Topology is as per the below points:
- R1 and R2 have BGP adjacency across 1.1.1.0/30 network.
- R1 has LAN interfaces 192.168.3.1/28 and 192.168.3.250/30.
In the below examples we look at "matching the /30 routes only within the /24 range" as one example and "matching the smaller routes within the /24" as a second example. The last example we "match routes that are /28 to /29 in length from within the /24" by chaining both le and ge together.
Match all /30 routes within larger supernet
I wanted to write a prefix-list that would:- Advertise /30 routes only from within the 192.168.3.0/24 supernet (i.e don't advertise 192.168.3.0/28 but advertise 192.168.3.250/30)
The below achieves that:
R1#router bgp 1
no synchronization
bgp log-neighbor-changes
network 1.1.1.0 mask 255.255.255.252
network 192.168.3.0 mask 255.255.255.240
network 192.168.3.252 mask 255.255.255.252
timers bgp 5 20
neighbor 1.1.1.2 remote-as 2
neighbor 1.1.1.2 soft-reconfiguration inbound
neighbor 1.1.1.2 route-map BGP-OUT out
no auto-summary
!
route-map BGP-OUT permit 10
match ip address prefix-list TEST
!
ip prefix-list TEST seq 5 permit 192.168.3.0/24 ge 30
In the above we match /30 routes only within the /24 supernet. Below is the output proving it. You can see that we don't advertise the 192.168.3.0/28 route we have.
R1#show ip bgp neigh 1.1.1.2 advertised-routes
BGP table version is 6, local router ID is 192.168.3.253
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
*> 192.168.3.252/30 0.0.0.0 0 32768 i
Total number of prefixes 1
Match all smaller routes within larger supernet
I wanted to write a prefix-list that would:
- Advertise all smaller routes within a specific supernet (192.168.3.0/24)
The below achieves that:
R1#router bgp 1
no synchronization
bgp log-neighbor-changes
network 1.1.1.0 mask 255.255.255.252
network 192.168.3.0 mask 255.255.255.240
network 192.168.3.252 mask 255.255.255.252
timers bgp 5 20
neighbor 1.1.1.2 remote-as 2
neighbor 1.1.1.2 soft-reconfiguration inbound
neighbor 1.1.1.2 route-map BGP-OUT out
no auto-summary
!
route-map BGP-OUT permit 10
match ip address prefix-list TEST
!
ip prefix-list TEST seq 5 permit 192.168.3.0/24 le 32
In the above I match routes that have a subnet mask between /24 and /32. Below is evidence of this being true (note that both the /28 and /30 routes are advertised):
R1#show ip bgp neigh 1.1.1.2 advertised-routes
BGP table version is 8, local router ID is 192.168.3.253
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
*> 192.168.3.0/28 0.0.0.0 0 32768 i
*> 192.168.3.252/30 0.0.0.0 0 32768 i
Total number of prefixes 2
Match /28 to /29 routes from within a supernet
I wanted to write a prefix-list that would:
- Advertise only routes that are between /28 and /29 in length from within a specific supernet (192.168.3.0/24)
The below achieves that:
R1#router bgp 1
no synchronization
bgp log-neighbor-changes
network 1.1.1.0 mask 255.255.255.252
network 192.168.3.0 mask 255.255.255.240
network 192.168.3.252 mask 255.255.255.252
timers bgp 5 20
neighbor 1.1.1.2 remote-as 2
neighbor 1.1.1.2 soft-reconfiguration inbound
neighbor 1.1.1.2 route-map BGP-OUT out
no auto-summary
!
route-map BGP-OUT permit 10
match ip address prefix-list TEST
!
ip prefix-list TEST seq 5 permit 192.168.3.0/24 ge 28 le 29
In the above I match routes that have a subnet mask between /28 and /29 only from within the /24 supernet. Below is evidence of this being true (note: that we only advertise the /28 route as it is the only one meeting the criteria):
R1#show ip bgp neigh 1.1.1.2 advertised-routes
BGP table version is 8, local router ID is 192.168.3.253
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
*> 192.168.3.0/28 0.0.0.0 0 32768 i
Total number of prefixes 1
