WLAN Microsoft NLB Conundrum

Posted on 2014-02-03
Last Modified: 2016-11-23
Good afternoon All

I have 2 subnets x.x.32.x and x.x.35.x with a wlan in between, on the 35.x end I have a Dell 6224 and on the 32.x end I have a HP 3400cl.  I have a web host on the 35.x and a 2 x wcf apps server on the 32.x with MS load balancing - the web host needs to connect to the 32.x wcf servers.  Both ends are VMs on VMware hosts.

The web host can connect to the wcf if it used the NIC IP but is unable to connect to the NLB IP.  

On the Dell switch I can add static ARP entries but on the HP switch I only have the option of Proxy-ARP (unless someone knows different).
I cam across an option of adding a second NIC to the pair of WCF servers and altering the vSwitch but that will only work if I keep both servers on a single host, which sort of negates the reason for having them on separate hosts.

Could anyone give me a suggestion on how to achieve this connection without compromising my network or breaking anything else.?

Many thanks
Question by:Eric
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LVL 28

Expert Comment

ID: 39829566
What's the network topology at the 32.x site?  It's the layer 3 device that I'm concerned about.

Author Comment

ID: 39829652
Hi Asavener

The layer 3 device we use for routing is the HP3400cl - it routes the traffic through the Huawei to the 35.x site.

LVL 28

Expert Comment

ID: 39829804
So that's the device that won't let you add a static ARP entry for the NLB address?
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LVL 28

Expert Comment

ID: 39829827
OK, according to the documentation for that model switch, it should support static ARP entries, but they only get added to the ARP table when the interface comes up:

A static entry enters the ARP cache from the static ARP table (which is a separate table) when the interface for the entry comes up

Author Comment

ID: 39829882
Hi Asavener

There is no way to add static entries, and the dynamic entry only seems to pick up the host IP and mac not the load balancing IP and mac - in IPConfig it shows the nic as having 2 IP addresses not a real card and a NLB virtual card.  The wcf apps servers are virtual machines so they are not connected directly to the HP3400.
LVL 28

Accepted Solution

asavener earned 500 total points
ID: 39829943
I suggest you contact the vendor, because the documentation indicates that static ARP entries are possible.

This is a "normal error" for layer three devices.  Cisco has an article on why their devices reject dynamic ARP replies from NLB clusters.  (Basically, it's a mis-match between multicast MAC address in the ARP reply and the unicast IP address of the NLB cluster address.)

Unless you're able to add a static ARP entry on your router/layer3 switch, only local hosts will be able to communicate with the NLB cluster.

Here's a good article describing the problem:

Author Closing Comment

ID: 40292041
Many thanks for you help with this asavener, the links you supplied were very useful.

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