load balancing in BGP

[shahid.faisal]

Hi,

We have an IP pool of /24 from APNIC. We are connected with two ISPs each with a bandwidth of 2MB. BGP is configured and our network is advertised through these two ISPs.

My question is can we configure load balancing on BGP so that our both link can be utilized simultaneously for incoming and outgoing traffice, I know that we can control outgoing traffic using Policy Based Routing, but what about incoming is their any way.

Regards,

Faisal Shahid

[Xavier Hick]

Hi Faisal,

There is no way you can ensure the return traffic will be evenly load balanced. Similarly, without any config change, there is no way you can be sure the return traffic will take the same path as when being sourced - assymetric routing can happen - AS paths will not be necessarely of the same length when reaching the destination, meaning one path will be prefered.Furthermore, BGP attributes can be easily manipulated so there is really no way to guarantee this.

In my opinion, the best way you can achieve load balancing is by advertising one /25 to each ISP (split the /24 in two) and hope that the ISPs don't do aggregate based on the /24 or even less OR advertise the /25 to each ISP as well as the /24 but add your own AS# to the ASpath before advertising it to the ISP. This way, the ISPs should take into account the preference to reach each /25 through a single link, yet you ensure link failure will not affect connectivity by advertising the whole /24 as well.

Hope this helps,

Xavier

[Mahesh Gohil · Accepted solution]

Hello Faisal,

ISP will not accept /25 prefix from customer. The proposed solution is ok if you are connected to single ISP where ISP permit /25 as well as

/24 and advertise /24 to upstream. In this way traffic coming from internet to /24 pool and once it reach isp backbone it will be divided

to two different link.

Now you are connected to two different ISP so the only option left is take one /23 ip from apnic (I know they will easily give you if you utilization of existing /24) and divide it into two /24 and advt. each one to different ISP.

Hope this helps

Regards

Mahesh