GLBP Manual Load Balancing

[aformisani]

Hi

How can be possible to decide which particular Gateway an host have to use? With host-dependent algorithm, hosts have always the same AVF until redirect time expires. But how AVG decide to assign to an host a vMAC instead of an other?

In other world I need to configure manually the load balance: when an host make an ARP request to the AVG, this have to know exactly which vMAC assign as a gateway (so which forwarder the host have to use).

[Peter Paluch]

Hi Andrea,

I do not believe that this algorithm is publicly described or modifiable by user. It is strongly probable that the client's MAC address is hashed with respect to the number of currently active vMACs, and the resulting number is used as a pointer to the respective vMAC that will be assigned to the client. Perhaps a modulo operation of the client's MAC address with the number of active vMACs is used for this, but this is a just an example and the GLBP may use a totally different approach.

Let me ask another question: why would you need to exactly know that? Can you describe how you would use that knowledge?

Best regards,

Peter

[aformisani]

Hi Peter

I would like to decide manually for each host which gateway to use because I have 4 Cisco core switches locate in 2 different place (2 Cisco in place 1 an the other 2 in place 2). Servers connected to them are in cluster between both locations. So they can have configurated the same gateway in both locations.

What I want is to configure GLBP in a way that AVG can assign to hosts located in place 1 a vMAC of the device lacated in the same Place 1 and  thus also for Place 2.

From what I know and as you said is not possible to customize in this way any load balancing algorithm of GLBP but maybe there is a way to better control the relation between source MAC of the host and destination vMAC of the AVF.

Thanks for your answer

Best Regards

[Jon Marshall]

Andrea

If you want to manually pick which router is used then GLBP is possibly not the best solution. Have you considered using MHSRP where you can have 2 or more VIPs in the same subnet ?

The downside to MHSRP is that you manually need to configure the IP settings because you have different default-gateways within the same vlan and so cannot use DHCP to hand this out. This may or may not be an issue for you as these are servers so you may well manually configure the IP settings.

Jon

[aformisani]

Hi Jon

yes I was thinking of MHSRP but my problem is that the servers in both locations need to be configured with the same IP Address gateway. So I should have anyway only one Virtual IP that is the only gateway for all servers.

Just to be clear there is just one IP address configured as gateway in all servers in the same network.

Thanks for your advice.. let me know if I misunderstood something

Regards

[Jon Marshall]

Andrea

No, you understood what i was proposing.

Basically if you can only have one gateway then there is no way i know of to do this. GLBP is designed to automatically load balance between a group of servers. If you have to manually assign which gateway to use then you are kind of negating one of the main benefits.

If you cannot use MHSRP then you just have to accept that there will be inter-site traffic for that vlan when it needs to route traffic to remote subnets and size the link accordingly.

Actually you might well be better off with normal HSRP. Because with HSRP at least you know that at least some of the servers are taking the shortest path ie.

site1 - 20 servers

site2 - 20 servers

site 1 is HSRP active

With this setup you have deterministic traffic ie. you know only 20 servers (in site 2) have to use the interconnect link to get to their HSRP active gateway.

Using the same example above with GLBP, you have no idea how that traffic is going to be spread. Worst case scenario is the 20 servers in site1 use site2 as their gateway and the 20 servers in site2 use site1 as their gateway. So it could be a lot more inefficient in terms of interconnect traffic.

Jon

[aformisani]

Thanks Jon

in fact in this case this was my doubt about GLBP. But on the other hand with HSRP all traffic will be managed by just one router between four available.

[Jon Marshall]

Andrea

What is the reason for every server having to have the same default-gateway in that vlan ?

Jon

[aformisani]

Hi Jon

Anything is installed yet but they told me that the servers have to be in cluster so probably for that.

I think there will be not reason to have only one gateway but I just wanted to be prepared for the worst situation.

Andrea

[Jon Marshall]

Andrea

Definitely worth having a chat with them because i suspect the clustering just needs L2 adjacency ie.same vlan which obviously has nothing to do with the default-gateway.

With 4 "routers" MHSRP is definitely something that could make the traffic paths more efficient.

Jon

[aformisani]

Yes Jon I think so.. thanks for your advise.