4.2 Steps to OSPF Operation
4.2.6 Step 5: Maintaining routing information
When there is a change in a link-state, the router uses a flooding process to notify the other routers in the network of the change. In general, the flooding process is as follows:
  1. A router notices a change in a link-state and multicasts an LSU packet that includes the updated LSA entry to 224.0.0.6, the "all DR Routers" (and BDR) address.
  2. The DR acknowledges the receipt of the change and floods the LSU to others on the network using the OSPF multicast address 224.0.0.5, the "all SPF Routers" address as shown in the figure. After receiving the LSU, each router responds to the DR with an LSAck.
  3. If a router is connected to another network, it floods the LSU to other networks by forwarding the LSU to the DR of the multi-access network, or adjacent router if in a point-to-point network. The DR, in turn, multicasts the LSU to the other routers in the network.
  4. When a router receives the LSU that includes the changed LSU, the router updates its link-state database. It then computes the SPF algorithm with the new information to generate a new routing table. After a short delay, it switches over to the new routing table.

If a route already exists in a Cisco router, the routing table is used as the SPF is calculating. But if the SPF is calculating a new route, the use of the routing table occurs after the SPF calculation is complete.

Each LSA entry has its own aging timer. The default timer value is 30 minutes. After an LSA entry ages, the router that originated the entry sends an LSU to the network to verify that the link is still active. This validation method saves on bandwidth compared to distance vector routers, which frequently send their entire routing table to the broadcast address.

When each router receives the LSU, it does the following (see the Figure):

  • If the entry already exists and the received LSU has the same information, it ignores the LSA entry.
  • If the entry already exists but the LSU includes new information, it sends an LSAck to the DR, adds the entry to its link-state database, and updates its routing table.
  • If the entry already exists but the LSU includes older information, it sends an LSU with its information.

Note that there are different types of LSAs. In this chapter, the LSAs discussed are the router link LSA, which is an LSA about a link and its status, and the network LSA, which the DR sends out. The network LSA describes all the routers attached to a multiaccess segment. The next chapter discusses other LSA types.