BGP AS-Path

[Anukalp S]

Hi, Could you please share difference b/w local-pref inbound and AS-path prepend inbound. In which scenarion these two can be used.

Also if we applying AS-path prepend inboud on my router so would ASN be of ISP or my side ASN to lengthen path.

[Jon Marshall]

The two are used for different things.

AS path prepending affects traffic coming into your AS and local preference affects traffic going out of your AS. 

If you want to use AS path prepending you should really only use your own AS to add to the path although it is not clear exactly what you are trying to do. 

Jon

[Anukalp S]

Hi.. I know this facts that  AS path prepending affects traffic coming into your AS and local preference affects traffic going out of your AS. 

But what if i use AS-path prepend in inbound direction, so in this situation how AS-path prepend in inbound direction is different from Local-pref inbound.

[Hector Gustavo Serrano Gutierrez]

Hi @Anukalp S,

If the Router is receiving the same route via two different BGPs peers, the BGP Path Attributes of both routes are compared in a specific order to choose the "best" route that will be installed in the Routing Table via BGP.

Take a look at BGP Best Path Selection Algorithm.

Local Preference is compared earlier in the process than AS-path to elect the "best" route. 

Yes, can inbound prepend Autonomous Systems (AS) in the AS-path attribute to influence the election of the "best" route in your network, but Local Preference is compared first than AS-path.

Additionally, if your AS would forward these routes to another AS, the AS-path would be carried and you would be indirectly influencing other ASes as well.

Local Preference is local to an AS (not advertised to eBGP peers) and that is the reason it is most commonly use to influence traffic going outbound direction from your network.

I hope this helps.

Cheers.