Subject: IP multicast traffic flow

From: Sven Laemmermann <[email-masked]>
Date: Wed, 13 Sep 2000 17:56:48 +0200

Dear BGMP community:

I have a question regarding the multicast traffic flow in a
bi-directional shared multicast traffic distribution tree built
by BGMP. In order to illustrate my point consider the following
example.

Assume that A, B, C, D and E are domains whose border gateway
routers are peered as shown below.

               A         B
                \       /
                 \     /
                  \   /
                   \ /
                    C
                   /
                  /
                 /
                D (root domain for 224.0.128.1)
                 \
                  \
                   E

Domain D allocated the IP multicast address range 224.0.128.0/24
using MASC. Suppose there is a receiver of the group 224.0.128.1
in domain B and a sender in Domain A. Since the group address
224.0.128.1 is in domain D's address range, D will be the group's
root domain.

MY QUESTION IS: Assume the group 224.0.128.1 has only one receiver
(which is located in domain B) and one sender (which is located in
domain A). Is the multicast traffic generated by the source in A also
forwarded to root domain D (after all prunes took place)? If not, how
gets domain D the traffic back if it receives a join from domain E?

Thanks,
Sven