routing updates - multicast or broadcast


Wonder if someone could explain to me the advantage of the newer routing protocols sending their routing updates via multicast rather than broadcast? Is it just to lower the hit on everyone else on a broadcast multi access network?

Also, something else I'm not sure on, in RIP when the broadcasts are sent, are the broadcasts forwarded by the routers that recieve them? Same question with multicasts, eg OSPF? My feeling is the answer to RIP is no, they don't forward, and the reason why I'm thinking that is because RIp uses routing by rumour.... which would seem to suggest that OSPF etc DO forward the multicasts.

If anyone can clear up my obvious stupidities I'd be very greatfull!


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Robin Peters
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Broadcasting has higher overhead as all hosts need to process the broadcast packets they received, even they may not be interested in the packets. On the other hand, a host drops a multicast packet at Layer 2 if it has not subscribed to that multicast group.

Regarding the forwarding of routing updates, it depends on the routing protocol types, rather than whether broadcast or multicast is used.

A router running a link-state routing protocol (e.g. OSPF) forwards the LSAs it received to all neighbors (flooding).

A router running a distance vector routing protocol (e.g. RIP) sends routing updates to its neighbors based on the route information in its routing table.


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thanks for that kplab

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