From: suri tk (surendratk@hotmail.com)
Date: Wed May 07 2008 - 11:42:24 ART
I dont have any practical experience with configuring BGP. But here is
what i could think of.
Lets say our AS is connected to another AS. You would not want the
connected AS (could be a ISP)
to become transit AS for a particular prefix and you also dont have
access to their network to make any configs.
One option is filtering unwanted traffic at your edge. But that still
causes traffic to traverse through the neighboring AS. If your ISP is
charging $$ based on how much traffic he forwards to you, this needs to
be avoided.
In this scenario you could use NO_EXPORT community. The neighboring AS
would not export the prefix to other connected AS'
I have made some assumptions here and i would be glad to be corrected by
folks who have some real-life experience with this.
Thanks,
surendra
Todd, Douglas M. wrote:
> All depends on what you are trying to accomplish:
>
> Transitive/non-transitive - well-known/not-well-known - Internet
Architectures
> (cisco press) goes through all this very well. The ultimate bgp book.
>
> Thoughts:
>
> Typically you will see senerios like, dont all this route to export from
your
> domain. Do not use filter-lists, prefix-lists, to accomplish this task. Or
don't
> be a transit for your ISP. Here you might put the no-export on your routes
> (inbound on ISP routes, outbound for other routes). There for your other
ibgp
> routers will note export those routes outside your domain.
>
> no-advertise - means do not advertise this to any other peer. If used
inbound,
> routes are not advertised to any peer. Outbound, peer/s do not advertise
those
> routes to anyone else.. no-export is somewhat the same, but the radius of
the
> route is much less. Ie., no-export can be multiple hops within YOUR AS.
> no-advertise is next hop only (depending on in vs out appliacation). (more
can
> be added here,but you get the idea).
>
> Local-as: kind of combines no-advertise an no-export to some degree. Routes
do
> not leave your as (works in confederations issues ). Routes float around
in
> your AS only. Do not leave you AS.
>
> Additive: keeps the original community, but "ORS" the additional.
Basically, if
> no additive the inbound community is over written with what you set.
>
> AA:NN(New format) - great for tagging routes for policies. Look at this as
a
> route tag for filtering etc.
>
> Dont forget to use the send communty on your neighbors.
>
> Internet - well... not much other use....
>
> None - null out any inbound community seen...
>
> Here is the basics for setting the community, replace with a match if you
need
> to match what you set.
> (config-route-map)#set community ?
> community number
> aa:nn community number in aa:nn format
> additive Add to the existing community
> internet Internet (well-known community)
> local-AS Do not send outside local AS (well-known community)
> no-advertise Do not advertise to any peer (well-known community)
> no-export Do not export to next AS (well-known community)
> none No community attribute
> please correct/add where necessary...
>
> Thanks
> DMT
>
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Ashwani
> Sent: Monday, May 05, 2008 4:07 PM
> To: Cisco certification
> Subject: Help understanding BGP community
>
> Hello Experts,
>
> Can any one please explain how to use BGP community? I know the basic
> understanding of community to influence inbound traffic , why cant we use
MED or
> AS_ PATH prepend to do that? What's advantages or disadvantages of one over
> another?
>
> Any help with example will be highly appreciate.
>
> Thanks,
> Ashwani
>
>
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