BGP conditional advertisement is another feature of Cisco IOS. It controls the way it advertises the BGP network by considering whether it has or has not a BGP network in its table.
There are 2 command about conditional advertisement.
- neighbor advertise-map <advertise route> exist-map <exist-route>
= BGP will advertise <advertise-route> if the router has an <existing route> in its BGP table
- neighbor advertise-map <advertise route> non-exist-map <non-exist-route>
= BGP will advertise <advertise-route> if the router do not have an <non-existing route> in its BGP table
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Here is the sample configuration of BGP conditional advertisement.
R1 <-----> R2
R1
router bgp 100
no synchronization
bgp log-neighbor-changes
network 10.10.18.0 mask 255.255.255.0
network 10.10.19.0 mask 255.255.255.0
neighbor 192.168.1.2 remote-as 100
neighbor 192.168.1.2 advertise-map Conditional-bgp exist-map test
access-list 1 permit 10.10.19.0 0.0.0.255
access-list 2 permit 10.10.18.0 0.0.0.255
!
route-map Conditional-bgp permit 10
match ip address 1
!
route-map test permit 10
match ip address 2
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R1#sh ip bgp
BGP table version is 8, local router ID is 10.10.19.1
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
*> 10.10.18.0/24 0.0.0.0 0 32768 i
*> 10.10.19.0/24 0.0.0.0 0 32768 i ( advertise out to R2)
Check R2 Routing table
R2
R2#sh ip route
Gateway of last resort is not set
10.0.0.0/24 is subnetted, 6 subnets
O 10.10.10.0 [110/11] via 192.168.1.1, 01:39:45, FastEthernet0/0
O 10.10.12.0 [110/11] via 192.168.1.1, 01:39:45, FastEthernet0/0
O 10.10.14.0 [110/11] via 192.168.1.1, 01:39:45, FastEthernet0/0
O 10.10.16.0 [110/11] via 192.168.1.1, 01:39:45, FastEthernet0/0
O 10.10.18.0 [110/11] via 192.168.1.1, 00:00:26, FastEthernet0/0
B 10.10.19.0 [200/0] via 192.168.1.1, 00:00:08
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When network 10.10.18.0/24 is deleted from R1 BGP table, R1 stops advertising 10.10.19.0/24
debug ip bgp updates
*Mar 1 01:55:12.003: BPG(0): Condition test changes to Withdraw
*Mar 1 01:55:12.007: BPG(0): Condition test changes to Withdraw
*Mar 1 01:55:12.007: BGP(0): net 10.10.19.0/24 matches ADV MAP Conditional-bgp: bump version to 11
*Mar 1 01:55:12.219: BGP(0): nettable_walker 10.10.19.0/24 route sourced locally
*Mar 1 01:55:12.223: BGP(0): 192.168.1.2 10.10.19.0/24 matches advertise map Conditional-bgp, state: Withdraw
*Mar 1 01:55:12.223: BGP(0): 192.168.1.2 send unreachable 10.10.19.0/24
*Mar 1 01:55:12.223: BGP(0): 192.168.1.2 send UPDATE 10.10.19.0/24 -- unreachable
R2#sh ip route
Gateway of last resort is not set
10.0.0.0/24 is subnetted, 4 subnets
O 10.10.10.0 [110/11] via 192.168.1.1, 01:41:26, FastEthernet0/0
O 10.10.12.0 [110/11] via 192.168.1.1, 01:41:26, FastEthernet0/0
O 10.10.14.0 [110/11] via 192.168.1.1, 01:41:26, FastEthernet0/0
O 10.10.16.0 [110/11] via 192.168.1.1, 01:41:26, FastEthernet0/0
C 192.168.1.0/24 is directly connected, FastEthernet0/0
R2#
http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a0080094309.shtml
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