BGP regex examples for filtering AS_PATHS


Here are good ones: 


.*                      Matches anything 

.+                      Match at least one character 

^$                      Match routes local to this AS 

_1800$                  Originated by 1800 

^1800_                  Received from 1800 

_1800_                  Via 1800 

_790_1800_              Passing through 1800 then 790 

_(1800_)+               Match at least one of 1800 in sequence 

_\(65350\)_             Via 65350 (confederation AS) 

^[0-9]+$                Match AS_PATH length of one 

^[0-9]+_[0-9]+$         Match AS_PATH length of two 

^[0-9]*_[0-9]+$         Match AS_PATH length of one or two 

^[0-9]*_[0-9]*$         Match AS_PATH length of one or two, and will also match zero 

^[0-9]+_[0-9]+_[0-9]+$  Match AS_PATH length of three 

_(701|1800)_            Match anything which has gone through AS701 or AS1800 

_1849(_.+._)12163$      Match anything of origin AS12163 and passed through AS1849

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!x

router bgp EEE
 neighbor X.Y.Z.U prefix-list pfx-FILTER in

ip prefix-list pfx-FILTER seq 5 deny A.B.C.D/22			!deny abcd/22
ip prefix-list pfx-FILTER seq 10 permit 0.0.0.0/0 le 32 	!permit anything else

! pfx-FILTER denys that network A.B.C.D/22 is learned of the bgp neighbour X.Y.Z.U.
! 

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router bgp BBB
 neighbor X.Y.Z.U prefix-list pfx-FILTER out
!
! neighbor 157.161.190.193 filter-list 1 out			! AS-Filter-List 


ip prefix-list pfx-FILTER seq 5 deny A.B.C.D/22			!deny abcd/22	
ip prefix-list pfx-FILTER seq 10 permit 0.0.0.0/0 le 32		!permit anything else

! ip as-path access-list 1 permit ^$ 				! Filter only allows updates origination from local
								! AS

! pfx-FILTER denys that network A.B.C.D/22 is propagated to the bgp neighbour X.Y.Z.U.

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