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BGP metrics with ISP

Posted on 2011-04-23
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Hello Experts,

Case study Scenario:
RTR-A and RTR-B are in the same AS and have IBGP between each other. They both have EBGP peering with ISP-A.

When dealing with MEDS, is it correct to assume that if I set an outgoing route-map on RTR-A and RTR-B for different metric values to ISP-A, the ISP has to be willing to cooporate or "accept" those values?
Meaning, this is not something I can just do on my side without talking with them first?  
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Question by:trojan81
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by:rfc1180
ID: 35455093
>Meaning, this is not something I can just do on my side without talking with them first?  
typically, you can assume they will accept.

The concept is this, MED is an optional nontransitive BGP attribute. Think of MED as a  hint to your external upstream neighbors about the preferred path into your AS that has multiple entry points in the ISP network. Additionally, the MED is also an external metric of a route,  the lower MED value is, the more preffered it is over a higher value. Rememeber, you have no control on the inbound traffic to your network; there are methods to deploy to "influence" inbound traffic and MED is one of those methods. If the you are setting metrics on the outbound prefixes and traffic does not appear to be working as expected, I would make a call into your ISP to get information. The ISP might have different routing policies set and might perfer other methods over the other; however, I have yet to come across this.

Billy
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by:trojan81
ID: 35456980
If I go under the bgp process and set a routemap that designates a metric value on both my routers (hinting to my ISP which router to prefer into my AS with the lower metric), I'm essentially sending them a value and it's up to them if they want to accept it or not, correct?

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rfc1180 earned 2000 total points
ID: 35457121
That is correct, as stated, in most cases they will accept it; if not contact them and ask them what their routing policy is; they will either allow you to send communities to allow their policies to adjust local_pref on their network and/or allow you to send a metric.

Billy
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