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BGP - weight or as-path prepend

My scenario is as follows;

Site A - AS100

Site B - AS200

Site C - AS300

Sites B and C are connected to site A via DS3. This is the prefeered route when the two sites need to pass data. Sites B and C are also directly connected via a T1 link. This is not a preferred route because of bandwidth limitations even though they are directly connected.

What I would like to do is use the T1 link as a back up in the event that the primary DS3 circuit at either sites B or C goes down. It appears to me that if I do not influence the BGP decision making process that it will prefer the T1 link to pass traffic between sites B and C instead of the DS3 link.

My question is should I use the "set as-path prepend" command and route maps at sites B and C to influence the routing of traffic to each site or should I use the "neighbor x.x.x.x weight" command? I guess I need to know if the weight option will still influence route selection even though the paths are not of an equal cost.

Suggestions?

Thanks,

bcbv

3 REPLIES
Bronze

Re: BGP - weight or as-path prepend

Hi

I would use the as-path prepend to achieve this. Weight is a local to the AS attribute and it is also a cisco proprietary parameter.

Hope that helps

Roger

Cisco Employee

Re: BGP - weight or as-path prepend

To control the redundancy using multihomed bgp network, the best way is using "set as-path prepend" command. So use that command to create and propagate a longer AS path (by way of prepending its own AS number more than once)

Here is the best link which discuss "Redundancy in a Multihomed BGP Network".

http://www.cisco.com/warp/public/459/hsrp_bgp.html

Also

http://www.cisco.com/warp/public/615/tac_elearn/ecourse/BGP__1/html/loframe16778560.htm

OR

You can use the weight attribute as well to assign weight to inbound bgp updates from two different sources. Visit

http://www.cisco.com/warp/public/459/bgp-toc.html#weight

Bronze

Re: BGP - weight or as-path prepend

Weight is the first metric Cisco routers look at when deciding which BGP routes are preferred, so it will work in your case. You just have to keep in mind that it's a Cisco proprietary attribute. I'd recommend using either weight or local preference over as-path prepends in your case -- path prepending is generally used when you're sending routes to the Internet and you want to control how non-directly connected AS's send traffic to you.

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