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GLBP with NAT query

Posted on 2010-11-21
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Last Modified: 2012-05-10
Hi guys,

We have a few sites which use host-dependent GLBP to load balance traffic over two links, these sites are usually 'tails' and have NAT configured on them for clients, etc.

Now for some of these sites, the links are unequal cost, i.e. one could be a 40/40 fiber and the other an old ADSL1 link. Many times traffic, such as voice goes through the slower link, and if the link is congested - it affects voice quality and frequent dropouts.

I changed GLBP to weighted, with ratios of 1:5 and that seems to have improved things. However I have noted the following statement on some sites:

"Host dependent load balancing will need to be used when using stateful Network Address Translation (NAT) because it requires each host to be returned the same virtual MAC address each time it sends an ARP request for the virtual IP address
What this means is that Host-dependent ensures a host uses one and only one gateway for its traffic. In case of round-robin/weighted, a host sends some packets through 1st gateway and some packets of the same session to the 2nd gateway and this doesn’t work well with NAT"


Now I know the default ARP timeout for Cisco is 4 hours. Not all our switches are cisco, however would having weighted GLBP affect windows based clients ? I haven't had any noise from the sites where I changed the configs, but just need to know in case it might be causing minor disruptions.

Cheers,
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Question by:demon777
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jimmytheshark earned 2000 total points
ID: 34359083
I dont think your definition of weighted GLBP is correct..   a client will not send some packets one way and some the other,  as the client only cares about where the next hop comes from (this is why GLBP is not 100% true load balancing)

Let say we have a 8/2 weight on 2 GLBP routers..  the AVG will issue the mac address of the first router for the first 8 requests, then issue the mac for the second router for the following 2, that another 8 to the first router again.

So, so long as the clients maintain a connection their ARP cache will not time-out and you shouldnt run into problems.

You may also want to throw some QoS on your VOIP,  or at least a second GLBP group for VOIP traffic.

Hope this helps

J
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by:demon777
ID: 34510265
Thanks I think this is correct - I am yet to hear back of any problems with the sites in question, and could not find anything in the documentation to suggest weighted utilizes ARP on every new request.
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