RE: BGP Communities values

Kelvin,

As Alex has already mentioned, providers will allow customers to send communities that do a number of things, giving the customer control of how the carrier treats the routes they receive from their customers. Common uses are for pre-pending of AS, local preference assignment, and temporary black holing. With respect to the IE, the thought process is to define intuitive route tags from upstream peers that are either within the same AS or different AS and to make route decisions based on those edge tags anywhere within your network.

e.g. AS 54 routes from BB1 are tagged with 54:1 and AS 54 routes from BB3 are tagged with 54:3. Those tags are used on interior routers to prefer one path over the other back to AS 54.

-ryan

-----Original Message-----
From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of Kelvin Yeo
Sent: Thursday, May 28, 2009 4:59 PM
To: 'Dufour, Andre'; 'Cisco certification'
Subject: RE: BGP Communities values

Andre, thanks for your response.

Let me clarify further.

If I need to tag a prefix with a community values, what should it be? Should
this value make sense when the adjacent AS received it?

Ie 200:300

200 will be the originating AS while 300 is the next hop AS? Or is there any
rationale I should consider when choosing the values?

Kelvin

-----Original Message-----
From: Dufour, Andre [mailto:Andre.Dufour_at_PAETEC.com]
Sent: 29 May 2009 04:45
To: Kelvin Yeo; 'Cisco certification'
Subject: RE: BGP Communities values

Kelvin,

To give a short summary, you can use communities with BGP in a similar
manner that you would use tags in IGPs.

The format that you are describing is the "new" community format. The new
community is really just to make things easier to manage. You can use the
AS#:number to make things easier than just a random number like 234353442,
etc.

Please let us know if there's a more specific question.

Regards,
Andre
 

-----Original Message-----
From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of
Kelvin Yeo
Sent: Thursday, May 28, 2009 4:35 PM
To: 'Cisco certification'
Subject: BGP Communities values

Anyone can explain the idea behind assigning the communities values such as
below

300:200 or 123:456

Why is 300/123 assigned as AA while 200/456 assigned as NN in the AA:NN
format? I mean, is there a rule to follow or design purposes? What exactly
is it telling us?

Kelvin

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Received on Thu May 28 2009 - 17:13:58 ART