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New Member

MultiCast on Cisco Blade Centre Switches


I have an IBM blade centre with Cisco 3012 switch modules installed.

The server team want to use NLB (Network Load Balancing) to load balance connections to the servers on the blades and have asked me to confirm the switches support multicast as they want to deploy NLB in multicast mode rather than unicast.

My first thought was it wouldn't be a problem, the servers are in the same VLAN and the switches wouldn't have a problem dealing with muticast packets at layer 2. But when I checked the configuration guide:

I found a note saying:

Note The Catalyst Switch Module 3012 does not support IP multicast routing.

Does this mean the switches don't support multicast at all, even within a single VLAN or is it simply that they will not route multicast traffic for onward transmission?


Everyone's tags (2)
Hall of Fame Super Blue

Re: MultiCast on Cisco Blade Centre Switches

I haven't used them but looking at the configuration guide you linked to i would say it is multicast routing that is not supported ie. routing between vlans for multicast.

If you need multicast within a vlan then you need two things -

1) IGMP snooping which according to the guide is enabled globally and per vlan


2) you need something to make the IGMP queries so that IGMP snooping can work. PIM enabled on an interface usually does this if you are using mutlicast routing but as you can't use this then within a vlan you can use the IGMP snooping querier function.

See your configuration guide for details.

Like i say, i have not configured it on those switches but it should work.


New Member

Depending on what is upstream

Depending on what is upstream from your chassis switches you may need some special configuration at your core to support this. Windows NLB traffic is not like standard multicast traffic, it actually uses a multicast mac address with a unicast IP address which certain switches will just drop as the traffic isn’t "normal".


You will likely need to create static ARP entries for this to work correctly as well as mac table entries. Generally you would do this at the core or where the routing logic is for your chassis servers and map the downstream interfaces to your chassis switches.


Something like:

arp 03bf.0axx.xxxx ARPA

mac-address-table static 03bf.0axx.xxxx vlan 1 interface gi0/1 gi0/2


You should not have to do anything on your chassis switches (I am assuming they are L2 only and routing logic is handled upstream). If you google "Windows NLB setup Multicast Cisco", you will fine a decent amount of information on this topic if you need some further reading :)


Again, this isn’t standard multicast traffic so you are correct in saying that @ L2 you should be fine, its when you hit L3 is where you run into issues, but not in the conventional sense of having to enable PIM, setup RPs etc. So as far as multicast routing support goes you don’t even need to worry about that for this type of application.


Hope that helps.

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