GLBP with NAT query
Posted on 2010-11-21
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.