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6500 ARP replies to request from a MS NLB host

Howdy,

We are experiencing an unusual problem (aren't they all) and was wondering if anyone has an explanation or workaround. We have problems with some new Windows 2003 servers running Multicast Network Load Balancing. Based on some packet captures it appears as though the 6500 MSFC is not responding to ARP Requests from the NLB adaptor. The packet contains an ethernet source address of the NICs globally unique MAC, while the ARP Request has a Source Address of the Multicast MAC (01.00.5e.whatever). Does anyone know if the 6500 will ignore the ARP Request because of this discrepancy ? The 6500 is not replying for some reason and any hints would be greatly appreciated. From reading around here it seems like Kevin D is having fun with MS Multicast NLB as well, ever run into this Kevin ?

Thanks a bunch for any and all assistance.

Best Regards,

Dave

4 REPLIES
Hall of Fame Super Silver

Re: 6500 ARP replies to request from a MS NLB host

Dave

I have not run into this particular issue. But I do know from some other issues that Cisco will not respond to an ARP request when the source MAC is a multicast. Cisco regards it as a violation of addressing standards for any end station to source things from a multicast MAC. I am not aware of a workaround for this.

HTH

Rick

New Member

Re: 6500 ARP replies to request from a MS NLB host

Thanks for the reply Rick. Do you have any Cisco references that discuss this issue ? I spent an hour or so searching yesterday but could not find anything specific.

Dave

Hall of Fame Super Silver

Re: 6500 ARP replies to request from a MS NLB host

Dave

When I start to look I am surprised at how little I really have found. Perhaps the best reference is this (though it is from a Microsoft oriented source rather than a Cisco source):

Multicast Mode

The other NLB mode is multicast mode. Microsoft would prefer to set the default mode to multicast, but there are problems because certain Cisco devices don t allow multicast MAC addresses to be associated with a unicast IP address and the Cisco device will not insert a multicast MAC address its ARP table if its associated with a unicast IP address.

from this URL:

http://www.isaserver.org/pages/article_p.asp?id=1057

A Cisco reference about it being a non standard implementation is here out of the TAC Case Collection (though it does not discuss it in relation to ARP):

http://www.ciscotaccc.com/lanswitching/showcase?case=K17817118

I thought I remembered a discussion in one of these forums where one of the Cisco engineers had something to say about it, but I can not find that discussion now that I look for it.

So I know it is the case, but have very little in the way of documentation to support it.

HTH

Rick

New Member

Re: 6500 ARP replies to request from a MS NLB host

Didn't have a chance to look at the links provided, but I vaguely recall from somewhere that a static ARP entry was a workaround.

Vyto

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