Using AS-Path attribute to influence routing over multiple regions

Question

We have standard approach to influence routing using AS-Path. I could see we are unable to do on AWS and when we have connection homed in different regions. Is there a way we can use AS-Path to influence traffic instead of any other best path selection algorithm?

khanakee · Accepted Answer

From AWS Doc: 
When you have multiple virtual interfaces in the same associated Region, you can set the AS_PATH attribute to prioritize which interface AWS uses to route traffic. However, AS_PATH prepending doesn’t work when the Direct Connect connections are in different associated AWS Regions than the VPC.
https://docs.aws.amazon.com/directconnect/latest/UserGuide/routing-and-bgp.html#private-routing-policies

However you can use AS-Path as your primary best path selection algorithm firstly by setting equal preference with Local preference BGP communities values e.g. (7200:100) on all the connections(VIFs). Which leads to tie in path selection hence AS-Path can be evaluated as a tie breaker. By also advertising your prefixes with different AS-Path length will influence traffic flow from AWS based on AS-Path.

TonyG · Answer

The best practice from AWS when it comes to multi-region traffic engineering is to use communities.
This article goes into the same scenario you described https://repost.aws/knowledge-center/direct-connect-bgp-communities
also look at https://docs.aws.amazon.com/directconnect/latest/UserGuide/routing-and-bgp.html#local-pref-bgp-communities

Imran · Answer

Hi khanakee,
So if I correctly understood this is that if I set the same BGP local preference community across all the VIFs, then I can use AS-Path to influence how routing takes place from AWS side. Is that correct?

Khai anis · Answer

On the multi region with direct connect usage u need to use : 
- Local preference , 
- bgp communities to influence routing

Using AS-Path attribute to influence routing over multiple regions

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