Re: vlans multicast and igmp problems

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Zoilo Gomez wrote:

Using tcpdump, I see that the IGMPv2 queries being sent out on the server via the vlan101 interface, however nothing ever arrives on the client. ICMP (ping) and other traffic over the VLAN are working fine.

Does anybody have a clue what could be the problem?


I don't know if this is your problem, but Cisco IOS has a bug in earlier revs that incorrectly counts vlan tagged frams that are 64 octets, as 'runts' (but, forwards them anyways) - I beleive it's expecting a 68 octet minimum in the vlan tagged case. But the second part is that with this bug, although frames are being forwarded, multicast is not, which will lead to the case where ping and such works but igmp for example does not. This will also break carp and other multicast reliant protocols as well. This bug exists in 2003 - 2004 year release verisons of IOS and I don't know if it was ever documented by cisco but it's for real and is resolved in later revs. One clue if your switch supports this, is flashing amber on the switchport(s) of your 8021q enabled router and excessive runt frames noted in the interface statistics of the switch.



Does anyone have multicast+igmp running over Linux vlans?


Yes, lots and lots, but the above fix was necessary when we upgraded to 2970 switches....


Mike-
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