BGP – How to display incoming/outgoing routes before/after filtering

Below you’ll find some simple BGP commands that you can use in order to check your policy-control rules (filter-list, distribute-list, route-map, etc.) locally, when you don’t have access to the peer neighbor.

This is our initial network….

Incoming Routes : Before filtering (soft-reconfiguration in must be enabled *)

R1#sh ip bgp nei 10.10.10.2 received-routes
% Inbound soft reconfiguration not enabled on 10.10.10.2

R1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#router bgp 1
R1(config-router)#neighbor 10.10.10.2 soft-reconfiguration inbound

R1#sh ip bgp nei 10.10.10.2 received-routes
BGP table version is 8, local router ID is 1.1.1.1
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
*> 20.20.0.0/16     10.10.10.2               0             0 2 i
*> 20.20.20.0/24    10.10.10.2               0             0 2 i

Total number of prefixes 2

Incoming Routes : After filtering

R1#sh ip bgp nei 10.10.10.2 routes
BGP table version is 8, local router ID is 1.1.1.1
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
*> 20.20.0.0/16     10.10.10.2               0             0 2 i
*> 20.20.20.0/24    10.10.10.2               0             0 2 i

Total number of prefixes 2

Outgoing Routes : Before filtering

R1#sh ip bgp
BGP table version is 8, local router ID is 1.1.1.1
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
*> 1.0.0.0          0.0.0.0                            32768 i
*> 1.1.0.0/16       0.0.0.0                            32768 i
s> 1.1.1.0/25       0.0.0.0                  0         32768 i
*> 1.1.1.0/24       0.0.0.0                            32768 i
*> 20.20.0.0/16     10.10.10.2               0             0 2 i
*> 20.20.20.0/24    10.10.10.2               0             0 2 i

Outgoing Routes : After filtering

R1#sh ip bgp neighbors 10.10.10.2 advertised-routes
BGP table version is 7, local router ID is 1.1.1.1
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
*> 1.0.0.0          0.0.0.0                            32768 i
*> 1.1.0.0/16       0.0.0.0                            32768 i
*> 1.1.1.0/24       0.0.0.0                            32768 i

Time to add some filtering….

Let’s apply an inbound prefix-list first:

R1(config)#ip prefix-list R2-IN permit 20.20.0.0/16

R1(config)#router bgp 1
R1(config-router)#neighbor 10.10.10.2 prefix-list R2-IN in

Do an inbound soft reconfig :

R1#clear ip bgp 10.10.10.2 soft in

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Incoming Routes : Before filtering

R1#sh ip bgp nei 10.10.10.2 received-routes
BGP table version is 10, local router ID is 1.1.1.1
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
*> 20.20.0.0/16     10.10.10.2               0             0 2 i
*  20.20.20.0/24    10.10.10.2               0             0 2 i

Total number of prefixes 2

Incoming Routes : After filtering

R1#sh ip bgp nei 10.10.10.2 routes
BGP table version is 10, local router ID is 1.1.1.1
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
*> 20.20.0.0/16     10.10.10.2               0             0 2 i

Total number of prefixes 1

So filtering works fine on the inbound.

Let’s apply an outbound prefix-list now :

R1(config)#ip prefix-list R2-OUT permit 1.0.0.0/8 le 16

R1(config)#router bgp 1
R1(config-router)#neighbor 10.10.10.2 prefix-list R2-OUT out

Do an outbound soft reconfig

R1#clear ip bgp 10.10.10.2 soft out

Outgoing Routes : Before filtering

R1#sh ip bgp
BGP table version is 10, local router ID is 1.1.1.1
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
*> 1.0.0.0          0.0.0.0                            32768 i
*> 1.1.0.0/16       0.0.0.0                            32768 i
s> 1.1.1.0/25       0.0.0.0                  0         32768 i
*> 1.1.1.0/24       0.0.0.0                            32768 i
*> 20.20.0.0/16     10.10.10.2               0             0 2 i

Outgoing Routes : After filtering

R1#sh ip bgp nei 10.10.10.2 advertised-routes
BGP table version is 10, local router ID is 1.1.1.1
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
*> 1.0.0.0          0.0.0.0                            32768 i
*> 1.1.0.0/16       0.0.0.0                            32768 i

So filtering works fine on the outbound too.

You can also use the following commands in order to test your policy-controls on your BGP table before you actually apply them.

R1#sh ip bgp ?
community        Display routes matching the communities
community-list   Display routes matching the community-list
filter-list      Display routes conforming to the filter-list
prefix-list      Display routes matching the prefix-list
quote-regexp     Display routes matching the AS path "regular expression"
regexp           Display routes matching the AS path regular expression
route-map        Display routes matching the route-map

* : Because soft-reconfiguration creates an extra copy of all routes received per neighbor, you want to be careful when implementing this feature in a production network or a network with many routes/neighbors. Route-refresh is a much better solution; you just miss the functionality of viewing the incoming routes before filtering.

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