BGP Regular Expression

On R4, I only want to learn routes from R5 that originated from AS100 and deny everything else. One way of accomplishing this is to use a BGP filter-list. We will use the topology in figure 1.

Figure 1

Current BGP Config with the filter-list already applied.

R4#show running-config | section router bgp
router bgp 300
 no synchronization
 bgp log-neighbor-changes
 network 172.16.1.0 mask 255.255.255.0
 network 172.17.1.0 mask 255.255.255.0
 network 175.175.1.0 mask 255.255.255.0
 neighbor 192.168.4.2 remote-as 100
 neighbor 192.168.4.2 weight 100
 neighbor 192.168.4.2 maximum-prefix 100
 neighbor 192.168.6.2 remote-as 300
 neighbor 192.168.6.2 filter-list 1 in
 no auto-summary
R4#


Verify debug:

R4#show debugging
IP routing:
  BGP updates debugging is on for address family: IPv4 Unicast


Verify AS Path Access-List:

R4#show ip as-path-access-list
AS path access list 1
    permit ^100$

Perform a route refresh for peer 192.168.6.2:

R4#clear ip bgp 192.168.6.2 in
R4#

Confirms AS Path Access-List is working as expected from debug:

*Mar  1 01:00:14.715: BGP(0): 192.168.6.2 rcvd UPDATE w/ attr: nexthop 192.168.6.2, origin i, localpref 100, metric 0
*Mar  1 01:00:14.719: BGP(0): 192.168.6.2 rcvd 176.176.1.0/24 -- DENIED due to: filter-list;
*Mar  1 01:00:14.723: BGP(0): 192.168.6.2 rcvd 172.19.1.0/24 -- DENIED due to: filter-list;
*Mar  1 01:00:14.727: BGP(0): 192.168.6.2 rcvd 172.18.1.0/24 -- DENIED due to: filter-list;
*Mar  1 01:00:14.731: BGP(0): 192.168.6.2 rcvd UPDATE w/ attr: nexthop 192.168.6.2, origin i, localpref 100, metric 0, path 100
*Mar  1 01:00:14.735: BGP(0): 192.168.6.2 rcvd 9.9.9.0/24
*Mar  1 01:00:14.743: BGP(0): 192.168.6.2 rcvd UPDATE w/ attr: nexthop 192.168.6.2, origin i
R4#, localpref 100, metric 0, path 100
*Mar  1 01:00:14.747: BGP(0): 192.168.6.2 rcvd 8.8.8.0/24
*Mar  1 01:00:14.751: BGP(0): 192.168.6.2 rcvd 7.7.7.0/24
*Mar  1 01:00:14.751: BGP(0): 192.168.6.2 rcvd UPDATE w/ attr: nexthop 192.168.6.2, origin ?, localpref 100, metric 0, path 100
*Mar  1 01:00:14.751: BGP(0): 192.168.6.2 rcvd 177.177.1.0/24

Verifying BGP database:

R4#show ip bgp
BGP table version is 32, local router ID is 175.175.1.1
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
* i7.7.7.0/24       192.168.6.2              0    100      0 100 i
*>                  192.168.4.2                          100 100 i
* i8.8.8.0/24       192.168.6.2              0    100      0 100 i
*>                  192.168.4.2                          100 100 i
* i9.9.9.0/24       192.168.6.2              0    100      0 100 i
*>                  192.168.4.2              0           100 100 i
*> 172.16.1.0/24    0.0.0.0                  0         32768 i
*> 172.17.1.0/24    0.0.0.0                  0         32768 i
*> 175.175.1.0/24   0.0.0.0                  0         32768 i
* i177.177.1.0/24   192.168.6.2              0    100      0 100 ?
*>                  192.168.4.2                          100 100 ?
R4#

Further Reading:

Using Regular Expressions in BGP

Juniper Using Regular Expressions