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:
- 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.

- 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.

- 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.

- 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.
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