IGMP snooping

Unanswered Question
Feb 19th, 2010

I need some clearification.  If i have igmp snooping enabled on a 7609 router.  I have configured a vlan where other routers connect to and are then connected to the network.  This in a sense creates a lan.  When i do a show ip mroute I get serveral multicast addresses that are pruned on the end router.  I thought snooping would not allow the traffic to get to the end router.  It seems like pruning is done at the end router not the vlan at the 7609.  Is this normal?

I have this problem too.
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Ganesh Hariharan Fri, 02/19/2010 - 11:42
I need some clearification.  If i have igmp snooping enabled on a 7609
router.  I have configured a vlan where other routers connect to and
are then connected to the network.  This in a sense creates a lan. 
When i do a show ip mroute I get serveral multicast addresses that are
pruned on the end router.  I thought snooping would not allow the
traffic to get to the end router.  It seems like pruning is done at the
end router not the vlan at the 7609.  Is this normal?

Hi,

IGMP snooping, as implied by the name, is a feature that allows an Ethernet switch to "listen in" on the IGMP conversation between hosts and routers. When a Switch hears an IGMP report from a host for a given multicast group, the switch adds the host's port number to the IGMP list for that group. And, when the switch hears an IGMP leave, it removes the host's port from the IGMP list.

IGMP Snooping provides the ability to “prune” multicast traffic so that it travels only those end destinations that require that traffic.  In effect, it yields the best of both unicast and multicast addressing, and reduces the amount of traffic on the Ethernet LAN.

Hope to help !!

Ganesh.H

pjbaca777 Fri, 02/19/2010 - 11:49

That part of snooping is clear.  Now you say "conversions between a host and a router"  I have a 7609 router with a 48 port switch blade with one vlan tied to many ports.  Then a 3845 router connects to a port and gets authenicated and is placed in that vlan.  Can my 3845 router now become the host?  And if so why do i see multicast traffic get pruned at the 3845 router.  I would figure the 7609 router with the snooping would hear that the 3845 would not need to send traffic to it and prune.  I see the pruning done at the 3845.  That seems to go against the purpose of pruning

Edison Ortiz Fri, 02/19/2010 - 12:02

3845 is acting like a PIM router, not an IGMP host. It's supposed to gather all Mcast groups on the table and it receivers downstream do not need the Mcast group, it will prune it.

What you are seeing is expected on a Mcast design.

pjbaca777 Fri, 02/19/2010 - 12:11

Oh...i guess i have been chasing a ghost.  Now, is there a way to control that.  This becomes an issue when we run the 3845 routers through a microwave.  The microwave gets saturated.  Maybe to block all multicast except for the need traffice and pim.

pjbaca777 Fri, 02/19/2010 - 12:35

Thanks for the link...here is a question.  I see the SP code when i do show ip mroute.  So that tells me that i am pruning the traffic at the pim router.  Now is the multicast traffic still showing up at the uplink interface to the 7609 then pruning?  I have a test bed router connected to the vlan sitting on the 7609 and i see a lot of traffic inbound on the uplink port.  The seems to tell me that the multicast traffic getting to the port..this in a sense will not work with our microwave since the microwave with connected directly the the 7609 and then the other end will be the pim router.

Edison Ortiz Fri, 02/19/2010 - 14:09

SP means is pruning a mcast sparse group.

You need to provide more information about your topology and mcast design in order to provide more information regarding the expected behavior.

Diagrams and config will definitely help.

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