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load balancing over two serial lines

I've got two serial connections to internet, in different networks (different ISP, different subnets). I would like to exploit both these lines, at the same time, to share the load, but also i want to make sure that each of these interfaces will route all the traffic in case the other line goes down.

How can i configure the router in order to make this possible ???

Please, help me, i am in desperate need !!!!

Genti

2 REPLIES
Silver

Re: load balancing over two serial lines

I think you need 2 default route to point to the serial interfaces i.e

ip route 0.0.0.0 0.0.0.0 s0

ip route 0.0.0.0 0.0.0.0 s1

When the line protocol of a serial interfaces goes down, the route is purged from the routing table, and when both are present, the router load balance between the two route since they have the same distances.

Silver

Re: load balancing over two serial lines

You do not mention how you are currently configured, but the following are a few key considerations: If you are using BGP as a multihomed AS (and if the acronyms are strange, then you are not) then you can get some degree of load balancing by setting up two static routes, one to each serial interface. This only works if link failure is detectable at the link level (point to point T1 or frame relay with end-to-end keepalives).

Assuming you are NOT using BGP, then you have an additional challenge. Each link is associated with a different set of IP addresses identifying you and each link will ONLY route traffic to its set of your addresses to you. So if a link goes down, any connections on the failed link must be reestablished using an IP address from the other ISP. This can be done using NAT, there is an example in chapter 8 of my book (see listings 8-25 & 8-26, also on my website) that you may be able to adapt and there may also be examples on cisco.com if you look hard enough :-) My book also discusses some of the reasons you may not want to take this approach. Keep in mind that just as in the BGP case, you must be able to detect at the link level that a link has failed.

Good luck and have fun!

Vincent C Jones

www.networkingunlimited.com

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