Altering LSA Retransmissions

Cisco routers have the capability to alter the timing in which they retransmit LSAs on a perinterface basis. When a router runs OSPF and when it transmits an LSA to a neighbor, the normal operation of OSPF is to hold that LSA until the router receives an acknowledgment that the LSA was received successfully. By default, a router waits 5 seconds for the acknowledgment and, if needed, the LSA is retransmitted. In certain instances, this waiting period is not long enough for the round trip when a slow serial link or perhaps a virtual link is involved. The need to adjust the waiting period prior to a retransmission has been addressed in the ip ospf retransmit-interval command, which is deployed on a perinterface basis, as shown in Example 3-1.

Example 3-1 Adjusting the Waiting Period Prior to LSA Retransmission

HAL9000(config)#int e0

HAL9000(config-if

)#ip

ospf ?

authentication-

key

Authentication password (key)

cost

Interface cost

dead-interval

Interval after which a neighbor is declared dead

demand-circuit

OSPF demand circuit

hello-interval

Time between HELLO packets

message-digest-

key

Message digest authentication password (key)

network

Network type

priority

Router priority

retransmit-interval

Time between retransmitting lost link state advertisements

transmit-delay

Link state transmit delay

HAL9000(config-if

)#ip

ospf retransmit-interval ?

<1-65535> Seconds

If you decide to use this command, you need to alter the timer values on both ends of the link to ensure the smooth operation of OSPF.

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