Hi Giuseppe,

Attached is a problem we have with VLAN configuration of Cisco Switches. I would be grateful if you could send your answer to my email address clfranjo@yahoo.es.

Many thanks,

Carlos.

Giuseppe/Jerry:

Perhaps it would be better if I gave you the requirement. Kindly read the entire post. Thank you.

The objective of the as path ACL should be to match prefixes we receive from 25525 that originate in AS 59591, which can get prepended, and then is advertised directly to us by AS 25525, which may also get prepended.

So there will only be 2 different AS numbers in the AS path: the originating one (59591), which will probably be prepended in the received advertisement and the directly connected AS (25525), which advertises it to us.

The route table looks something like this:

*  x.x.x.x    z.z.z.z           0             0 25525 59591 59591 59591 59591 i
*>y.y.y.y    z.z.z.z            0            0 25525 59591 59591 59591 59591 i

Given this requirement, which as path list is correct? And can you explain why? Please make your explanation dummy-proof because I am a regexp dummy, for sure.

Giuseppe:

I know I posted a similar question a couple months back. I didnt quite underatand the answers then, and thats why Im reposting. I dont use bgp regexp too much - almost never. When I get more time, Ill read up on them.

Thanks

Hello Joe,

no problem, I was just noting the fact

if the objective is to match this AS path:

0 25525 59591 59591 59591 59591 i

you can use both

ip as-path access-list 2 permit 25525+ 59591+

ip as-path access-list 3 permit ^(25525_)+(59591_)+$

the first match because it finds the substring 25525 59591 inside the AS path string

the second matches because it matches the whole AS path string 25525 59591 59591 59591 59591 as a case included in

^(25525_)+(59591_)+$  the leading ^ means starting point in AS path one or more occurrences of ASN 25525 followed by one or more occurrences of ASN 59591.

Hope to help

Giuseppe

I will say the 2nd list, ip as-path access-list x permit ^(25525_)+(59591_)+$, is the correct one based on the following condition

"So there will only be 2 different AS numbers in the AS path: the originating one (59591), which will probably be prepended in the received advertisement and the directly connected AS (25525), which advertises it to us."

$ check the originating AS path

^ check the connected AS path

Regards,

jerry

Jerry:

Awesome answer! That is what I am looking for...the difference between the 2...

A last quick question...

What does adding "( )" and a "_" do?

Thanks!

Jerry, you are the man....!

Stick around this board more often...youll be on eof the bright stars on here.

Thanks