route balancing problem

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Jan 19th, 2006
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i have a problem. My client has got to links from two diff ISP's . one is 128 kbps and the other one is 512 Kbps. they r terminated at cisco 1721 the whole lan traffic goes to Pix 515e, there they r nated to single Public Ip , then that traffic goes to the router.

Now the problem is that client wants the traffic to do load balancing ,so that he can use both the links . he also wants that he can switch from one link to another link without any config changes in case if one link goes down.

Please tell me the possible ways to do this as i think the problem is that the pix is doing nat to a single ip , so my router will see only a single ip so how can we implement load balancing or Pbr.

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stephtchoko Thu, 01/19/2006 - 02:44
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1- You can use pbr for upload balacing, with route-map.

Criteria in your route-map can be packet length coming from your public ip address.


route-map isp permit 5

macth ip length 1500

set ip next-hop x.x.x.x (Isp with 512kbps)

route-map isp permit

set ip next-hop y.y.y.y(Isp with 512kbps)

2- you need bgp to advertise this public address this to your isp who doesn't advertise it.That will help your for dowlink recovery if this link (for isp that provide this address come down).

3-If you can implement bgp, you can say to your isp who provide you this ip address to advertise this adress with high weigth for link with high bandwith (512 kbps).

Best regards.

olorunloba Thu, 01/19/2006 - 07:22
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I'm assuming that you are not using BGP. Rather than do policy routing, you might want to "divide the internet into two". I.e. Rather than use a default route, use the following two routes.

ip route

ip route

You can then use object traking with static routes to ensure that the static routes fail over to each other. Check the following link for example configuration.

Despite these, I wonder if the router is also doing some form of address translation. Realising that you are using two different ISPs would imply that the address ranges would be different. You therefore need a way to translate to the ISP address based on the packet destination. A lot of ISPs would drop packets if the source address is not in their adress range. Please provide more information on your scenario.

diptanshusingh Thu, 01/19/2006 - 19:54
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yes you are right.the two different Isp's has got Two different ranges.all the nating part is done by pix which is behind the router. router only sees the public ip .

sroberts7758 Wed, 02/15/2006 - 14:34
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I have a similar issue, I'm trying to route traffic over one ISP for traffic to the X.45.87.0 network, and all other traffic through the "default" isp. I believe my problem is with the outbound NAT/PAT. How were you able to do this?

Thank you.


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