BGP: Regular Expressions for AS-PATH Filtering

Symbol	Description
|	logical OR
.	match any
[x..y]	match one in range
^	match beginning of string
$	match end of string
_	match any delimiter/white space
( )	group as a single atom
*	match 0 or more instances of previous atom
?	match 0 or 1 instance of previous atom
+	match 1 or more instances of previous atom
\	escape character; if followed by a number n, points to the nth atom

Example 1: Advertise routes with empty AS-PATH (internal routes)

router bgp 123  neighbor 5.6.7.8 remote-as 387  neighbor 5.6.7.8 filter-list 1 out ! ip as-path access-list 1 permit ^$ !match "blank" atom at the "start" and "end" of string !

Example 2: Accept only default routes, preferring primary route based on AS PATH:

	AS387 (primary ISP)
	/
AS123
	\
	AS462 (backup ISP)

router bgp 123  neighbor 1.2.3.4 remote-as 462  neighbor 1.2.3.4 route-map FILTER in  neighbor 5.6.7.8 remote-as 387  neighbor 5.6.7.8 route-map FILTER in ! route-map FILTER permit 10 ! default routes from primary ISP (AS 387) accepted are preferred (larger weight)  match ip prefix-list DEFAULT_ONLY  match as-path 10  set weight 150 ! route-map FILTER permit 20 ! default routes from backup ISP are accepted, with lower preference than primary ISP routes  match ip prefix-list DEFAULT_ONLY  set weight 100 ! ip as-path access-list 10 permit _387$ ip prefix-list DEFAULT_ONLY seq 10 permit 0.0.0.0/0 !

Example 3: AS PATH Filtering with AS Path Prepending:

- customer in AS123 is performing AS-PATH pre-pending

AS123
10.0.0.1	\
	\
	AS462

router bgp 387  neighbor 10.0.0.1 remote-as 213  neighbor 10.0.0.1 filter-list 10 in ! ip as-path access-list 10 permit ^123(_123)*$ ! accepts "123", "123 123", or "123 123 123"

Example 4: AS PATH Filtering with AS Path Prepending, multiple customers:

- multiple customers performing AS PATH Prepending

Customer 1
	\
Customer 2 -	AS387
	/
Customer 3

!

router bgp 387  neighbor 10.0.0.1 remote-as 123  neighbor 10.0.0.1 filter-list 10 in  neighbor 20.0.0.1 remote-as 456  neighbor 20.0.0.1 filter-list 10 in  neighbor 30.0.0.1 remote-as 789  neighbor 30.0.0.1 filter-list 10 in ! ip as-path access-list 10 permit ^([0..9]+)(_\1)*$ ! accepts repeating instances of "123", "456", and "789" ! does not accept strings non-repeating strings (e.g. "123 123 100") !

Notes:
atom 1 = at least one instance of a number at the beginning of the string;
atom 2 = 0 or more instances of a whitespace and atom 1 until the end of the string.