Convergence

10 Mar

Convergence
Each routing protocol has a different method of updating the routing table, affecting the time to converge the routing tables. Some new concepts are introduced in the following comparison. Theconcepts are explained in depth in the chapters that concentrate on the specific protocols.

OSPF Convergence
The steps for OSPF convergence are as follows:
1. When a router detects a link failure, the router sends an LSA to its neighbors. If the router is on a multiaccess link, it sends the update to the designated router (DR) and the backup designated router (BDR), not to all neighbors.
2. The path is removed from the originating router’s tables.
3. On receipt of the LSA, all routers update the topology table and flood the LSA out its interfaces.
4. The routing protocol runs the Dijkstra algorithm to rebuild the routing table.

For OSPF, convergence is detection time, plus LSA flooding, plus 5 seconds before computing the
topology table. This amounts to a few seconds.

IS-IS Convergence
The steps for IS-IS convergence are as follows:
1. When a router detects a link failure, an LSP is sent to its neighbors. If the router is on a multiaccess link, the update is sent to the designated intermediate system (DIS the IS-IS term for a designated router), not to all neighbors.
2. The path is removed from the originating router’s tables.
3. On receipt of the LSP, all routers update the topology table and flood the LSP out its interfaces, except for the interface that received the LSP.
4. Each router runs the Dijkstra algorithm to rebuild the forwarding table.

For IS-IS, convergence is detection time, plus LSP flooding. The time to converge the network amounts to a few seconds. If convergence is deemed to be the topology table being updated, this could take longer.

BGP Convergence
BGP convergence is different, depending on whether IBGP or EBGP is being run. Reliability is far more important to EBGP than how long it takes to update the routing table, whereas IBGP needs to ensure a faster convergence to remain synchronized with the interior routing protocol.

When a neighbor is no longer available, the BGP router tries to reconnect to its neighbor. If this fails, the session is formally closed and the information from the router is removed from the database. An update is sent to all neighbors.

166 Chapter 5: IP Link-State Routing Principles

Foundation Summary

The “Foundation Summary” section of each chapter lists the most important facts from the chapter. Although this section does not list every fact from the chapter that will be on your exam, a wellprepared candidate should, at a minimum, know all the details in each “Foundation Summary” before going to take the exam.

Table 5-2 is a summary of IP routing protocols and the update timers.

IP Routing Protocols

Table 5-3 summarizes the major differences between distance vector routing protocols and link-state routing protocols.

Table 5-3 Distance Vector Routing Protocols Versus Link-State Routing Protocols

Distance Vector Routing Protocols

168 Chapter 5: IP Link-State Routing Principles

Table 5-4 summarizes the differences between RIPv1 and OSPF. RIPv1, as the first distance vector routing protocol, and OSPF, as the first link-state routing protocol, are very familRIPv1 Versus OSPFiar to most in the networking industry and thus easily used as examples for comparison.

Table 5-4 RIPv1 Versus OSPF

Table 5-5 summarizes the major differences between all available IP routing protocols.
Table 5-5 Comparison Chart for IP Routing Protocols

summarizes

Foundation Summary 169

Table 5-5 Comparison Chart for IP Routing Protocols (Continued)

Routing Protocols


170 Chapter 5: IP Link-State Routing Principles

Q&A
As mentioned in the introduction, “All About the Cisco CCNP, CCDP, and CCIP Certifications,” you have two choices for review questions. The questions that follow next give you a bigger challenge than the exam itself by using an open-ended question format. By reviewing now with this more difficult question format, you can exercise your memory better and prove your conceptual and factual knowledge of this chapter. The answers to these questions are found in Appendix A.

For more practice with examlike question formats, including questions using a router simulator and multichoice questions, use the exam engine on the CD.

1. What is the routing algorithm used in OSPF?
2. State one method by which a link-state routing protocol attempts to reduce the network overhead.
3. What is the purpose of the Dijkstra algorithm?
4. Name two link-state IP routing protocols.
5. Name the TCP port used by BGP-4.
6. State the metric used by OSPF.
7. How often does Integrated IS-IS send out new LSAs?
8. State one way that OSPF is an improvement over RIPv1.
9. State one key attribute of OSPF.
10. State one key attribute of IS-IS.
11. State one key attribute of BGP-4.
12. What is the default hello update timer for IS-IS on broadcast media?
13. On a broadcast link, how long does OSPF wait by default before it determines that a neighboris dead?
14. What is iBGP?
15. When does OSPF send updates?
16. When does BGP send updates?
17. What is a topological database?
18. What is an adjacent neighbor?
19. What is a triggered update?
20. What is required for iBGP to operate?

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