[c-nsp] Multicast clarification

[Arie Vayner (avayner)]

Martin,

Without this functionality the routers would not be able to detect new
sources as the switch would not replicate the stream, and we would never
know about this source.

If both routers are talking PIM between them then only one of the them
would be the DR (Designated Router), and only this router would be
forwarding packets received on this LAN domain.
Take a look here (search for designated router):
http://www.cisco.com/en/US/docs/ios/ipmulti/configuration/guide/imc_basi
c_cfg_ps10591_TSD_Products_Configuration_Guide_Chapter.html


Arie

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Martin Barry
Sent: Friday, May 07, 2010 15:25
To: cisco-nsp at puck.nether.net
Subject: [c-nsp] Multicast clarification

Just trying to understand a traffic pattern and my Google-fu is failing
to
turn up anything that covers this query.

Scenario:

- 3750 stack with 2 members
- 2 routers forwarding different multicast streams into the same VLAN

The switch correctly shows both routers:

#sh ip igmp snooping mrouter 
Vlan    ports
----    -----
   4    Gi1/0/25(dynamic), Gi2/0/25(dynamic)


The switch will take multicast traffic from router A and forward it to
those
ports where it has snooped an IGMP join. It also appears to forward the
multicast out router B's port.


Questions:

Is this correct? Is multicast forwarded out all mrouter ports on the
same
VLAN except the ingress port? Is there a way to prevent this from
happening?

cheers
Marty
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