Conditional BGP Advertisement

Introduction:
This we can use to advertise the prefixes automatically when one peer goes down rather manually advertise the prefixes.
Design Notes:
In this example I’m trying to advertise 10.0.0.0/16 through AS 65001 but if the peer (203.143.0.5) goes down automatically advertise the 10.0.0.0/16 through AS 65002(203.143.0.1).
To make the configuration works we need to match against a prefix that advertise from that(203.143.0.5) peer. But to make sure that learned through that peer we can use AS-Path match or community based match. Here in this example I’m matching against 172.16.0.0/24 advertise through 65001 and match against the as path ^ 65001.
Configuration Notes:
This is the only configuration different from normal one:
neighbor 203.143.0.1 advertise-map otherblock non-exist-map[exist-map] inblock
Better notes on this:

Normal Status:

R2#show ip bgp neighbors 203.143.0.1 | inc Cond
Condition-map inblock, Advertise-map otherblock, status: Withdraw
R0#show ip route
Gateway of last resort is not set
B    172.16.0.0/16 [20/0] via 203.143.0.2, 00:08:41
     203.143.0.0/30 is subnetted, 1 subnets
C       203.143.0.0 is directly connected, FastEthernet0/0
B    192.168.0.0/16 [20/0] via 203.143.0.2, 00:08:11

R2#show ip bgp neighbors 203.143.0.1 advertised-routes
BGP table version is 5, local router ID is 203.143.0.6
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? – incomplete
   Network          Next Hop            Metric LocPrf Weight Path
*> 172.16.0.0       203.143.0.5              0             0 65001 i
*> 192.168.0.0/16   0.0.0.0                  0         32768 i
Total number of prefixes 2

When the BGP PEER Down:

R2#show ip bgp summary
Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
203.143.0.1     4 65002     113     141        7    0    0 00:13:20        0
203.143.0.5     4 65001     117     121        0    0    0 00:00:52 Active
R2#show ip bgp neighbors 203.143.0.1 advertised-routes
BGP table version is 7, local router ID is 203.143.0.6
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? – incomplete
 Network          Next Hop            Metric LocPrf Weight Path
*> 10.0.0.0/16      0.0.0.0                  0         32768 i
*> 192.168.0.0/16   0.0.0.0                  0         32768 i
Total number of prefixes 2
R2#show ip bgp neighbors 203.143.0.1 | inc Cond
  Condition-map inblock, Advertise-map otherblock, status: Advertise

Configuration:

R2#
hostname R2
interface FastEthernet0/0
 ip address 203.143.0.2 255.255.255.252
 duplex auto
 speed auto

interface FastEthernet0/1
 ip address 203.143.0.6 255.255.255.252
 duplex auto
 speed auto
!
router bgp 65003
 no synchronization
 bgp log-neighbor-changes
 network 10.0.0.0 mask 255.255.0.0
 network 192.168.0.0 mask 255.255.0.0
 neighbor 203.143.0.1 remote-as 65002
 neighbor 203.143.0.1 soft-reconfiguration inbound
 neighbor 203.143.0.1 advertise-map otherblock non-exist-map inblock
 neighbor 203.143.0.5 remote-as 65001
 neighbor 203.143.0.5 soft-reconfiguration inbound
 neighbor 203.143.0.5 prefix-list allow_65001 out
 neighbor 203.143.0.5 route-map inbound_65001 in
 no auto-summary
!
ip route 10.0.0.0 255.255.0.0 Null0
ip route 192.168.0.0 255.255.0.0 Null0
!
ip as-path access-list 1 permit ^65001
ip prefix-list allow_65001 seq 5 permit 10.0.0.0/16
ip prefix-list allow_65002 seq 5 permit 192.168.0.0/16
access-list 1 permit 10.0.0.0 0.0.255.255
access-list 2 permit 172.16.0.0 0.0.255.255
route-map otherblock permit 10
 match ip address 1
!
route-map allow_65001 permit 10

route-map inbound_65001 permit 10
 set community 4259905537
route-map inblock permit 10
 match ip address 2
 match as-path 1 ! to match the as-path
end
R2 END#

R1#show run!
!
interface FastEthernet0/0
ip address 203.143.0.5 255.255.255.252
 duplex auto
 speed auto
!
router bgp 65001
 no synchronization
 bgp log-neighbor-changes
 network 172.16.0.0
 neighbor 203.143.0.6 remote-as 65003
 no auto-summary
!
ip forward-protocol nd
ip route 172.16.0.0 255.255.0.0 Null0 
R1 END#
R0#show run
ip cef
interface FastEthernet0/0
ip address 203.143.0.1 255.255.255.252
 duplex auto
 speed auto

router bgp 65002
 no synchronization
 bgp log-neighbor-changes
 neighbor 203.143.0.2 remote-as 65003
 neighbor 203.143.0.2 soft-reconfiguration inbound
 no auto-summary