BGP and null path announcement

[zillah2004]

http://info.connect.com.au/docs/routing/general/multi-faq.shtml

((How do I configure BGP so that bad things don't happen?))

Does the command "ip as-path access-list 1 permit ^$" from the link above, stand for a null path (^$) ?

[pkhatri]

Yes.

The regular expression ^$ matches the empty string.

This is useful when matching on prefixes that are locally generated within an AS. They will carry an empty AS-PATH until they leave the AS. Therefore, ^$ can be used within an as-path access-list to match on locally generated routes ...

Hope that helps - pls do rate posts that help.

Regards,

Paresh

[mheusinger]

Hello,

basically this allows to sort out those routes learned from another AS. It is the easiest and most flexible way of preventing to become transit AS between two ISPs for a customer.

The statement would allow only those routes to be announced, which are locally originated within the customer AS.

Hope this helps! Please rate all posts.

Regards, Martin