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.
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| Figure 1 |
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
