Load Balancing between 2 ISPs

Unanswered Question
Nov 10th, 2008

I have one 2600 Router and two ISP with 2Mbps each pipe. and it is connected via serial interface. As of now, i just created two default routes, one with AD of 0 and one with AD of 10. Is there anyway that I could use those two links at the same time so I could have a total of 4Mbps. What configuration should I need to make this work ?

I have this problem too.
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Giuseppe Larosa Tue, 11/11/2008 - 00:50

Hello Jaime,

to load balance over the two links:

first of all you need two default routes with the same AD value.

the most difficult part is to handle NAT.

If you have your own public IP address block you can think of doing load-balancing otherwise if are the ISPs that give a public ip address block the problem is that each ISP will give you IP addresses of their own and are ready to route to them.

In these cases is usually easier to have a primary /backup solution.

see the following document:

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_white_paper09186a0080091c8a.shtml

If you have your own IP address block it is easier and you can use eBGP as explained by Satish

Hope to help

Giuseppe

Sushil Kumar Katre Tue, 11/11/2008 - 02:01

Hi Jaime,

As explained by Giuseppe there is a problem with NAT if you are using Global IP Addresses from the service providers.

If you don't have your own IP address block and still want to use both the links then you need to invest on additional router.

You'll terminate ISP A on Router A and ISP B on Router B. Both these routers will be connected over ethernet.

From your lan all traffic would come to lets say Router A, where you'll have two default routes with same AD, hence traffic will be loadbalanced. Each router will have its OWN NAT table, router A - Nat to ISP A IP address and router B - NAT to ISP B IP address.

In case ISP B goes down, Router A will not come to know about this and will still forward traffic to ROuter B which will be dropped. In order to overcome this, configure SAA probes with and track the connectivity to ISP B. When ISP B goes down, the default route on Router A pointing towards ISB B will be removed and all the traffic would be sent over ISP A.

Drawback - You need to invest on another router.

Here's the link on SAA probes with static routes -

http://www.cisco.com/en/US/docs/ios/12_3/12_3x/12_3xe/feature/guide/dbackupx.html

-> Sushil

tcordier Tue, 11/11/2008 - 08:03

Guys - I am sure I am being thick, but I do not see the NAT problem in above scenario. If he creates two NAT inside statements using the serial interface of each ISP as global address (PAT), what would be the issue? The router will load-balance (per-flow) and translate the traffic to ISP 1 using the WAN IP address provided by ISP 1. All traffic translated in that flow would flow back to that interface, and the same would happen to traffic sent out the WAN interface for ISP 2. Load-balancing based on two equal-cost default routes as suggested by Giuseppe would be sufficient. I would think that a 2600 router is not equipped with sufficient memory and processing power to hold a full Internet routing table anyway, which would be needed to use BGP multi-homed load-balancing.

- Thomas

thotsaphon Tue, 11/11/2008 - 10:29

Thomas,

"If he creates two NAT inside statements using the serial interface of each ISP as global address (PAT), what would be the issue?"

Creating Nat statements with each ISP interface doesn't mean that the router will know which is the correct interface. It means the router will use the IP address assigned to that interface to do NAT. This will result the same as you directly assign the ip address of each ISP interface.

Think of when one ISP interface was down. The router goes reading the Nat statements. all right. that's a problem. You may use the ip address of the broken-down interface to do NAT. The router then sends the packets. The packets are finally dropped by the other ISPs.

Why do we need to use BGP? all right, Can you control in-bound traffics if you just use default routes and NAT. No. you can't. BGP will help you out.

You may also receive the only default route from ISP although you're running BGP.

Hope this help

Toshi

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