Loadsharing with glbp

[suthomas1]

Hi,

Attached is a diagram, would it be fine if glbp is run across the devices R1 & R2 at location B.R1 & R2 are 2 different service providers link which end up on the L3 switch.Would this work for outwards loadbalancing.All hosts are behind the switch?

Kindly suggest the problems with this setup also ..as it seems glbp has issues with address resolution.

Thanks.

[deepak george]

Hello All,

I've got a scenario here in the attachment.

I've got two ISP's configured on two different routers. I'm running static routing on the WAN side. I would like to load share the traffic via both the internet links. How can this be achieved?Can GSLB be used or MHSRP or are there any other methods.I can use a layer 3 switch instead of a layer 2 switch between firewalls and routers.

NAT would be done on the firewall.

Thanks 

[lamav]

"If you statically route from an L3 switch to GLBP addresses, GLBP will only see the source MAC for the L3 switch."

Joseph:

Your point is well taken, but the inability to load balance in this case is not so much a function of GLBP "seeing" one address more than it is a result of the fact that, since there is only one host, it will not continuously send out new ARP requests for the gateway. It will do it once and cache it.

What GLBP "sees" are new ARP requests when they come in and it is that they will not be seeing if there is only one host on the LAN.

HTH

Victor

[Joseph W. Doherty · Accepted solution]

Victor, you're tecnically correct, since GLBP replies to host ARPs with its virtual gateway MACs, just one host MAC would not preclude load balancing.

For instance, if our single host was dealing with GLBP configured for round-robin load balancing and it ARPed before sending each IP packet, we would effectively have achieved load balancing (similar to CEF's packet-by-packet).

Practically, though, since hosts don't usually ARP for every IP packet, we're not going to see concurrent load balancing when there is only one host MAC.

Also from a practical level, there's reasons why IP doesn't normally ARP per packet. Further, although IP itself doesn't require flow packet sequencing, it's something we try to maintain.

So, for practical reason, there's a problem when GLBP only "sees" one host MAC although you're correct that it's more to do with GLBP only seeing ARPs from the one host. Because, in a round-robin GLBP configuration, when the host's ARP time-outs, GLBP will move the host to the other gateway, but during the ARP cache time period, only one GLBP gateway will be used.

Since GLBP does respond to ARPs, all flows from one host will go to the same gateway. Further, ARP times can run into minutes. Given these facts, many routing protocols might effectively load-balance better than GLBP. Given the L3 switch, in this instance, this is one reason why I recommended routing vs. using GLBP.

[lamav]

Joseph:

I agree with what you have said in all your previous posts, including the one in which I made a clarification.

I was just splitting hairs, I guess, and trying to expound on one specific concept regarding the reason why only one host will not result in leveraging GLBP's load balancing function.

Thanks

Victor