BGP-Regular-Expression

Using regexp with as-path access-list are one of the coolest features of BGP. The show ip bgp regexp command is good way to test your regular expression

Here is what I have currently on R1's bgp table:

R1#show ip bgp | be Ne
Network Next Hop Metric LocPrf Weight Path
*> 100.3.0.0/24 172.12.123.3 0 0 300 i
*> 100.3.1.0/24 172.12.123.3 0 0 300 i
*> 100.3.2.0/24 172.12.123.3 0 0 300 i
*> 100.6.0.0/24 172.12.123.3 0 300 600 i
*> 100.6.1.0/24 172.12.123.3 0 300 600 i
*>; 100.6.2.0/24 172.12.123.3 0 300 600 i
*> 100.6.3.0/24 172.12.123.3 0 300 600 1000 1200 i
*> 100.6.4.0/24 172.12.123.3 0 300 600 1000 1200 i

Suppose I want to match routes that contain one AS or two AS but no more. I could do this:

R1#show ip bgp regexp ^[0-9]*$|^[0-9]*_[0-9]*$

Network  Next Hop Metric LocPrf Weight Path
*> 100.3.0.0/24 172.12.123.3 0 0 300 i
*> 100.3.1.0/24 172.12.123.3 0 0 300 i
*> 100.3.2.0/24 172.12.123.3 0 0 300 i
*> 100.6.0.0/24 172.12.123.3 0 300 600 i
*> 100.6.1.0/24 172.12.123.3 0 300 600 i
*> 100.6.2.0/24 172.12.123.3 0 300 600 i

How about paths that only contain at least one 4-digit AS# (why? i have no clue but here's how)
R1#show ip bgp regexp _[0-9][0-9][0-9][0-9]_

Network Next Hop Metric LocPrf Weight Path
*> 100.6.3.0/24 172.12.123.3 0 300 600 1000 1200 i
*> 100.6.4.0/24 172.12.123.3 0 300 600 1000 1200 i


http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094a92.shtml#

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