This article describes how to discard any prefix received from a BGP peer with more than certain number of autonomous systems in its AS-Path.

BGP allows numerous attributes (including AS-path) to be attached to every advertised IP prefix. The total length of the AS-path attached to a single IP prefix can be very large.

Alpha is advertising two prefixes to Beta. Prefix 77.15.23.0 will be discarded if it contains more than six  autonomous systems in the AS-path.

lab@Alpha> show route advertising-protocol bgp 89.170.10.1

inet.0: 10 destinations, 10 routes (10 active, 0 holddown, 0 hidden)
Prefix 				Nexthop 			MED 	Lclpref 	AS path
* 77.15.23.0/24 		Self 							6485 4856 1845 1052 1052 65421 65421 3378 [12741] I
* 144.10.98.0/24 		Self 							6485 4856 1845 [12741] I


lab@Beta > show route protocol bgp
77.15.23.0/24       *[BGP/170] 00:00:39, localpref 100
		       AS path: 6485 4856 1845 1052 1052 65421 65421 3378 12741 I, validation-state: unverified
		     > to 89.170.10.2 via ge-0/3/2.0
144.10.98.0/24      *[BGP/170] 00:00:39, localpref 100
                       AS path: 6485 4856 1845 12741 I, validation-state: unverified
                     > to 89.170.10.2 via ge-0/3/2.0

The following policy will be applied in Beta to discard prefixes (in this case: 77.15.23.0/24) with more than six autonomous systems in the AS-path.

lab@Beta > show configuration policy-options | display set
set policy-options policy-statement block-very-long-paths term blocking-1 from as-path too-many-AS
set policy-options policy-statement block-very-long-paths term blocking-1 then reject
set policy-options as-path too-many-AS ".{6,}"

lab@Beta > show configuration protocols bgp | display set
set protocols bgp group external-bgp type external
set protocols bgp group external-bgp import block-very-long-paths
set protocols bgp group external-bgp peer-as 12741
set protocols bgp group external-bgp neighbor 89.170.10.2

Verification after applying the policy:

lab@Beta > show route protocol bgp
144.10.98.0/24       *[BGP/170] 00:08:33, localpref 100
                        AS path: 6485 4856 1845 12741 I, validation-state: unverified
                      > to 89.170.10.2 via ge-0/3/2.0

lab@Beta > show route protocol bgp hidden
77.15.23.0/24        [ BGP ] 00:08:38, localpref 100
                       AS path: 6485 4856 1845 1052 1052 65421 65421 3378 12741 I, validation-state: unverified
                     > to 89.170.10.2 via ge-0/3/2.0