Multicast MAC addresses and IGMP snooping

[mmelbourne]

We have a pair of Sup720s which have static ARP entries configured which map a unicast IP address onto a multicast MAC address so that packets destined for this destination are sent to multiple hosts for redundancy (e.g. clustered services).

For this to work as intended, we have found we needed to turn off IGMP snooping on the switches switching it on on both switches caused a problem, but one switch was OK).

With IGMP snooping enabled will all other multicast MAC addresses be dropped unless a CAM entry is added through the snooping of a IGMP 'join' message?

Currently, no static CAM entries have been added; the packets with multicast MAC address destinations are flooded to all ports in the VLAN. Is there a way to have IGMP snooping and static ARPs which map to multicast MACs?

[umedryk]

As far as I know, you cannot have IGMP snopping and stat ARP map to multicas MACs.

[alagrawa]

Hi,

are all the servers in the cluster connected to the same switch? If igmp snooping is enabled, either the server has to send a join or you can configure static cam entries for the server cluster in addition to the static arp entries.

regards

-Alok

[mmelbourne]

The servers are connected to different switches. If static unicast IP-to-multicast MACs are needed (as they are for load-balancing certain servers) and IGMP snooping is enabled then we're saying that static CAM entries are required (or igmp snooping needs to be disabled). I see in Win2k3, that Network Load Balancing is more useful for use in this type of environment, because cluster members can send IGMP joins, but this necessarily isn't true for Win2k or other clustered boxes for other vendors.

[secnet]

I have been trying to setup Windows NLB by using IGMP on two 6509s connected via a trunk. I found that when all the 3 NLB servers are on the same switch, they can use multicast and they all see each other, however when one is on the other switch, it never sees the remaining two. Is there a restriction on the catalayst switch that prevents it from passing multicast traffic accross trunked ports or associating a multicast MAC address to a trunked port?

Thanks

Tony K.