How to Configure Conditional Advertisement on Border Gateway Protocol (BGP)

Overview

This document demonstrates how to configure conditional advertisment on Border Gateway Protocol (BGP).

Firewall FW-A have two routes 55.55.55.0/24 and 100.100.100.0/24.

Firewall FW-C have one route 60.60.60.0/24.

Requirement is that if FW-B is having routes 100.100.100.0/24, 55.55.55.0/24 in its local-rib it should not advertise 55.55.55.0/24 route to FW-C but it should advertise 100.100.100.0/24. If route 100.100.100.0/24 is not in local rib of FW-B only then it should advertise 55.55.55.100 route to FW-C.

How to Achieve This?

This can be achieved by configuring conditional advertisement on FW-B. With the help of conditional advertisement we can be specific on which route should be advertised when some network/route is down. To accomplish this, we have to configure non-existent filter. Below, we specify that if 100.100.100.0/24 doesn’t exist, then advertise what ever we have specified in Advertise filters.

FW-A have 55.55.55.0/24 and 100.100.100.0/24 in its local rib.

FW-A advertising both route to 55.55.55.0/24 and 100.100.100.0/24 to FW-B.

FW-B have received both route from FW-A.

FW-B sending only 100.100.100.0/24 route to FW-C.

FW-C getting 100.100.100.0/24 from FW-B.

On FW-A 100.100.100.0/24 went down.

FW-A advertising only 55.55.55.0/24.

FW-B got 55.55.55.0/24 from FW-A.

FW-B started adverting route 55.55.55.0/24 to FW-C.

FW-C got 55.55.55.0/24 from FW-B.