Prefix list

In this scenario I want to filter 192.168.3.x /25 , /26, /27 networks form R1 route update.
I can use access list but I need to set the rules for every network. In prefix list it will easy to filter.

R2(config-router)#do sh ip bgp
BGP table version is 8, local router ID is 2.2.2.2
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
*> 192.168.1.0      10.10.10.1               0             0 1 i
*> 192.168.2.0      10.10.10.1               0             0 1 i
*> 192.168.3.0/25   10.10.10.1               0             0 1 i
*> 192.168.3.128/26 10.10.10.1               0             0 1 i
*> 192.168.3.192/27 10.10.10.1               0             0 1 i
*> 192.168.4.0/25   10.10.10.1               0             0 1 i
*> 192.168.4.128/26 10.10.10.1               0             0 1 i


When we want to exclude the all of 192.168.3.0 networks, first we need to set the deny rule and permit rule for other networks. If we don't set permit rule all network will be excluded because we need to notice all deny rule is always under the rule.

First we apply the prefix list.

ip prefix-list NEI_1 seq 5 deny 192.168.3.0/24 ge 25 le 27
ip prefix-list NEI_1 seq 10 permit 0.0.0.0/0 le 32

And apply the prefix list to BGP.

router bgp 2
 no synchronization
 bgp router-id 2.2.2.2
 bgp log-neighbor-changes
 neighbor 10.10.10.1 remote-as 1
 neighbor 10.10.10.1 prefix-list NEI_1 in
 no auto-summary

Now we can check BGP network, 192.168.3.0 network is gone.

R2#sh ip bgp

   Network          Next Hop            Metric LocPrf Weight Path
*> 192.168.1.0      10.10.10.1               0             0 1 i
*> 192.168.2.0      10.10.10.1               0             0 1 i
*> 192.168.4.0/25   10.10.10.1               0             0 1 i
*> 192.168.4.128/26 10.10.10.1               0             0 1 i

If we want to set the prefix list 192.168.x.x/16 and filter all /25 network.
we can play the range setup from ge to le. 'ge' for minimum and 'le' for maximum.

ip prefix-list NEI_1 seq 5 deny 192.168.3.0/16 ge 25 le 25

Now all of /25 network have been removed from routing table.

R2#sh ip bgp

   Network          Next Hop            Metric LocPrf Weight Path
*> 192.168.1.0      10.10.10.1               0             0 1 i
*> 192.168.2.0      10.10.10.1               0             0 1 i
*> 192.168.3.128/26 10.10.10.1               0             0 1 i
*> 192.168.3.192/27 10.10.10.1               0             0 1 i
*> 192.168.4.128/26 10.10.10.1               0             0 1 i



R2#sh ip route bgp
     192.168.4.0/26 is subnetted, 1 subnets
B       192.168.4.128 [20/0] via 10.10.10.1, 00:00:57
B    192.168.1.0/24 [20/0] via 10.10.10.1, 00:00:57
B    192.168.2.0/24 [20/0] via 10.10.10.1, 00:00:57
     192.168.3.0/24 is variably subnetted, 2 subnets, 2 masks
B       192.168.3.192/27 [20/0] via 10.10.10.1, 00:00:57
B       192.168.3.128/26 [20/0] via 10.10.10.1, 00:00:57