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| 6.2 |  | OSPF Operation |
| 6.2.6 |  | Step 5: Maintain routing information |

When an OSPF router has installed routes in its routing table, it must diligently maintain routing information. When there is a change in a link-state, OSPF routers use a flooding process to notify other routers on the network about the change. The dead interval from the Hello protocol provides a simple mechanism for declaring a link partner down. If RTB does not hear from RTA for a time period exceeding the dead interval, usually 40 seconds, RTB declares its link to RTA down.

RTB then sends an LSU packet containing the new link-state information, but to whom?

- On a point-to-point network, no DR or BDR exists. New link-state information is sent to the 224.0.0.5 multicast address. All OSPF routers listen at this address.
- On a multiaccess network, a DR and BDR exist and maintain adjacencies with all other OSPF routers on the network. If a DR or BDR needs to send a link-state update, it will send it to all OSPF routers at 224.0.0.5. However, the other routers on a multiaccess network are adjacent only to the DR and the BDR and therefore can send LSUs only to them. For that reason, the DR and BDR have their own multicast address, 224.0.0.6. Non DR and non BDR routers send their LSUs to 224.0.0.6, or all DR and BDR routers.

When the DR receives and acknowledges the LSU destined for 224.0.0.6, it floods the LSU to all OSPF routers on the network at 224.0.0.5. Each router acknowledges receipt of the LSU with an LSAck.

If an OSPF router is connected to another network, it floods the LSU to other networks by forwarding the LSU to the DR of the multiaccess network. It could also flood the LSU to an adjacent router if in a point-to-point network as shown in Figure . The DR multicasts the LSU to the other OSPF routers in that network.

Upon receiving an LSU that includes new information, an OSPF router updates its link-state database. It then runs the SPF algorithm using the new information to recalculate the routing table. After the SPF hold timer expires, the router switches over to the new routing table.

If a route already exists in a Cisco router, the old route is used while the SPF algorithm is calculating the new information. If the SPF algorithm is calculating a new route, the router will not use that route until after the SPF calculation is complete.

It is important to note that even if a change in link state does not occur, OSPF routing information is periodically refreshed. Each LSA entry has its own age timer. The default timer value is 30 minutes. After an LSA entry ages out, the router that originated the entry sends an LSU to the network to verify that the link is still active.

| Interactive Media Activity Drag and Drop: OSPF States Upon completion of this activity, the student will be able to identify the different OSPF neighbor states.