Asymmetric Routing in BGP

Routing & Switching BGP
1

Hi Sir,

i am connected to multiple BGP upstream provider and i want to use the 1 upstream for outgoing traffic and 2 upstream for incoming traffic … So i mean if i use the asymmetric routing in the edge , is there any performance issue will happen .?

Thanks

2

Hello Narad

If I have understood correctly, you have three connections to the Internet at your network edge, and you want to use one of those for outgoing traffic, and the other two for incoming traffic, correct?

From a practical standpoint, if you’re running BGP on the edge of your network, this can be done in cooperation with your ISP. If the links are from different ISPs it may be a little bit tricky, but you can still do it if you let your ISPs know what you want to do.

From a technical point of view, what would be the benefit of the problems involved with routing traffic in this way? To be honest, I can’t think of a benefit for such an arrangement. Can you share with us the reasoning behind such a configuration?

Problems that can arise include:

  • Services such as VPNs as well as firewalls that may be at the edge of your network will have trouble functioning in such an environment because these features rely on “seeing” all the traffic in both directions in order to function properly. Voice services may also suffer in such a network edge configuration.
  • By using links in only one direction, you are leaving a lot of bandwidth unused. For example, the link that is serving outbound traffic only is not using any of its incoming bandwidth that is available. Similarly, the incoming links won’t use any of their outgoing bandwidth.
  • Such an arrangement will also lack any redundancy. Since you have three links, using all three links in both directions will give you redundancy in the event that a single link fails.

For more information on how to configure your network edge using BGP and multiple ISPs, take a look at this lesson:

I hope this has been helpful!

Laz

3

Lets say that u have 4 public prefixes and you r peered with 2 upstream ISP and we as a customer are receiving the default route from the both ISP , in that case How will control the out going traffic ??.

By using PBR i can achieve it but that does the maximum load on my router .
what the recommended solution is going to be ??

4

Hello Narad

In this case you must deal with two things:

  1. Outgoing traffic
  2. Advertised prefixes for incoming traffic.

If we’re talking about running BGP at the edge of your network, then there are several things you can do. Always keep in mind that in such cases, you are always in control of outgoing traffic. You can route outgoing traffic however you like. Use PBR, use equal cost load balancing with BGP, or use primary and backup paths by modifying weight and other BGP attributes. Perform it in such a way so that you will have both redundancy, as well as high capacity, taking advantage of both links to the Internet.

Concerning the four prefixes you want to advertise, here you must decide how you want your incoming traffic to be distributed. How it will be distributed will affect how you advertise your prefixes. You must coordinate with your ISPs so that you will be getting the traffic you expect. Remember, that the ISPs have ultimate control over your incoming traffic, even though you can attempt to influence that incoming traffic in various ways. More about this can be found at the following post:

I hope this has been helpful!

Laz