Learning a New Route

When the 2500 receives a network LSA update from the designated router, it goes through the following logical steps:

1 The router takes the first entry from the update—the first network with information about the state of its link.

2 The router verifies the type of LSA is one that can be accepted by this router.

3 Having ascertained that it is a valid LSA that it can receive, the router issues a lookup to its topological database.

4 If the LSA entry is not in the topological database, it is flooded immediately out all the OSPF interfaces, except for the receiving interface.

5 If the LSA entry is in the topological database, further questions are required.

6 The router determines whether the new LSA has a more recent (higher) sequence number?

7 If the sequence numbers are the same, the router calculates the checksum for the LSAs and uses the LSA with the higher checksum.

8 If the checksum numbers are the same, the router checks the MaxAge field to ascertain which is the more recent update.

9 Having found that the latest LSU is the one that was received, the router determines whether it has arrived outside the wait period, before another computation is allowed (minsLSarrival).

10 If the new LSA entry passes these tests, it is flooded out all the OSPF interfaces, except for the receiving interface.

11 The current copy replaces the old LSA entry. If there was no entry, the current copy is just placed in the database.

12 The received LSA is acknowledged.

13 If the LSA entry was in the database, but the LSA that has just been received has an older sequence number, the process will ask whether the information in the database is the same.

14 If the information is the same and the new LSA has an older sequence number, the process just discards the packet. It may be old news, but there is no inconsistency in the database.

15 If the information is different and the newly received LSA has an older sequence number, however, the receiving router discards the LSA update. It issues its own LSA out the receiving interface to the source address of the out-of-date LSA. The logic is that the sending router either has bad or old information and must be updated because its topological database is obviously not synchronized with the rest of the area.

This ensures that any packets that get out of sequence will be verified before action is taken. It also attempts to rectify a problem that it sees—that of multiple routers offering different paths because their topological databases are completely confused.

16 After the initial flood, things calm down, and updates are sent only when there are changes in the area or when the 30-minute timer goes off. This timer ensures that the databases stay synchronized.

This shows some of the internal complexity of OSPF. As you can see, the internals are extremely detailed. Therefore, the design of any OSPF network should be very carefully thought out. The configuration of the routing protocol, on the other hand, is incredibly straightforward.

Figure 5-9 shows a logical flowchart of how the OSPF topological database is updated.

As you have seen, different types of LSA are propagated through the network. Figure 5-10 illustrates graphically the flooding of an LSA through the network.

Figure 5-9 Updating the Topological Database

Figure 5-9 Updating the Topological Database

Figure 5-10 Flooding LSAs Throughout the Area

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