How to modify AS path??

Hello all,

is there a way to modify (NOT prepend) AS path on cisco routers? I want
to "clean" some ASN's off the AS-PATH.

I know it would be dangerous in internet-exposed network because of the
loops, but this one is not connected to internet (not by bgp at least).

 

If the route map says "set as-path ...", instead of "set as-path prepend
....", I'd expect it to replace the AS path instead of prepending to it.

 

On 18.12.2004 02:28 Barry Margolin wrote

Well, you won't be successfull. The set command only does

set as-path

To modify an autonomous system path for BGP routes, use the set as-path
command in route-map configuration mode. To not modify the autonomous
system path, use the no form of this command.

set as-path {tag | prepend as-path-string}

no set as-path {tag | prepend as-path-string}
Syntax Description

tag
----
Converts the tag of a route into an autonomous system path. Applies only
when redistributing routes into BGP.

prepend as-path-string
-----------------------
Appends the string following the keyword prepend to the autonomous
system path of the route that is matched by the route map. Applies to
inbound and outbound BGP route maps.




Arnold

 

Hm.. I expected that too, but it seems it was wrong since only things
you can do with "set as-path" is path prepending and using tag values so
both of that options are out.

The main problem was that I had a device at edge of the network (non-
cisco device) that was speaking BGP but didn't have a concept of
confederation which were heavily used in other parts of the network.

To summarize: I had to accept NLRI's with my own (actually outside - not
confederation identifier) AS in the path. I finally solved it using
"neighbor x.x.x.x allowas-in 1".

I still don't get it why there's no command to modify AS path....

 

On 18.12.2004 12:28 Ivan Ostreš wrote

You don't need it and it almost only would cause harm There are
already enough knobs to twiddle on.

The AS path is constructed by many. Hence it does make sense that you
only can change your part.




Arnold

 

It is a requirement of confederations that every bgp speaker in the
network understands them. Your non-confedaration router has to be
configured outside the AS, e.g. a private AS stub.

Remember AS-path is BGP's main loop-detection mechanism...imagine the
chaos if inexperienced engineers started messing with global routes -
more trouble than it's worth.

Ben

 

It IS confgured like that, the only problem was with customer routes
received by this "stupid" router when those routes were propagated to
routers in the confederation. But, I've fixed that so it isn't really a
problem anymore.

I know very well it is loop-detection mechanism and looks like a good
one. I don't think there would be bigger chaos (that it is now) if
changing AS-path would be possible. It would need some additional
filtering by ISPs (which should be in place anyway).

 

On 20.12.2004 09:27 Ivan Ostreš wrote

ymmv, but with AS-path mangling filtering would be absolutely useless as
the meaning of the AS path is void. BGP life would be a nightmare.



Arnold

 

Yes, it's virtually the same reason many people dislike NAT...kind of
defeats one of the original purposes of ip.

 

On 20.12.2004 23:52 Ben wrote

WSell, I wouldn't even compare both situation to each other. While
NATting is a more or less elegant solution for saving IPv4 address
space, mangling with AS paths is like playing with dynamite ... as
always ymmv,



Arnold