Re: Why MSDP?

["Stephen Sprunk" <ssprunk@cisco.com>]

Thus spake "Ali Boudani" <aboudani@irisa.fr>
> 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.

You're missing what MBGP does.  For mcast forwarding, all routers need
an RPF table.  Normally, the RPF table is identical to the unicast (BGP)
routing table; however, in some cases we want RPF and unicast to be
different.  For that, we have MBGP.

> 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)

Remember that the PIM-SM shared tree is limited to a single domain.  To
find sources outside your domain, you need a global shared tree; that's
the purpose of MSDP.

Think of the split between PIM and MSDP as similar to the split between
ISIS/OSPF/EIGRP/RIP and BGP -- we use different protocols within and
between routing domains.

S

     |          |         Stephen Sprunk, K5SSS, CCIE #3723
    :|:        :|:        Network Design Consultant, ANS
   :|||:      :|||:       RCDN2 in Richardson, TX
.:|||||||:..:|||||||:.    Email: ssprunk@cisco.com