OSPF broadcasts

[annu86]

Hi guys,

i have a doubt about ospf.

In a broadcast network, all the packets are multicasted, imagine a 3 routers conencted ina network, how can multicast work in that scenario.

I mean before the actual LS update flows, DB sequences needs to be synchronized, in a multicasting flow, router's B can be recieved by router C..and vice-versa, this will make them reset the adjacency and go again for establishing master-slave relation-ship..

please guide me, how does that happens,, if i am wrong, then how actually the adjacency is formed in a broadcast network

[deilert]

In situations like this there is a command 'ip ospf network point to multipoint' on the interface level

[rjackson]

It sounds like you are talking about a broadcast network and just trying to understand how multiple routers can build relationships with each other using multicasts.

I think the key is in the data. The destination address of the packet is a multicast address but the data in the packet uniquly identifies each router by router-id and source address.

[arto.oltaci]

On a Broadcast network, OSPF elects a DR for this purpose. Read up about DR election, and DR,BDR and DROther communication.

The summary is that the non-DR routers send their updates to the multicast address of 224.0.0.6 (AllDRouters). The DR and BDR listen to this address.

The DR takes the following action:

- reflood the LSA to 224.0.0.5 AllSPFRouters

The BDR takes following action:

- nothing

- unless it did not see the DR reflood the LSA within a few seconds, in which case he'll take over DR role