Information Technology Reference
In-Depth Information
Figure 4-3
Conditional Advertisement to Track the Existence of a Prefix
R3
AS 300
.3
192.168.13.0/24
AS 100
OSPF
172.16.0.0/16
.1
10.0.0.0/16
192.168.12.0/24
R2
R1
Example 4-19 shows a sample BGP configuration on R1. Both the prefix to be advertised
(172.16.0.0) and the prefix tracked (10.0.0.0) are injected into the BGP RIB. The private
prefix is then blocked from being advertised to R3 with the prefix list Block10. The exist
map Prefix10 tracks the existence of 10.0.0.0/16, which is learned from OSPF. When the
match returns true (status: Advertise), AS300-out is executed. When 10.0.0.0/16 is gone
from OSPF (status: Withdraw), 172.16.0.0/16 is not advertised or withdrawn.
Example 4-19
Sample BGP Configuration on R1
router bgp 100
network 10.0.0.0 mask 255.255.0.0
network 172.16.0.0
neighbor 192.168.13.3 remote-as 300
neighbor 192.168.13.3 prefix-list Block10 out
neighbor 192.168.13.3 advertise-map AS300-out exist-map Prefix10
no auto-summary
!
ip prefix-list Block10 seq 5 deny 10.0.0.0/16
ip prefix-list Block10 seq 10 permit 0.0.0.0/0 le 32
ip prefix-list adv-out seq 5 permit 172.16.0.0/16
ip prefix-list Private10 seq 5 permit 10.0.0.0/16
!
route-map Prefix10 permit 10
match ip address prefix-list Private10
!
route-map AS300-out permit 10
match ip address prefix-list adv-out
