BGP Advertisements

[Brandon Baker]

We currently have 2 ISP providers and advertise the same /20 to both of them but the load balancing of incoming traffic is not to our liking. If we advertise 4 /22 blocks instead of one large /20 to one of our ISPs and basically set up route-maps to only allow 2 or 3 of the /22s through one of the ISPs will this work or would we have to advertise a /20 since that is the block we have through Arin?

[Richard Burts]

I would suggest that you think very carefully about the implications of using route maps to only allow some parts of your network through one of the ISPs. This would eliminate the possibility of redundancy and failover if the other ISP were having problems.

You are not required to advertise a /20 since that is the block you have through Arin - but I would suggest that you probably do want to advertise the /20. If you want traffic for certain parts of your network to prefer one ISP over the other it is certainly possible to advertise a more specific block to that ISP (in addition to advertising the /20 to both ISP). I have some customers who do this and it works well. You advertise the complete block to all ISP that you connect to so that there is possibility of failover if one ISP has a problem. Then you advertise a more specific block (size of the block is something that you can work out) to one ISP so that traffic to that group of addresses will come through that ISP.

HTH

Rick

[Brandon Baker]

My plan would be on the route map to just give a couple of the blocks an as-path prepend so that it will recognize our other connection as the better path.  Basically you are saying we should advertise the 1.1.16.1/20(for example) 1.1.16.1/22, 1.1.20.1/22, 1.1.24.1/22, and 1.1.28.1/22 correct? If we do this then in my route map I would do the following to force to traffic to our other provider

route-map OUTBOUND permit 10
 match ip address X (1.1.16.1/22)
 set as-path prepend AS AS AS AS

route-map OUTBOUND permit 20
 match ip address X (1.1.20.1/22)
 set as-path prepend AS AS AS AS

route-map OUTBOUND permit 30

This should allow all traffic to come through except for

1.1.16.1/22 and 1.1.20.1/22 correct? (but would still be allowed to come in if our other path was down)

[Richard Burts · Accepted solution]

You could do it as you suggest. But to me that is more complex than it needs to be. I do not see much reason to advertise all /22 to both ISP. So let me try to explain my approach in a slightly different way.

I suggest that you need to advertise the /20 to both ISP so that either of them can carry the load. I then suggest that you decide which provider you wish to use for the first /22 (1.1.16.1/22 in your example) and advertise that block only to that provider. Then you choose which provider you wish to use the second block and advertise that block only to that provider. After you have done this traffic will flow naturally to the provider with the most specific route and if that provider is having a problem then the traffic will flow to the alternate provider.

HTH

Rick

[Brandon Baker]

Great Thank you Rick

[Richard Burts]

I am glad that my suggestion was helpful. Thank you for using the rating system to mark the question as answered. That will help other readers of the forum to know that there is helpful information in the discussion.

HTH

Rick