BGP - Selection of Inbound Traffic

[zekebashi]

Hello experts, 

We have two data centers at two separate locations (Loc A & Loc B). Each data center has an ASR (edge router: ASR_A and ASR_B) connected to a different ISP(Integra & L3). We architected the infrastructure where if traffic is initiated (egress traffic) from location A, the ingress traffic should take the path to the same ASR at location A(ASR_A), thus, the same ISP. So, for example, if I am at location A and made an ftp request from my PC through ASR_A to download some files from a server located at Amazon’s cloud, then the traffic/packets should return to ASR_A and not ASR_B.

However, we found out that the traffic path isn’t working this way. The way it works is that when traffic or data request is initiated from location A the returned packets come to ASR_B.

I was wondering if someone can shed some light on what would be the best way to influence the traffic returned to end up a specific ASR/ISP? For example, the traffic initiated from location A(ASR_A)  to Amazon would return to location A (ASR_A) and not ASR_B.

Thanks in advance.

Best, zK 

[e.ciollaro]

Hi zekebashi,

I suppose that you have a direct link between the two sites, you are using the same ip address space in both location and you are advertising this class using BGP. Is it correct ?

If so I think you have to logically divide your address space between the two sites and advertise these subnets with different BGP metrics in each site. One of the most effective way is to configure BGP in each site to advertise each subnet but prepending your AS (more then once, typically 4 or 5 times)  to the subnet used in the other site.

In any case influencing return traffic in BGP is an issue because it depends on many parameters that are out of your control. It is always possible that ISP configure their network to prefer a path instead of another causing return traffic to be asymmetric. Ther are a lot of post in this forum about this problem.

enrico