BGP - regexp practice part 1

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
R1#

Well that's it for now. I'll come up with some more later.