Re: Why MSDP?

[Selina Priestley <selina@cisco.com>]

I am just learning about Multicast, so folks correct me if I am wrong:

I *believe* that MBGP carries the source networks that will be sending
multicast traffic (there is no tree because BGP is basically fancy
distance vector).  These networks are announced in MBGP whenever the 
sources associated network is up, whether the source is sending traffic 
or not.  This is used for reachability info only - how do I forward away 
from this source? or send joins/prunes toward the source?

MSDP is used to send information about specific sources, *when they
are actually sending traffic*.  This is used to start building trees
in the first place (assuming you have local receivers).  Then you use
the MBGP info to send your join to the source.

Networks are up for days, weeks, maybe even months.  Sources are sending
traffic for hours tops.

This may be what you're saying below, but I can't be sure.  :)

Selina


>Hello,
>I think I understand but just correct me if I am wrong.
>
>the multicast topology advertised by MBGP is not the Trees but it is just
>advertising the network reachability of the connected multicast routers. that
>mean instead of advertising all the routes in a domain , we just advertise the
>routes between multicast routers and subnetworks.
>MBGP is not advertising the trees in the domains and that is why we should sen
>d
>another kind messages when a source appers. because this source will construct
> a
>new multicast tree in the domain using PIM-SM or other protocol.
>Of course, MBGP like BGP cant support a lot of changes in the topology in a
>domain and that is why , multicast trees (reachability) should not transported
>via MBGP.
>we should found something else (and that is why we are using MSDP)
>
>Is that true? (I hope I am right :-) )
>
>thanx
>Ali
>
>