BGP Multicast routes

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Jun 20th, 2010
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Hi,


When we enable a network in BGP process

network a.b.c.d mask 255.255.255.0 nlri unicast multicast


For normal unicast : it will advertize a subnet a.b.c.d / 24 to it's peer but what's the deal with multicast?


It seems  me like  "When there is a user who wants to receive a stream of any particular multicast IP address ( Mult_Addr_1)  who is connected to the LAN which this router is advertizing in our case a.b.c.d/24 - BGP will inform it's peers that it needs a stream of Mult_Addr_1. A kind or way informing that it has a route to Mult_Addr_1."



When we peer with BGP only for multicast as follows:


neighbor p.q.r.s remote-as xyz nlri multicast


What actually takes place how does it helps or enables  Multicast routing or forwarding for multicast packets? ( or does it helps PIM )


Please share the experience.

Thanks in advance

Subodh

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Giuseppe Larosa Mon, 06/21/2010 - 08:06
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Hello Subodh,


we use BGP in inter AS multicast to advertise NLRI of possible multicast sources not multicast groups.


This is done in order to pass the RPF check on border router:

the border router will accept incoming multicast traffic if the source can pass the RPF check.


One effect of the capability to differentiate NLRI per sub address family is that you can dedicate a link to multicast traffic and you can keep multicast traffic separated from unicast traffic.


MSDP can be used to advertise the existence of sources but it is not enough to pass RPF check


So this is an help in passing RPF check



Hope to help

Giuseppe

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