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Using otv with Microsoft NLB

Hi All,

We're encountering the situation that some hosts try to access a service which is loadbalanced by microsofts NLB and unfortunately they can't reach them. The individual servers behind the loadbalancer are all reachable, but not via the loadbalancer address.

I found the following bit of information regarding this behaviour:

There they discuss a configuration possibility. However, somewhat later they state that the configuration option has not yet been implemented.

I'm wondering whether anybody knows if this option has already been implemented in a software release, and if so how to configure this, or whether there are other possibilities to get this to work.

Thanks in advance.


Configuring access to MS NLB is actually quite simple with a few "high-level" steps:

  • Every layer-3 hop connecting to the subnet of the NLB needs a static ARP entry, mapping the NLB's IP address to the layer-2 multicast address of the NLB. This is because a Cisco device rejects an ARP-reply containing a multicast hardware address for an IP unicast address.
  • Every Layer-2 instance between the layer-3 hop and the the NLB-nodes should have a static mac-address table entry mapping the layer-2 multicast address to all ports where members of the NLB-cluster can be reached.

Which hosts can't reach the NLB? Hosts on the same broadcast domain (IP subnet) or hosts from other broadcast domains?

Google for "Microsoft NLB" and "Cisco", there are a couple of Microsoft Knowledge Base articles and Cisco configuration guides, explaining the basic steps.

Rgds, MiKa


That goes for the 'normal' IOS based switches. However on a Nexus with OTV implemented there's more to it as the OTV blocks this traffic.

According to the article they're busy making an exceptionlist for this arp blocking on the otv link, but as far as I can see this has not yet been implemented.


Did you ever managed to solve this? 

it seems like i have the same problem as you have.


Hi Andrea,

We did manage to fix it, but I can't remember how we did it (some kind of workaround). Since I'm not working there anymore I can't look it up for you.

I'm sorry.

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