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duplicate multicast packets

I have a customer experiencing duplicate multicast packets at the receiver when routed across vlans. This is not a problem when the packets are not routed. The network is fairly simple with one router, Cat 6500. All VLAN interfaces are configured exactly the same. I tried the "mstat" command but did not get the negative packet count which would indicate dupicate packets (attached file). Any help would be appreciated.

Community Member

Re: duplicate multicast packets

Are you sure that duplicate packets are being seen at the receiver? If you're sniffing via a span port on the 6500, and the multicast traffic is going through a transit VLAN, you may be seeing duplicate packets on the span, depending on how the span was set up. By default, the span will get copies from frames both rx and tx to the VLAN you're spanning. That can result in duplicate frames in the sniffer trace.

Not second-guessing you if you've already proven that you get 2 copies of the frame at the receiver, just thinking out loud as I've run into the duplicate frame issue before.

Community Member

Re: duplicate multicast packets

There is no transit vlan. The 6500 is directly connected to both vlans and is the only router in the picture. I do have a trace showing duplicate packets but I think they captured it using a hub connected along with the receiver not by spanning a port on the 6500.

It is a very simple design with 1 router routing between all vlans. I thought the "mstat" command would have shown the duplicate packets to me.

Thanks for you input.

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