Conditional Advertisement in BGP:

In the above example, we have to set R2 as a route reflector and other two as client R1 & R2. We have passed a loopback on every router. And above you can see that a link with 90MB is for ABC city and 50MB link for XYZ city. Neighbor ships between these routers via loopbacks and internal communication through OSPF (open shortest path first protocol). 

Route reflector R2 having 192.168.1.0 route from anywhere and he forward that route to R1 client & R2 client (advertise). Now both R1 and R3 use our path for traffic transfer. Now R1 to R2 link is down, but after that reachability of R1 to R2 via R3 is still enabled.

But the issue is here that R1 forward traffic also goes on R3 mean now for both link traffic flow on a single link so that on the same link surly congestion will occur. Because ABC city use 90 Mb link and XYZ use 50 MB link. Now for ABC traffic comer from XYZ link and that link is only 50Mb but it needs now 140 MB link for traffic flow for both link. Due to this, both sites will be suffered. 

Now condition applies for this situation if the link of R1 to R2 will be down then route reflector R2 does not advertise the route to ABC router R1 mean we use here BGP conditional advertisement.

We do R1, R2 network advertise in BGP, that is present in routing table & after this in BGP table also. Then we make a condition when R1 and R2 network in the BGP table at that time being R2 advertise it to R1, when not present in the BGP table then not advertise it to R1. This is the way of conditional advertisement. Now we are going to apply condition on a router.

Make Access List: first we make an access list on route reflector router with the external route permit and internal also permit.

Make Route Map: we make a route map for exit routes.

After the create access list and route map for both exist routes then we will call route map on neighbor command as per above you can see. After the call the route map we checked with the R1 to R2 link down and neighborship gone deleted.

Above is the all command configuration for conditional advertisement whichever we have used in BGP. First, run BGP with 100 autonomous systems then describe neighborship with the same Autonomous system IBGP neighborship formed. Network pass external & internal both. And call the route map in neighbor command.

Note* all the above configuration is done on route reflector router only.