Bgp best path selection in two different autonoums system and describe

[nambirajanece55]

Hi Team,
Pls help on this , which is in two different AS how to we select best path for inbound and outbound traffic

[M02@rt37]

Hello @nambirajanece55 

In a network with two routers situated in different AS, the process of selecting the best path for inbound and outbound traffic involves the use of the BGP.

For outbound traffic (originating from the local AS), administrators can influence the path by configuring outbound traffic policies on their BGP router. One common method is to manipulate the AS path attribute, wherein the local AS number is prepended multiple times to make the route less preferable, steering traffic towards an alternate path.

On the other hand, for inbound traffic (coming from the remote AS), administrators in the remote AS can use attributes like Multi-Exit Discriminator (MED), or BGP communities to influence the inbound path. Lower MED values, and community tags are utilized to make certain routes more preferable, guiding incoming traffic through specific paths. These BGP mechanisms enable network administrators to have control over the flow of traffic, optimizing network performance and ensuring efficient utilization of network resources.

BGP Best Path Selection: https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html

[David Ruess]

Hello,

For BGP route inbound/outbound influencing you can use the following:

Inbound: AS_Path (Prepending), MED, Comminutes

Outbound: Weight, Local Preference, Communities, AS_Path

Here is a helpful chart:

*AS_PATH can be used for INBOUND or OUTBOUND traffic depending on direction applied.

M02@rt37 I don't think Local Preference is used for inbound traffic (from remote AS) as its not shared with the remote AS and stays local. LP is usually used for outbound (traffic being sent from Local AS) traffic manipulation.

-David

[M02@rt37]

Hello @David Ruess 

Yes, Local Preference is the second BGP attribute and is used to choose the exit path for an autonomous system. The BGP preference has to be set inbound on routes being received to influence the outbound routing behaviour.