How to configure the BGP conditional advertisement feature

[Gagank]

Hello Everyone, i am a service deliver engineer and having a project to deliver for my own organization.

Scenario:

1. We have two ISP and two different subnets in LAN. One subnet will pass through primary ISP and another will pass through secondary. 
2. In Achieving point 1 i configured PBR route-maps with access-List and segregate the traffic with set-ip next hop command.
3. Now i want - if my primary link to ISP goes down, traffic from primary subnet will shift to secondary link and same for Secondary LAN subnets.

[cofee]

There is a feature in BGP that can be used for conditional advertisement.

Below are the features/options that can be used for conditional advertisement:

>advertise-map

>non-exist-map

>exist-map

> inject-map

These features work in conjunction and the logic is just like if and else in programming. For example, only advertise network A to ISP x when network B is available in bgp rib or vice versa.

[Gagank]

Thanks buddy, I will try this and let you know.... One question here.. is there any chance for collision with pbr applied on local router interface.

[cofee]

Hi,

Correct me if I am wrong, but based on your explanation it appears you have a PBR that's doing load balancing between 2 WAN circuits for your internal subnets going out to internet, if that's the case then when a circuit goes down then you lose your "next ip hop" and PBR becomes invalid at that time and router will route packets based on global RIB.

For this you don't need any conditional advertisement unless you are receiving specific routes from your ISP or if you are advertising prefixes to your ISPs, but if you just have default routes just to access internet then you shouldn't need any of this.

[Richard Burts]

The title of the original post is pretty clear in suggesting that they are looking for conditional advertisement. And so the response explaining about conditional advertisement is appropriate. But as I read the original post I believe that conditional advertisement is not what they are really looking for. The original post describes setting up PBR to separate traffic being sent to their two ISP and asks for a solution to provide failover if one ISP has problems and would send that traffic to the other ISP. Would conditional advertising help do that?

If you think about it using PBR controls how they send traffic to the ISP. And conditional advertisement controls what they advertise to the ISP and therefore controls how the ISP sends traffic to them. And that does not provide the failover for PBR. I would suggest that what they are looking to achieve in reacting to a failure and sending traffic to the other ISP can be achieved using the verify-availability parameter in the PBR set ip next-hop command and perhaps some floating static routes.

HTH

Rick