BGP Conditional Advertisement

This BGP feature is able to filter a subnet advertisement based on a certain match (AS-PATH, subnet in the routing table...). In our example below, we can image that AS100 is an ISP1 and AS200 is an ISP2. In most of the case, we monitor only the status of the interface which is directly connected. But in our case, we will monitor the presence of the subnet 20.20.20.0/24 which is advertised by the ISP1.
By default, we only advertise the subnet 50.50.50.0/24 to ISP1 and if the subnet 20.20.20.0/24 disappears, we announce it also to ISP2. We can typically use this feature when ISP2 is more expensive than ISP1.

• Configuration:

R1:
interface Loopback50
 ip address 50.50.50.50 255.255.255.0
!
interface Ethernet0/0
 ip address 192.168.50.1 255.255.255.0
!
interface Ethernet0/1
 ip address 192.168.150.1 255.255.255.0
!
router bgp 50
 network 50.50.50.0 mask 255.255.255.0
 neighbor 192.168.50.2 remote-as 100
 neighbor 192.168.150.3 remote-as 200
 neighbor 192.168.150.3 advertise-map NOT_ANNOUNCE_R3 non-exist-map ADVERTISE
!
ip prefix-list LO20 seq 5 permit 20.20.20.0/24
!
ip prefix-list LO50 seq 5 permit 50.50.50.0/24
!
route-map NOT_ANNOUNCE_R3 permit 10
 match ip address prefix-list LO50
!
route-map ADVERTISE permit 10
 match ip address prefix-list LO20
R2:
interface Loopback20
 ip address 20.20.20.20 255.255.255.0
!
interface Ethernet0/0
 ip address 192.168.50.2 255.255.255.0
!
interface Ethernet0/1
 ip address 192.168.100.2 255.255.255.0
!
router bgp 100
 network 20.20.20.0 mask 255.255.255.0
 neighbor 192.168.50.1 remote-as 50
 neighbor 192.168.100.3 remote-as 200
R3:
interface Ethernet0/0
 ip address 192.168.100.3 255.255.255.0
!
interface Ethernet0/1
 ip address 192.168.200.3 255.255.255.0
!
interface Ethernet0/2
 ip address 192.168.150.3 255.255.255.0
!
router bgp 200
 neighbor 192.168.100.2 remote-as 100
 neighbor 192.168.150.1 remote-as 50

• Initial behavior:

R3 sees subnet 50.50.50.0/24 only from R3
R3#sho ip bgp
     Network          Next Hop            Metric LocPrf Weight Path
 *   20.20.20.0/24    192.168.150.1                          0 50 100 i
 *>                   192.168.100.2            0             0 100 i
 *>  50.50.50.0/24    192.168.100.2                          0 100 50 i
We can check that R1 is not advertising the subnet to R3:
R1#sho ip bgp neighbors 192.168.150.3 advertised-routes
     Network          Next Hop            Metric LocPrf Weight Path
 *>  20.20.20.0/24    192.168.50.2             0             0 100 i
R1#sho ip bgp neighbors 192.168.50.2 advertised-routes
     Network          Next Hop            Metric LocPrf Weight Path
 *>  50.50.50.0/24    0.0.0.0                  0         32768 i
R1#sho ip bgp neighbors 192.168.150.3 | in Conditio
  Condition-map ADVERTISE, Advertise-map NOT_ANNOUNCE_R3, status: Withdraw

• Now the subnet 20.20.20.0/24 is removed from the routing table:

R2#int lo20
shut

We can check that R1 is now advertising the subnet to R3:
R1#sho ip bgp neighbors 192.168.150.3 | in Conditio
  Condition-map ADVERTISE, Advertise-map NOT_ANNOUNCE_R3, status: Advertise
R1#sho ip bgp neighbors 192.168.150.3 advertised-routes
     Network          Next Hop            Metric LocPrf Weight Path
 *>  50.50.50.0/24    0.0.0.0                  0         32768 i

R3#sho ip bgp
BGP table version is 18, local router ID is 192.168.200.3
 *>  50.50.50.0/24    192.168.150.1            0             0 50 i
 *                    192.168.100.2                          0 100 50 i