BGP Conditional Advertisement
The use of BGP conditional Route advertisement is mostly seen on the multi-homed environments i.e. sites having a connection to two different ISPs.
BGP Advertise-map & Non-Exist Map
BGP will advertise all the routes to its neighbours if no filtering has been done. BGP conditional advertisement feature makes use of BGP advertise-maps and non-exist-maps to control the route advertisements.
If a route prefix is not present in the output of the non-exist-map command, then the route specified by the advertise-map command is announced. This feature is useful for multi-homed networks, in which some prefixes are advertised to one of the providers only if information from the other provider is not present. This will help us identify partial reachability.
We will take the help of the below example to further understand the behaviour:
With the BGP conditional advertisement feature, you can now accomplish these tasks on R2:
- If 1.1.1.1/32 exists in R2’s BGP table, then do not advertise the 2.2.2.2/32 network to R3.
- If 1.1.1.1/32 does not exist in R2’s BGP table, then advertise the 2.2.2.2/32 network to R3.
CONFIGURATION ON R1:
network 1.1.1.1 mask 255.255.255.255
neighbor 9.9.12.2 remote-as 200
Related- BGP Table-Map
CONFIGURATION ON R2:
network 2.2.2.2 mask 255.255.255.255
neighbor 9.9.12.1 remote-as 100
neighbor 9.9.23.3 remote-as 300
neighbor 9.9.23.3 advertise-map Advertise non-exist-map Non-Exist
!
access-list 10 permit 2.2.2.2
access-list 20 permit 1.1.1.1
!
route-map Non-Exist permit 10
match ip address 20
!
route-map Advertise permit 10
match ip address 10
CONFIGURATIONS ON R3:
neighbor 9.9.23.2 remote-as 200
Related- BGP Suppress Map & Attribute Map
VERIFICATION:
We see route of 1.1.1.1 is installed in BGP table of R2 and hence route of 2.2.2.2 isn’t advertised to R3.
R2#sh ip bgp
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i – IGP, e – EGP, ? – incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 1.1.1.1/32 9.9.12.1 0 0 100 i
*> 2.2.2.2/32 0.0.0.0 0 32768 I
R2#sh ip bgp neighbors 9.9.23.3 advertised-routes
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i – IGP, e – EGP, ? – incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 1.1.1.1/32 9.9.12.1 0 0 100 i
Total number of prefixes 1
R3#sh ip bgp
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i – IGP, e – EGP, ? – incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 1.1.1.1/32 9.9.23.2 0 200 100 I >>>> Only 1.1.1.1/32 is received and not 2.2.2.2
Now we will shut down Lo0 interface on R1 to stop the advertisement of 1.1.1.1 to R2 and hence will see 2.2.2.2 is received on R3 now from R2.
R2’s BGP Table:
Status codes: s suppressed, d damped, h history, * valid, > best, i – internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i – IGP, e – EGP, ? – incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 2.2.2.2/32 0.0.0.0 0 32768 i >>>>> No 1.1.1.1/32 route in the BGP table
R2#sh ip bgp neighbors 9.9.23.3 advertised-routes
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i – IGP, e – EGP, ? – incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 2.2.2.2/32 0.0.0.0 0 32768 i
Total number of prefixes 1
We see now R3 will receive the route of 2.2.2.2 from R2.
R3#sh ip bgp
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i – IGP, e – EGP, ? – incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 2.2.2.2/32 9.9.23.2 0 0 200 I
Related- Advertisement Control in BGP With Incorrect Route Map Name
ABOUT THE AUTHOR
I am here to share my knowledge and experience in the field of networking with the goal being – “The more you share, the more you learn.”
I am a biotechnologist by qualification and a Network Enthusiast by interest. I developed interest in networking being in the company of a passionate Network Professional, my husband.
I am a strong believer of the fact that “learning is a constant process of discovering yourself.”
– Rashmi Bhardwaj (Author/Editor)
