OSPF: Designated Routers and Backup Designated Routers

OSPF makes broadcast network adjacencies more efficient by using Designated Routers (DR).  Without DRs, each router attached to the same OSPF broadcast network would require a neighborship to every other router on the same segment.  Each pair of routers would have to exchange their LSDBs with one another and with enough routers simply the act of exchanging LSAs amongst other routers could congest the network.

OSPF eliminates this problem with the DR, where all neighbors on a segment pair with the DR and a Backup DR only and that’s it.

DRs and BDRs communicate changes in the network via LSAs being sent to the 224.0.0.5 (all OSPF routers) multicast group.  Every OSPF router on  the LAN segment will process the LSU and send back a unicast LSAck to the DR and BDR.

If nonDr and nonBDR routers need to send an update, they do so using the 224.0.0.6 (All DR and BDR routers) multicast group.  The DR and BDR immediately forward the same LSU to the 224.0.0.5 (All OSPF router) multiaccess group for processing, the act of resending the SAME LSU back to the original sender is an Acknowledgement itself that it was received by the DR and BDR.  Subsequently all routers processing the update will send back a Unicast LSAck to the DR and BDR except for the originating nonDR or DROTHER router.

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