AS-PATH used to control inbound route in bgp

[wfqk]

Hi, Please see picture attached. Both R1 and R2 are in one AS of ISP, and both R3 and R4 are in other AS of company. Usually in company side, we use local-preference to control inbound routes to decide which path is primary or secondary. My question is, If we ues as-path to control inbound routes, it would be Ok? I tested it, and it can work, but I do not know whether it has some disadvantage? Anyone has this kind of experience? Thank you

[Reza Sharifi · Accepted solution]

Hi,

AS path pre-pend is a common way to influence inbound routes coming to you from your provider and to prefer certain provider.  You basically pre-pend your AS number a few times (3 to 5 times) to make the path longer via your backup provider.

HTH

[wfqk]

Thank you for your reply. We usually think AS-path, MED are used in outbound route, and weight, local-preference are used in inbound route. Based on your reply, now we can not only use AS-path, MED in outbound routes, but also can use AS-path, MED in inbound routes. Do you agree? 

[Jon Marshall]

AS-path and MED affect inbound traffic not inbound routes. 

Jon

[wfqk]

I agree with your comment. That means AS-path and MED are used to change outbound routes, and then affect inbound traffic. All these are one concept.

My original topic is whether AS-path also can be used to change inbound routes. and then affect outbound traffic. I tested it. it can work. so Reza's comment should be right.

[Jon Marshall]

I have never used it like that and in my experience it is not common at all. 

The only routes you should be prepending your AS to are your own in which case you should not be using it to modify routes that do not belong to you, which kind of negates the whole idea of an AS. 

I have never seen it used like that, as you say weight and local preference are the tools to use. 

I guess if you were not a transit AS so you were not advertising those routes to another AS you could use it, assuming it works which you say it does, but still I wouldn't do it. 

Jon

[wfqk]

Thank you!