Unequal Load Balancing across GRE Tunnels

Unanswered Question
Mar 10th, 2009

Hi,

I've two Cisco router which have a double interconnection across two different WAN.

''''''''''''''''''''''''''-----------tunnel 1--------------

'''''''''''''''''''''''''''''|______WAN 1_______|

Network_A-----ROUTER A ROUTER B---Network_B

'''''''''''''''''''''''''''''|______WAN 2_______|

'''''''''''''''''''''''''''-----------tunnel 2----------------

(Let's see the network sheme enclosed)

The WAN 1 provides 2 Mbps whereas the WAN 2 provides 6 Mbps.

A GRE tunnel is set up through the WAN 1 (tunnel 1).

Another GRE tunnel is set up through the WAN 2 (tunnel 2).

I configured a routage algorithm (OSPF) across each tunnel in order to broadcast all the networks set up on the two routers.

So, The two roads (Tunnel 1 and Tunnel 2) learnt by Router A to join the Network_B have the same weight.

There will be an equal load balancing, when Network_A will communicate with Network_B.

In other world, there will be as much traffic transmited through the tunnel 1 as the tunnel 2.

The problem is that the two physical links haven't the same bandwith.

So I would like to send 1/3 of the traffic in the Tunnel 1 and 2/3 in the Tunnel 2.

I would like to do that in a dynamic way, without use PBR which turn out to be too static.

Do you know if it exists a mean to do that (maybe with another routing algorithm??)??

Best Regards

Julien

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Giuseppe Larosa Tue, 03/10/2009 - 03:56

Hello Julien,

only EIGRP with the variance command can support this kind of load balancing over unequal cost paths.

to be noted that you can influence the EIGRP point of view in two ways:

delay

bandwidth

because EIGRP metric is cumulative on delay and inverse proportional to the lowest bandwidth in path you can:

assign to each interface a different BW

or

you can assign a greater delay to the slowest link.

I'm not sure if the GRE tunnels are needed in your scenario or are part of your attempt to achieve load balancing over the unequal links.

However, the considerations above can apply to the GRE tunnels or to physical interfaces

to be noted that the EIGRP feasibility condition needs to be satisfied (it should be in your simple topology)

to be noted that if you can use FR on the primary link you could think also to divide it into two logical links each with a CIR of 1Mbps

in this way you can build three equal cost paths and you can still use OSPF to balance over them

Hope to help

Giuseppe

Djule2804 Fri, 03/13/2009 - 05:22

Thanks very much for your response very helpful.

I did what you advice me in putting unequal cost path on the two link.

I changed the BW of the two Tunnels Interface and set up a variance in the EIGRP instance.

Joseph W. Doherty Tue, 03/10/2009 - 07:24

Besided Giuseppe's description, two other methods come to mind.

First, if you define 3 tunnels across WAN 2, you would have 4 equal cost OSPF paths, but the 3:1 load distribution you seek.

Second, you might be able to use OER/PfR to dynamically load balance across the two tunnels.

Djule2804 Fri, 03/13/2009 - 05:33

Thanks for your ideas joseph, your first ideas is good one. I succeded to set up the Giuseppe solution but your ideas would have been a good option.

I didn't know OER/PtR, I gonna try to find some information on these protocols. Is it availaible on 2811 and 3825 routers?

Joseph W. Doherty Fri, 03/13/2009 - 07:34

"Is it availaible on 2811 and 3825 routers? "

Yes, although it might not be in all feature sets. It also requires 12.3T or later, I believe.

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