Multicast

Unanswered Question
Apr 14th, 2009

We have a server that utilises multicast to create a virtual IP entity, from what I understand Cisco switches/router block multicast by default and therefore any traffic designated to the IP.

To resolve this issue we need to add the virtual IP entity NIC MAC address to the ARP table. Does this sound logical and how can I implement?

I have this problem too.
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austin0824 Wed, 04/15/2009 - 07:20

Thanks Toshi,

I have an ACE blade module in one of my 6509 switches. Is this going to achieve the same as what this MS NLB would? Do you recommend I continue using ACE or go for MS NLB?

Thanks

thotsaphon Wed, 04/15/2009 - 07:54

Austin,

Would you please explain what you are going to achieve for ACE with multicast? The link I provided is to give you ideas. Let's say 2 ports of ACE are using Multicast mac-addresses with the unicast virtual ip address. This will be fine if you are connecting to this ACE from the local segment. The problem is that when you try to connect this virtual ip address from the other segment. The switch will not accept an arp reply for a unicast IP address that contains a multicast MAC address. That is why the document says that to use static arp and static mac-address for that.

In case ACE is an another story please clarify.

HTH,

Toshi

chrismarget Tue, 04/14/2009 - 09:26

Routers won't forward multicast unless specifically configured to do so.

Switches *will* forward multicast without explicit configuration, but they depend on an IGMP querier (a router) being nearby to help. Otherwise they'll screw it up.

If you just want your multicast to be forwarded between servers in a single VLAN (not routed), then static MAC configurations will suffice.

Alternatively, you can fix the L2 feature that screws things up. That feature is IGMP snooping.

It screws things up because it relies on a nearby router to perform the IGMP query function.

So, you can create static mappings, or you can fool with IGMP snooping in one of the 3 following ways:

1) disable IGMP snooping either on the VLAN, or globally

2) turn on multicast routing on a router on the segment: one global command, and one interface command is required.

3) your switch may be able to perform the query function on its own, depending on the model. the command is:

ip igmp snooping querier

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