BGP Regular Expressions (Originated AS Expressions)

[kfarrington]

All,

I am a tad confused with the doc http://www.cisco.com/warp/public/459/26.html

Now, I have the following AS-Path Lists

network 10.0.0.0/8 is in AS 444

The prefix ASP is 222 333 444

I am in AS111.

Now if I want to stop all routes including the 10.0.0.0/8 from being advertised into my AS, do I use the following :-

ip as-path access-list 1 permit ^444$

router bgp 111

neighbor x.x.x.x remote-as 222

neighbor x.x.x.x route-map foo in

route-map foo deny 10

match as-path 1

route-map foo permit 20

---------------OR------------------

ip as-path access-list 1 permit _444$

router bgp 111

neighbor x.x.x.x remote-as 222

neighbor x.x.x.x route-map foo in

route-map foo deny 10

match as-path 1

route-map foo permit 20

What is the difference?

Many thx,

Ken

[Harold Ritter]

Ken,

The first one would deny as-path containing 444 only (ie: 444 would be denied but 222 333 444 would not)

The second one would deny any as-path ending with 444 (ie: 222 333 444 would be denied)

Hope this helps,

[kfarrington]

ta dude

[mheusinger]

Just a few remarks to what Harold wrote:

With regular expressions the router treats the AS path like a text string.

_ (underscore) searches for space, begin of string, end of string ... anything except numbers

^ means begin of string, so ^4 means an AS path starting with 4 (this includes 4 or 4123)

$ means end of string, so 1$ means f.e. "4321" or "3123 1"

When writing filters keep in mind that the originating AS will be at the end of the AS path and the AS you learn a prefix from will be the first in the AS path. Another thing to keep in mind is that due to AS path prepending any AS number could occur more than once.

Regards

Martin

[kfarrington]

Thx fella, for the extra comments on from Mr H.

Much apprciated