Flooding of multicast groups

[sbrobak]

Have a network based on Cisco switches. Usning it for transport of multicast TV. When performing some sniffer traces in some vlans, I see some periodic flooding of multicast groups. The traces show correct ip sources. But the mac adresse of the packets is the spanning-tree bridge id of the router (a CS3560)

VLAN0020

Spanning tree enabled protocol rstp

Root ID Priority 32788

Address 0015.fa7a.7a80

The stp looks ok, has been ok for over 1 week.

Think this is quite strange? Any ideas.

[tdrais]

Sounds like proper operation of a router running something like pim dense mode. Just like normal layer 3 traffic when a packet crosses a router it will be sourced from the router interface at the layer 2 level.

Depending on you multicast configuration you can prevent transmission on a interface in a number of ways. If all else fails a access list will work.

[sbrobak]

Thanks, but the router is using spare-mode only. Have a static rp configured on the box. Agree that I would see the routers mac (svi mac), but not the STP one. Strange to me. I'm looking for any loops at the moment or a bug.