dynamic route table updating </li></ul><ul><li>Two general classes exist </li></ul><ul><ul><li>Distance Vector (RIP, IGRP) </li></ul></ul><ul><ul><li>Link State (OSPF, EIGRP) </li></ul></ul><ul><li>Distance Vector protocols will be discussed in this chapter </li></ul>
    12. 12. Autonomous Systems AS 1 AS2 IGPs: RIP, IGRP EGPs: BGP <ul><li>An autonomous system (AS) is a collection of networks under a common administrative domain. </li></ul><ul><li>Interior Gateway Protocols (IGPs) operate within an AS. </li></ul><ul><li>Exterior Gateway Protocols (EGPs) connect different ASes </li></ul>
    13. 13. Routing Classes <ul><li>Classful </li></ul><ul><ul><li>Subnet mask not advertised with routes </li></ul></ul><ul><ul><li>Standard classful masks are used by routers </li></ul></ul><ul><ul><li>RIP, IGRP are classful </li></ul></ul><ul><li>Classless </li></ul><ul><ul><li>masks are advertised </li></ul></ul><ul><ul><li>VLSM is supported </li></ul></ul><ul><ul><li>OSPF, EIGRP are classless </li></ul></ul>
    14. 14. Administrative Distance <ul><li>A measure of trustworthiness for a route </li></ul><ul><li>For multiple routes to a remote network </li></ul><ul><ul><li>the route with the lowest AD is placed in routing table </li></ul></ul><ul><ul><li>if routes have same AD, other metrics can be used to determine best route </li></ul></ul><ul><ul><li>if routes have same AD and same metrics, load balancing is implemented </li></ul></ul>
    15. 15. Distance Vector Protocols <ul><li>RIP, IGRP </li></ul><ul><li>Distance – number of hops to destination </li></ul><ul><li>Vector – direction (path) to destination </li></ul><ul><li>Entire routing table is advertised to neighboring routers for update purposes </li></ul><ul><ul><li>called “Routing by rumor” because validity of route tables is assumed </li></ul></ul>
    16. 16. Distance Vector Issues <ul><li>Periodic table advertisement </li></ul><ul><li>Slow Convergence </li></ul><ul><li>Pinhole Congestion </li></ul><ul><li>Routing loops </li></ul><ul><li>Restricted metrics </li></ul><ul><li>Load balancing techniques </li></ul><ul><li>Note: Severity depends upon specific Distance Vector implementation </li></ul>
    17. 17. Slow Convergence Routing Loop
    18. 18. Pinhole Congestion
    19. 19. Distance Vector Problem Solutions <ul><li>Maximum hop count </li></ul><ul><ul><li>fix for count to infinity problem </li></ul></ul><ul><li>Split horizon </li></ul><ul><ul><li>fix for routing loop problems </li></ul></ul><ul><li>Route poisoning </li></ul><ul><ul><li>fix for inconsistent updates </li></ul></ul><ul><li>Holddowns </li></ul><ul><ul><li>prevent regular update messages from reinstating problem routes </li></ul></ul>
    20. 20. Discovering Routes
    21. 21. Discovering Routes
    22. 22. RIP Overview <ul><li>Hop count metric selects the path, 16 is unreachable </li></ul><ul><li>Full route table broadcast every 30 seconds </li></ul><ul><li>Load balance maximum of 6 equal cost paths (default = 4) </li></ul><ul><li>RIPv2 supports VLSM and Discontiguous networks </li></ul>
    23. 23. RIP Routing Configuration Router(config)# router rip Router(config-router)# network network-number* network network router RIP network network router RIP
    24. 24. RIP Version 2 <ul><li>Allows the use of variable length subnet masks (VLSM) by sending subnet mask information with each route update </li></ul><ul><li>Distance Vector – same AD, and timers </li></ul><ul><li>Easy configuration, just add the command “version 2” during router configuration </li></ul>router rip network version 2
    25. 25. Discontiguous Addressing <ul><li>Two networks of the same class separated by a different network address </li></ul> <ul><li>Classful RIPv1 and IGRP cannot support discontiguous subnets </li></ul><ul><li>Classless OSPF, EIGRP, and RIPv2 support discontiguous subnets </li></ul>
    26. 26. Passive Interface Command <ul><li>Allows a router to receive updates on an interface, but not send updates via that interface </li></ul><ul><ul><li>Router(config)# router rip </li></ul></ul><ul><ul><li>Router(config-router)# passive-interface serial0 </li></ul></ul>S0 Gateway Internet Updates X
    27. 27. Verifying RIP <ul><li>show ip protocols </li></ul><ul><li>show protocols </li></ul><ul><li>show ip route </li></ul><ul><li>debug ip rip </li></ul><ul><li>undebug all (un all) </li></ul>
    28. 28. IGRP <ul><li>Interior Gateway Routing Protocol - Cisco Proprietary </li></ul><ul><li>Uses bandwidth and delay of the line by default to select best path </li></ul>Autonomous System Router D Router B Router A Router C Requires Autonomous System number for activation
    29. 29. IGRP vs. RIP <ul><li>IGRP </li></ul><ul><ul><li>is more scalability than RIP </li></ul></ul><ul><ul><li>responds faster to network changes </li></ul></ul><ul><ul><li>uses a more sophisticated metric </li></ul></ul><ul><ul><li>supports multiple-paths </li></ul></ul>
    30. 30. Configuring IGRP Router(config)# router igrp autonomous-system Router(config-router)# network network-number* network network router IGRP 10 network network router IGRP 10
    31. 31. Verifying IGRP <ul><li>show ip route </li></ul><ul><li>show ip protocols </li></ul><ul><li>debug ip igrp transaction </li></ul><ul><li>debug ip igrp events </li></ul><ul><li>undebug all (un all) </li></ul>