Open Shortest Path First
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F I G U R E 4 . 3
LSA updates and flooding
Once the LSA updates have been flooded throughout the network, each
recipient must acknowledge that the flooded update was received. It is also
important that the recipient validate the LSA update.
LSA Acknowledgement and Validation
Routers receiving LSA updates must acknowledge the receipt of the LSA, but
they can do it using two forms:
Explicit acknowledgement The recipient sends a Link State Acknowl-
edgement packet to the originating interface.
Implicit acknowledgement A duplicate of the flooded LSA is sent back
to the originator.
Ethernet
Frame
Relay
Frame
Relay
3
3
1
2
2
1. Link s0/0 goes down.
2. RouterC sends LSU containing the LSA
for int s0/0 on multicast AIIDRouters
(224.0.0.6) to the DR and BDR.
3. RouterA floods the LSA to AIISPFRouters
(224.0.0.5) out all interfaces.
DR
RouterG
RouterF
DROther
DROther
RouterA
RouterB
RouterC
DROther
BDR
RouterD
RouterE
fe1/0
s0/0
fe1/0
fe1/0
fe1/0
fe1/0
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