Basic Multicast Routing

Unanswered Question
Feb 25th, 2007


I have a basic multicast routing situation, but I can't get it work.

I should route multicast traffic from VLAN x to VLAN y. Both VLANs are defined in same Catalyst 6000.

Catalyst Multicast config:

ip multicast routing

ip pim rp-address [C6000 address]

interface VlanX

ip address 10.X.X.1

ip pim sparse-mode

interface VlanY

ip address 10.Y.Y.1

ip pim sparse-mode

Do I have to give some more commands?



I have this problem too.
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Harold Ritter Sun, 02/25/2007 - 08:15

Your configuration looks good. Can you tell us more about the specific multicast application you are using. Some multicast applications issue IP packets with a TTL of 1 by default. Make sure that this is not the case.

Hope this helps,

tvaris Mon, 02/26/2007 - 06:06

I captured traffic and noted that in all packets the TTL field was 1. I think that it was the problem. Programmers will make changes for TTL field, and let see if it that was the case.

Does it mean that when the packet goes to the router, it reduce TTL from 1 to 0 and after that router drops the packet? And Multicast routing process dosen't never get the packet?

Thanks a lot, your post was very helpful.


Harold Ritter Thu, 03/01/2007 - 10:28


Your understanding is correct. Packets with TTL of 1 are not meant to leave the local subnet. The router will get them, decrement the TTL, sees it reaches 0 and then discard them.

Hope this helps,


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