Link state routing protokollerinin en onemlı farkı topology table a sahıp olmasıdır. Bu sayede dusen bır lınk ıcın, yedek linklerde bu tabloda tutuldugundan cok hızlı bır sekılde convergence saglanır.
10 Metrics In OSPF, all interfaces have a cost value or routing metric used in the OSPF link-state calculation. A metric value is configured based on bandwidth to compare different paths through an AS. OSPF uses cost values to determine the best path to a particular destination: the lower the cost value, the more likely the interface will be used to forward data traffic. To calculate the cost of a link a reference bandwidth is set. The reference bandwidth is referenced in kilobits per second and provides a reference for the default costing of interfaces based on their underlying link speed. The default interface cost is calculated as follows: The default reference-bandwidth is 100 000 000 kb/s or 100 Gb/s, so the default auto-cost metrics for various link speeds are as as follows: 10-Mb/s link default cost of 10 000 100-Mb/s link default cost of 1000 1-Gb/s link default cost of 100 10-Gb/s link default cost of 10 The reference-bandwidth command assigns a default cost to the interface based on the interface speed. To override this default cost on a particular interface,
18 OSPF uses IP multicast addressing to communicate with routing peers. This reduces the overhead of other devices on the same segment that are not running OSPF. OSPF has two reserved multicast IP addresses. The first is 220.127.116.11 and is used to communicate with all OSPF speakers. The second is 18.104.22.168 and is used in multi-access broadcast topologies in which a DR/BDR is required for proper OSPF operations. When an OSPF update is sent on an Ethernet topology, the destination MAC address is modified to use the reserved multicast range. The range has the first 24 bits of the MAC address, normally reserved for the manufacturer code, set to 01-00-5E. The remaining 24 bits of the MAC address are the lower 24 bits of the IP multicast address. With OSPF, the relationship between the IP multicast address and the MAC address is as follows: 22.214.171.124 and 01-00-5E-00-00-05: Any OSPF speaker 126.96.36.199 and 01-00-5E-00-00-06: Any DR/BDR
19 OSPF updates are sent using the IP header at the network layer. However, unlike RIP, OSPF does not use a transport-layer protocol. Instead, all OSPF updates are sent directly from the IP layer to the OSPF process. To accomplish this, reserved protocol number 89 in the IP header is allocated to identify OSPF traffic.
20 The OSPF header breaks down into the following fields: Version number — Identifies the version of OSPF that this packet pertains to. Type — The type of packet that is being received. There are five different types of packet, described on the next page. Packet length — The overall size of the packet. This does not include the IP header but does include all bytes in the OSPF update. Router ID — The Router ID of the sending router. Area ID — The area the router is sending the packet. All routers connected to a network must agree on which area the network resides in. Checksum — The CRC (similar to FCS) for the OSPF header. Authentication type — All OSPF protocol exchanges can be authenticated. This means that only trusted routers can participate in autonomous system routing. Authentication — When packets are sent with authentication invoked, this field is used to convey the authentication information. MD5 allows one authentication key to be configured per network. Routers in the same routing domain must be configured with the same key. When the MD5 hashing algorithm is used for authentication, MD5 is used to verify data integrity by creating a 128-bit message digest from the data input. The message digest is unique to that data. Data — This field varies depending on the type of OSPF packet being sent.
21 OSPF uses 5 different types of packets to establish and maintain router connectivity and network convergence. Hello packet — This packet is used to establish adjacencies with other routers that speak OSPF. It is also used to maintain neighbor connectivity by being propagated periodically, typically every 10 seconds. However, this value can be modified from 0 to 65 535 seconds. Database description — This packet conveys a summary of all networks in the router’s database. Typically this is the classless network, the router’s cost to access, and the sequence number associated with the network entry. Link-state request — When a neighbor router receives a database description packet, it compares the entry in its current link-state database with the information received. If a received network is not in the database or if the sequence number for a network is higher, the router generates a link-state request for more information about the network. Link-state update — When it receives a link-state request, the router responds with the complete link-state database entry. To accomplish this, the router generates a type 4 (link-state update) packet and forwards it back to the requesting router. Link-state ACK — Each newly received LSA must be acknowledged. This is usually done by sending link-state ACK packets. Many ACKs may be grouped together in a single link-state ACK packet.
24 There are three types of authentication supported by OSPF. They are: No authentication — The default and least secure Simple authentication — The first level of secure communications between OSPF speakers, yet not very secure MD5 authentication — The most secure communications between OSPF speakers and highly recommended Information about how to configure security is provided in the OSPF configuration section.
25 A router uses the OSPF hello protocol to discover neighbors. A neighbor is a router that is configured with an interface to a common network. The router sends hello packets to a multicast address and receives hello packets in return. In broadcast networks, a DR and a BDR are elected. The DR is responsible for sending LSAs that describe the network, which reduces the amount of network traffic. The routers attempt to form adjacencies. An adjacency is a relationship formed between a router and the DR or BDR. For point-to-point networks, no DR or BDR is elected. An adjacency must be formed with the neighbor. To significantly improve adjacency formation and network convergence, a network should be configured as point-to-point if only two routers are connected, even if the network is a broadcast media such as Ethernet. When the link-state databases of two neighbors are synchronized, the routers are considered to be fully adjacent. When adjacencies are established, pairs of adjacent routers synchronize their topological databases. Not every neighboring router forms an adjacency. Routing-protocol updates are only sent to and received from adjacencies. Routers that do not become fully adjacent remain in the 2-way neighbor state.
26 The hello packet consists of the following fields: Header — The standard OSPF header is identical for all five types of packets. The only modification is that the type field has the value of “1” to signify that this is a hello packet. Network mask — The network mask field contains the network mask for the interface that the packet is being sent on. Hello interval — The hello interval must match for all neighbors on the segment. By default, 10-second hello interval. This can be modified to a value between 0 and 65 535. Options — The options field is usually left blank. RTR Pri — The router priority field denotes the priority value seeded on the router for use in electing a DR and BDR. A priority of 0 means that the router can never be a DR or BDR in the network connected to this interface. Router dead interval — The default value is 40 seconds, or four times the update interval. If a neighbor does not send a hello packet within this interval, the router assumes that the neighbor is not active and purges all information that the neighbor has conveyed. Designated router — This field denotes the elected DR. Backup designated router — This field denotes the elected BDR. Neighbor — This field varies depending on the number of neighbors the router has learned of on the interface. The neighbor’s RID is conveyed in this field. Routers on this interface look for their RID, to ensure that the router that is sending the hello sees them.
27 In the figure above, the two routers have not formed an adjacency. The following steps describe how the adjacency is created and the actions that are required. Both routers are in a down state: neither router has sent any OSPF-related packets. The router on the left sends a hello packet with the standard header. In the hello information, the router inserts its RID and leaves the neighbor field blank because it does not know of any other router on the Ethernet segment. The right-side router responds with its own hello. However, this router’s hello contains not only its RID, but also the RID of the left router. When each router sees that the other router acknowledges its existence, the state changes from down to 2-way.
28 The DBD packet advertises a summary of all networks that the advertising router knows about. Along with the networks, the router advertises the associated subnet mask and sequence number. The receiving router compares the network, subnet mask, and sequence number with its existing topology database entries. If the advertised network is unknown or if the network is known but the advertised sequence is higher, the receiving router requests more information about the network so that it can add the network to its database. If the network is already known and the sequence number is lower, the receiving router sends back an LSU with more up-to-date information. If the network is already in the database and the sequence numbers are identical, then the receiving router discards the information.
29 In the figure above, the two routers have not formed an adjacency. The following steps describe how the adjacency is created and the actions that are required. The neighboring routers establish a master/slave relationship. During this step, the initial DBD sequence number is determined for the exchange state. The router with the highest RID becomes the master, and its initial sequence number is used. This is part of step 1. The right-side router sends its DBD packet, describing its link-state database. The sequence number negotiated in step 1 is used. The left-side router increments the sequence number and sends the DBD packet, describing its link-state database.
30 When it receives a DBD (type 2) packet, the router determines which networks it needs to add to its database. The receiving router then generates an LSR for these networks. The LSR identifies the networks for which the router wants full information.
31 When it receives an LSR (type 3) packet, the receiving router sends back the full topology database entry for the requested networks. The size of this packet varies depending on the interface MTU and administrator settings. The size of the packet is limited by the interface MTU.
32 The adjacency continues to be created with the following steps: Each router is responsible for maintaining a bit of reliability. Each responds to the DBD with an ACK packet. This ensures that each knows the other has received the information without error. In the example, the right side router asks for explicit information with the use of an LSR. Both routers would actually be sending LSRs. When the LSR is sent, the exchange state changes to the loading state. Each router responds to the LSR with one or more LSU packets. These packets contain explicit details about the requested networks.
33 The final steps for creating the adjacency are described below: The LSUs are sent and acknowledged by each router. After all LSUs have been received and ACKs sent, each router now has an identical link-state database. The state changes from loading to full. This means that each router is fully converged with the other’s database. To maintain the adjacency, the routers send periodic hellos to each other. The default interval is 10 seconds. If something changes, then only that change in the database is sent to the neighbor.
36 When the connection between two OSPF routers is a point-to-point link, there is no need for a DR or BDR. All packets are sent using the 188.8.131.52 IP multicast address. This implementation is typically used on serial interfaces; however, it can also be configured on point-to-point Ethernet segments, in which only two routers are connected.
37 A router uses the OSPF hello protocol to discover neighbors. A neighbor is a router that is configured with an interface to a common network. The router sends hello packets to a multicast address and receives hello packets in return. In broadcast networks, a DR and a BDR are elected. The DR is responsible for sending LSAs that describe the network, which reduces the amount of network traffic. The routers attempt to form adjacencies. An adjacency is a relationship that is formed between a router and the DR or BDR. For point-to-point networks, no DR or BDR is elected. An adjacency must be formed with the neighbor. To significantly improve adjacency formation and network convergence, a network should be configured as point-to-point if only two routers are connected, even if the network is a broadcast media such as Ethernet. In the example above, RTR-A is the DR and RTR-B is the BDR. Routers C, D, and E will only form adjacencies with RTR-A and RTR-B, not with each other. Not every neighboring router forms an adjacency. Routing protocol updates are only sent to and received from adjacencies. Routers that do not become fully adjacent remain in the 2-way neighbor state.
38 In the example above, RTR-C has a topology change that needs to be conveyed. The following steps occur: RTR-C sends its update to the DRs using IP multicast address 184.108.40.206. Both DRs receive the update. The BDR monitors to see if the DR sends out updates to all other routers, including the BDR. The DR takes the update from RTR-C and floods the change to all other routers on the segment, using IP multicast address 220.127.116.11. Note: DR and BDR election is not required in point-to-point networks.
39 When a new router becomes active in a multi-access broadcast topology, it generates a hello (type 1) packet. The multicast address used is 18.104.22.168, which is the “all OSPF devices” address. The new router’s hello does not contain any neighbor RIDs because it has not yet seen any neighbors on the link.
40 One of the already active routers generates a periodic hello. This hello also uses the IP multicast address 22.214.171.124. The new router not only sees its RID in the hello, but it also learns of the other devices on the segment based on their RIDs. In addition, the hello packet identifies the active DR and BDR for the link.
41 Because a DR and BDR already exist, the new router now advertises its networks to the DRs by using the IP multicast address 126.96.36.199 (all DRs). The routers, that are not DRs, ignore this update because they are only listening for the 188.8.131.52 IP multicast address.
42 When the DR receives the update and determines that the advertised network is a new entry in its topological database, it generates a message about the change to all devices on the segment. To send this update, the DR uses the IP multicast address 184.108.40.206 (all OSPF devices). The BDR does not send an update because the DR has performed its job by sending the update already. All routers, except the DR, send a type 5 (ACK) packet back to the DR to acknowledge receipt of the topology change; this includes the BDR and the new router that generated the update to start with.
43 A step-by-step example of a failing network is shown above. As soon as the router detects the failure of a link (a link-state change), it immediately sends an update to the DRs using the IP multicast address of 220.127.116.11. The DR compares the update received with its topology database and sees that there is a change. It generates an LSU and sends it to all OSPF speakers on the segment, using the IP multicast address of 18.104.22.168. All devices, including the BDR and the originating router, acknowledge the LSU. Any router that is connected to other networks forward the LSU packet to its downstream neighbors on those networks.
49 In the example above, rtr5 is reset. When it comes up, it sends an OSPF hello packet. The RID is set to 22.214.171.124. There are no neighbors in this hello packet because it does not yet know of any neighbors on the segment.
50 The next packet is an OSPF hello packet sent by rtr4. The RID is set to 126.96.36.199, and because rtr4 has seen a hello packet from rtr5, it populates the neighbor with RID 188.8.131.52. rtr5 does the same when it receives the hello from rtr4. When both routers have sent a hello packet with the neighbor address populated, the adjacency state is changed to 2-way.
51 Both router priorities are the same. In this case, the router with the highest RID will be the DR. In the example above, rtr4 sends a hello packet with both the DR and BDR set to 10.10.0.1. The hello packet sent from rtr5 has the DR set to 10.10.0.1 and the BDR set to 10.10.0.2.
52 The router with the higher RID becomes the master, and its sequence number (i.e., 77793 in this example) will be used.
53 rtr4 sends its DBD with the sequence number set by rtr5, including the DB summary.
54 rtr5 sends its DBD; the sequence number is incremented and the DB summary is included.
55 rtr5 sends an LSR to rtr4 for any LSA that it does not have. rtr4 does the same.
56 rtr4 responds with an LSU for the requested LSAs. At the same time, rtr5 responds to rtr4’s request.
57 rtr5 responds with an LS ACK. rtr4 acknowledges the LSU received from rtr5. The adjacency state is now full. rtr4 and rtr5 continue to exchange hellos to maintain the adjacency.
62 The OSPF environment is organized using two primary elements: Area — A grouping of contiguous OSPF networks and hosts. OSPF areas are logical subdivisions of OSPF autonomous systems. The topology of each area is invisible to entities in other areas, and each area maintains its own topological database. Autonomous System — A group of networks and network equipment under a common administration. Backbone area The OSPF backbone area, area 0.0.0.0, must be contiguous and all other areas must be connected to it. The backbone distributes routing information among areas. If it is not practical to connect an area to the backbone, the ABRs must be connected via a virtual link. Stub area A stub area is a designated area that does not allow external route advertisements. Routers in a stub area do not maintain external routes. A single default route to an ABR replaces all external routes. This OSPF implementation supports the optional summary route (type 3) advertisement suppression from other areas into a stub area. This feature further reduces topological database sizes as well as OSPF protocol traffic, memory usage, and CPU route-calculation time. NSSA Another OSPF area type is called an NSSA. NSSAs are similar to stub areas in that no external routes are imported into the area from other OSPF areas . External routes learned by OSPF routers in the NSSA are advertised as type 7 LSAs within the NSSA area and are translated by ABRs into type 5 external route advertisements for distribution into other areas of the OSPF domain. An NSSA cannot be designated as the transit area of a virtual link.
63 There are several terms used to define the function of the routers in an OSPF topology. The following functions are based on where the router is placed and not on the size or model of the router: Backbone router — A router that resides in Area 0 (backbone area) and only communicates with routers in the backbone area. This can include other backbone routers and ABRs. Area border router — Any router that has interfaces configured to support more than one area. Typically, this is between the backbone area and one more area; however, it is not uncommon that an ABR supports access between the backbone area and several other areas. When this type of configuration exists, care must be taken to ensure that the memory and CPU are not over-taxed. Intra-area router — A router that resides in an area other than the backbone area and only communicates with other routers in that area. This can include other intra-area routers or ABRs. Autonomous system boundary router — A router that connects the OSPF routing domain with other network protocols, static routes, or interfaces that are not participating in the OSPF process.
64 LSAs describe the state of a router or network, including router interfaces and adjacency states. Each LSA is flooded throughout an area. The collection of LSAs from all routers and networks form the protocol's topological database. The distribution of topology database updates takes place along adjacencies. A router sends LSAs when its state changes and according to the configured interval. The packets include information about the router's adjacencies, which allows the routers to construct their topological databases. When a router discovers a routing table change or detects a change in the network link state, information is advertised to other routers to maintain identical routing tables. Router adjacencies are reflected in the contents of LSAs. The relationship between adjacencies and the link states allows the protocol to detect non-operating routers. LSAs flood the area. The flooding mechanism ensures that all routers in an area have the same topological database. The database consists of the collection of LSAs received from each router that belongs to the area. OSPF sends LSAs for only the links that have changed and only when a change has taken place. From the topological database, each router constructs a tree of shortest paths, with itself as root. From this tree, OSPF can determine the best route to every destination in the network. The SPF tree is used to construct the routing table.
65 Type 1 (router) LSAs are generated by each router, no matter what area they reside in. Type 1 updates are not forwarded between areas by ABRs. The link-state ID is the advertising router’s RID.
77 Type 2 (network) LSAs are generated by DRs in multi-access networks, such as Ethernet or NBMA topologies. Type 2 LSAs are not forwarded by ABRs. The DR for the network originates the LSA. The DR originates the LSA only if it is fully adjacent to at least one other router in the network. The network LSA is flooded throughout the area that contains the transit network, and no further. The network LSA lists those routers that are fully adjacent to the DR; each fully adjacent router is identified by its OSPF RID. The DR includes itself in this list. The link-state ID for a network LSA is the IP interface address of the DR. This value, masked by the network's address mask (which is also contained in the network LSA) yields the network's IP address.
80 Type 3 (summary) LSAs are generated by ABRs to advertise networks in one area to another area. By design, the summary LSA should be a true summary network advertisement not just for the individual networks that it knows about. This requires manual summarization configuration on the router by the network administrator.
87 Stub areas must conform to the following attributes: The area must be a dead end. In the example above, the only reason to enter Area 2 is to access networks within Area 2. Traffic would not pass through Area 2 to get to any other location. Virtual links are not supported. Type 5 LSAs are blocked by the ABR, and a default route is advertised instead into the area. However, type 3 and 4 LSAs are still advertised. Stub area, no summary must conform to the following attributes: All attributes of a stub area are the same. By adding “no summary”, the ABR blocks type 3, 4 and 5 LSAs; instead it advertises a default route. The ABR originates a type 3 LSA into the stub area. The link-state ID is 0.0.0.0, and the network mask is set to 0.0.0.0. The industry term is “totally stubby”.
96 The sequence number field is a 32-bit integer referenced hex notation. It is used to detect old and duplicate LSAs. The larger the sequence number, the more recent the LSA. The sequence number starts at 0x80000000; however, this value is reserved and unused. This leaves 0x80000001 as the smallest value possible. This sequence number is referred to as the constant InitialSequenceNumber. A router uses InitialSequenceNumber the first time it originates an LSA. Afterward, the LSA's sequence number is incremented each time the router originates a new instance of the LSA. When an attempt is made to increment the sequence number past the maximum value of 0x7fffffff (also referred to as MaxSequenceNumber), the current instance of the LSA must first be flushed from the routing domain. This is done by prematurely aging the LSA and re-flooding it. As soon as this flood has been acknowledged by all adjacent neighbors, a new instance can be originated with the sequence number InitialSequenceNumber. Sequence numbers increment any time that an LSA is sent around about a specific network. This can be due to a change in the state of the network or because the 30-minute timer has expired and a refresh is necessary.
97 It is common for a router to receive self originated LSAs via the flooding procedure. A self-originated LSA is detected when either: The LSA's advertising router is equal to the router's own RID The LSA is a network LSA and its link-state ID is equal to one of the router's own IP interface addresses If the received self-originated LSA is newer than the last instance that the router actually originated, the router must take special action. The reception of such an LSA indicates that there are LSAs in the routing domain that were originated by the router before the last time it was restarted. In most cases, the router must then advance the LSA's LS sequence number one past the received LS sequence number and originate a new instance of the LSA.
98 The backbone area in an OSPF AS must be contiguous, and all other areas must be connected to the backbone area. Sometimes this is not practical or is unreasonably expensive to implement. Virtual links can be used to connect to the backbone through a non-backbone area. The figure above shows routers A and B as the start and endpoints of the virtual link and Area 0.0.0.1 as the transit area. To configure virtual links, the router must be an ABR. Virtual links are identified by the RID of the other endpoint, another ABR. These two endpoint routers must be attached to a common area, called the transit area. The area through which the virtual link is configured must have full routing information. Transit areas pass traffic from an area adjacent to the backbone or to another area. The traffic does not originate in, nor is it destined for, the transit area. The transit area cannot be a stub area or an NSSA. Virtual links are part of the backbone and behave as if they were unnumbered point-to-point networks between the two routers. A virtual link uses the intra-area routing of its transit area to forward packets.
IGMP Snoopıng ile birlikte 'Unknown Multicast Filter' enabled ozelligi bilinmeyen gereksiz multicast trafigini router uzerinden gecisini engellemek icin kullanılır. Ludovico Solution for Belgian Police Customer problem .
The autonomous system (AS) can be subdivided into areas that group
together contiguous networks, routers connected to these networks, and
attached hosts. Each area has its own topological database, which is
invisible from outside the area. Routers within an area know nothing of the
detailed topology of other areas. Subdividing the AS into areas significantly
reduces the amount of routing protocol traffic as compared to treating the
entire AS as a single link state domain.
Feature Updates Update type Transport Authentication Metric Metric type Topology size Convergence RIPv2 Periodic Broadcast/Multicast UDP Simple and MD5 Hops Distance vector IS-IS Incremental L2 Multicast Layer 2 Simple and MD5 Cost Link-state OSPF Incremental L3 Multicast IP Simple and MD5 Cost Link-state Small/Medium Slow Fast Large Fast Large
(2) Hello, RID= 184.108.40.206 I see neighbor 220.127.116.11 2-Way Hello (4) Hello, RID=18.104.22.168 I see neighbor 22.214.171.124
OSPF — Database Descriptor Packet Format Checksum Router ID Area ID AuType Version# 2 Packet length Authentication Authentication Interface MTU Options DD sequence number LSA header 0 0 0 0 0 M MS 0 31
126.96.36.199 188.8.131.52 E0 E0 (1) (2) (3) (4) (1) LSR: Send information for the Following networks… (2) LSR: Send information for the following networks… (3) LSU: Here is what you requested (4) LSU: Here is what you requested
Election of the DR and BDR in multi-access networks:
RTR-C 184.108.40.206 D 220.127.116.11 E 18.104.22.168 RTR-A (DR) 22.214.171.124 RTR-B (BDR) 126.96.36.199
Routers use the 188.8.131.52 IP address to send updates to the DRs.
The BDR monitors the DR to ensure that it sends updates.
The DR uses 184.108.40.206 to send updates to all OSPF routers.
RTR-C sends update to All DRs using IP address 220.127.116.11 RTR-A sends update to All OSPF routers using IP address 18.104.22.168
OSPF — Adding a Router to a LAN DR BDR New router * The new router uses IP address 22.214.171.124 to send a hello. All routers will see the hello. Hello, RID = 126.96.36.199 I see no others RID – 188.8.131.52 RID – 184.108.40.206 RID – 220.127.116.11
OSPF – Learning Which Is the DR/BDR in a LAN DR BDR New router * The new router waits to see if any other router speaks OSPF. If so, it checks to see if a DR and BDR are present. Hello, RID = 18.104.22.168 I see 22.214.171.124 and 126.96.36.199 RID – 188.8.131.52 RID – 184.108.40.206 RID – 220.127.116.11
OSPF — Advertising a New Network DR BDR New router * The new router sends LSAs about networks to the DR and BDR via the 18.104.22.168 (all DRs) multicast address. LSA 22.214.171.124 RID – 126.96.36.199 RID – 188.8.131.52 RID – 184.108.40.206
OSPF — Updating Peers about a Network Change DR BDR LSA 220.127.116.11 * The DR sends an update to all routers about the new network learned. It waits for an ACK from all routers. RID – 18.104.22.168 RID – 22.214.171.124 RID – 126.96.36.199 New router
OSPF — Network Change Flow DR BDR * The DR sends an update to all routers about the network change. It waits for an ACK from all routers. LSA 1 2 3 LSA 188.8.131.52 LSA 184.108.40.206
Open Shortest Path First (OSPF) Section 3 — Adjacency Case Study
Adjacency between rtr4 and rtr5 rtr5 rtr4 220.127.116.11 18.104.22.168 10.10.1.0/30 .1 .2 10.10.1.4/30 .5
In a large enterprise with many routers and networks, the LSDB and
routing tables become large. This is not advantageous because:
· Large routing tables consume memory and result in more CPU cycles
being needed to make a forwarding decision.
· Large LSDBs consume memory.
· The processing of LSAs is CPU-intensive.
Dividing the network into OSPF areas can reduce these undesirable side
Some advantages of implementing OSPF areas are as follows:
· Routers internal to the area incur less overhead.
· The impact of a topology change is localized to the area in which it
occurs. Although the change is advertised outside the area, the
processing of LSA, and consequent modification of the SPF tree,
requires less CPU overhead.
· With careful network address planning, networks within an area can be
advertised in the form of a summary. This reduces the amount of
processing on all routers external to the area, as well as reducing the
size of the routing table.
OSPF — Area Overview Area 0 Area 1 Area 2 Autonomous System
Areas allow for summarization
Reduced flooding of topology changes
Hierarchal topology design
RTR-A RTR-B RTR-C RTR-D RTR-E
OSPF — Types of Routers Area 0 Area 1 Area 2 Autonomous System
RTR-A is a backbone router.
RTR-B and RTR-C are ABRs.
RTR-D and RTR-E are intra-area routers .
RTR-A RTR-B RTR-C RTR-D RTR-E
OSPF — Link-State Advertisement Types Link-state type 1 2 3 4 5 7 8 9, 10, 11 OSPF function Router link states Network link states Summary link states ASBR link state External link advertisement NSSA external link state External attributes for BGP Opaque LSA
The objective of the passive interface is to enable an interface to advertise
into an OSPF domain while limiting its adjacencies.
When changing the interface type value to passive, it is advertised into the
OSPF domain as an internal stub network with the following behaviors:
• does not send hello packets into the OSPF domain
• does not receive hello packets from the OSPF domain
• does not form adjacencies in the OSPF domain
Circuitless IP (CLIP) is a virtual (or loopback) interface that is not associated with any physical port. You can use the CLIP interface to provide uninterrupted connectivity to your switch as long as there is an actual path to reach the device .
Open Shortest Path First (OSPF) Section 5 — OSPF Implementation
We can findout what is wrong from trace level 6 3 output : ers8600 :5/trace# level 6 3 ers8600 :5/trace# clear ers8600 :5/trace# info
At initial startup, routers transmit hello packets in an attempt to find other OSPF routers with which form adjacencies. After the hello packets are received, the routers perform an initialization process, which causes the routers to transition through various states before the adjacency is e stablished .
Although both routers can recognize each other and have moved beyond 2-way, the routers could be stuck in the ExStart/Exchange state. A mismatch in maximum transmission unites (MTU) sizes between the routers usually causes this type of problem. For example, one router could be set for a high MTU size and the other router’s default value is a smaller value. Depending on the size of the LSDB, the router with the smaller value may not be able to process the larger packets and thus be stuck in ExStart/Exchange state. To avoid this problem, ensure that the MTU size
value for both routers match. This problem is usually encountered during interoperations in networks with other vendor devices. Use the trace level 6 2 command to help troubleshoot this type of problem
Incoming OSPF database description (DBD) packets are dropped if their MTU size is greater than 1500 bytes. To allow the Ethernet Routing Switch 8600 to accept OSPF DBD packets with a different MTU size, enable mtu-ignore using the following command:
ERS-8606:5# config ip ospf interface <ipaddr> mtu-ignore
Interfaces which do not need to run the routing protocol, should be kept as externals. OSPF Announce Policies must then be applied to import RIP and local routes into the OSPF LSDB.
OSPF Passive interfaces are OSPF internal routes without forming adjacencies. No OSPF hellos are sent.
OSPF route summarization and black hole routes
When you create an OSPF area route summary on an area boundary router (ABR), be aware that the summary route can attract traffic to the ABR that it does not have a specific destination route for. If you have enabled ICMP unreachable message generation on the switch, this may result in a high CPU utilization rate.
To avoid such a scenario, Nortel recommends that you use a black hole static route configuration. The black hole static route is a route (equal to the OSPF summary route) with a next hop of 255.255.255.255. This ensures that all traffic that does not have a specific next hop destination route in the routing table is dropped by the hardware.
- Up to 512 routes (local + static + dynamically learned). The 5510 can support up to 512 routes, although in some instances the 5510 may only be able to scale to 64 routes depending on address distribution/sequence. However, any 5510 configuration supports a minimum of 64 routes, and in most cases will support many more routes (that is, up to 512). Nortel always supports the default route. The 5520 and 5530 also support 512 routes, and testing indicates that more than 512 routes are possible in some configurations, although 512 is the officially supported limit.
1) Regarding the statement "The 5510 can support up to 512 routes, although in some instances the 5510 may only be able to scale to 64 routes depending on address distribution/sequence.", are there any other factors that may limit the number of learned routes to 64.
In a situation like the above network, OSPF (Open Shortest Path First) routes can be summarized to reduce the routing table.
To distribute local attached interfaces into OSPF as a summary on the ERS (Ethernet Routing Switch) 8600 the following steps need to be performed. In this topology ERS 8600 will send the static routes to ERS 5510:
The below processes must be done : - Configuration of the ERS 8600 to be an ASBR (Autonomous System Border Router) - Creation of a policy that matches to the locally attached interfaces and distributes a summary - Configuration of the OSPF redistribution entry To summarize the routes, the local attached interfaces must not run OSPF.
In this example below the 3 local attached interfaces 192.168.4.0/24, 192.168.5.0/24 and 192.168.6.0/24 will be advertised as 192.168.0.0/16 into OSPF.
Configure the VLANs (Virtual LAN) 192.168.4.0,192.168.5.0 and 192.168.6.0 all with mask 24 vlan 4 create byport 1 vlan 4 ports add 2/4 member portmember vlan 4 ip create 192.168.4.173/255.255.255.0 vlan 5 create byport 1 vlan 5 ports add 2/5 member portmember vlan 5 ip create 192.168.5.173/255.255.255.0 vlan 6 create byport 1 vlan 6 ports add 2/6 member portmember vlan 6 ip create 192.168.6.173/255.255.255.0
A prefix list for all 192.168.x.x networks with a mask of 24 Networks with a mask of e.g. 17 or 30 (such as 192.168.7.1/30) will not be covered with this prefix In this case the "mask length from" and "mask length to" would need to be adjusted Ip prefix-list "192.168.0.0_16-24-24" add-prefix 192.168.0.0/16 maskLenFrom 24 maskLenTo 24 This is the prefix to advertise 192.168.0.0/16 as a summary ip prefix-list "192.168.0.0_16-16-16" add-prefix 192.168.0.0/16 maskLenFrom 16 maskLenTo 16 Create a policy ip route-policy "thePolicy" seq 10 create ip route-policy "thePolicy" seq 10 enable ip route-policy "thePolicy" seq 10 match-network "192.168.0.0_16-24-24" ip route-policy "thePolicy" seq 10 set-injectlist "192.168.0.0_16-16-16" General OSPF Config ip ospf admin-state enable The router needs to be ASBR ip ospf as-boundary-router enable ip ospf enable The redistribution policy ip ospf ip ospf redistribute direct metric 10 ip ospf redistribute direct route-policy "thePolicy" ip ospf redistribute direct enable
Before the summarization ip routes for 192.168.4.0, 192.168.5.0, 192.168.6.0 are seen as separately as below: 5510-24T#show ip route =============================================================================== Ip Route =============================================================================== DST MASK NEXT COST VLAN PORT PROT TYPE PRF ------------------------------------------------------------------------------- 0.0.0.0 0.0.0.0 22.214.171.124 10 1 T#1 S IB 5 10.10.10.0 255.255.255.0 10.10.10.53 1 10 ---- C DB 0 126.96.36.199 255.255.255.0 188.8.131.52 1 1 ---- C DB 0 192.168.4.0 255.255.255.0 10.10.10.173 20 10 10 O IB 20 192.168.5.0 255.255.255.0 10.10.10.173 20 10 10 O IB 20 192.168.6.0 255.255.255.0 10.10.10.173 20 10 10 O IB 20 Total Routes: 6 ------------------------------------------------------------------------------- TYPE Legend: I=Indirect Route, D=Direct Route, A=Alternative Route, B=Best Route, E=Ecmp Rou te, U=Unresolved Route, N=Not in HW
Q01832726 : In a SuperMezz R mode HA-CPU system configured with a dead interval of 3 seconds, when the Master is removed, OSPF neighborship is lost for interfaces configured with low timers (for example, 1 s Hello and 3 s Dead Interval). If failover is triggered by soft-resetting the Master CPU, or the dead interval is 10 s, this issue does not occur.
Workaround: Remove the Master CPU during a maintenance window or other low-traffic periods. Or, increase the dead-interval to 10 s.
Q01735063 : When the Link Aggregation Control Protocol (LACP) adds a new port to a link aggregation group (LAG), it brings all the ports of the LAG down, which brings the entire interface down. As a result, the multilink trunk is deleted and the VLAN interface is deleted. This causes OSPF to go down.
Q02008788 : In a square SMLT environment, if OSPF is disabled and re - enabled while the IST is down, the OSPF adjacency to one of the non-IST peer boxes may show ExStart state for 5 to 8 minutes. The condition does clear itself in that time frame, and will go to full adjacency.
HA-CPU for Layer 3 redundancy avoids disruption of network traffic when a
master CPU that is running OSPF fails over. It maintains an exact copy of
the OSPF instance of the master CPU on the HA-CPU. When the HA-CPU
initializes, all OSPF information on the master CPU is Table Synchronized
and all OSPF events are Event Synchronized to the HA-CPU. When a
master CPU failover occurs, the OSPF instance on HA-CPU resumes
without affecting router traffic and OSPF neighbors.
During HA-CPU to master CPU transition, it can take up to 3 seconds for the
new master CPU to transmit OSPF packets. Therefore, Nortel recommends
router dead intervals of 5 seconds or higher. (this value is for 8692SF)
OSPF MTU Size Problem Network AB Down Two way received Init Down Init Hello received Two way received Hello received ExStart ExStart Negotioation done Negotioation done Exchange Exchange Router A Router B Neighbor State Neighbor State (Packet too large, dropped) Sequence number mismatch ExStart ExStart Sequence number mismatch (Timeout expired) Hello (DR = B, seen = 0) Hello(DR = 0, seen = 0) Hello (DR = B, seen = A) Hello(DR = B, seen = B) Database Descr. (Seq = Y , Init, Master) Database Descr. (Seq = X , Init, Master) DD (Seq = Y , More, Slave) DD (Seq = Y+1 , Master) Retransmitted DD (Seq = Y , More, Slave) Database Descr. (Seq = Z , Init , Master)
TrapEnable - Indicates whether or not traps relating to the Spanning. Tree Protocol should be sent for this STG.
AutoVirtLinkEnable - Enables or disables automatic creation of virtual links.
SpfHoldDownTime - Allows the user to change the OSPF Hold Down timer value (3 to 60 seconds).
LastSpfRun - Indicates the time (SysUpTime) since the last SPF calculated by OSPF.
SPF Run - Allows you to initiate a new SPF run to update the routing table. This feature can be used when you need to immediately restore a deleted OSPF-learned route. It can also be used as a debug mechanism when the routing table’s entries and the link-state database are out of sync.