BGP Regular Expressions - explained with the examples

REMINDER of the META Characters

^ - START of Line
$ - END of Line
| - Logical OR
_ - ANY DELIMETER
? - ZERO instances of the PRECEDING character
* - ZERO OR MODE instances of the PRECEDING character
+ - ONE OR MORE instances of the PRECEDING character
(x) - Combine the enclosed String as a single entity
[x] - Wildcard where any position can match the position in AS-Path
. - Any Character


EXAMPLES

_65505$ - Prefixes that END with the AS 65505, meaning - they were originated by that AS

_65505_ - Prefixes that traversed the AS 65505

^$ - Locally Originated Prefixes (START and END of the line)

.* - ANY prefix (zero or more instances of ANY character)

^[0-9]+$ - All the prefixes from DIRECTLY CONNECTED ASs (meaning - they have only 1 AS in the AS PAth)


BEFORE CREATING THE AS-PATH ACL

If you want to STOP using the recursive algorithm in order to be able to control more complex  regular expressions

(config-router)#bgp regexp deterministic

Now you can actually DISPLAY the prefixes that match your condition in the AS-PATH before defining the AS-PATH ACL

#show ip bgp regexp REGULAR_EXPRESSION

*There is a TRICK here, you need to add a MEMORY location you want to temporarily place thhe results, so instead of the expression ^300$ you would have to type:

#show ip bgp regexp (^300$)(_\1)*$

You can also display the Filter List before applying it to the neighbor:

#show ip bgp filter-list 1