RE: BGP Config

It is your choice at this point. You have lots of options. You check with
both ISP's to see what communities they will accept to control route
selection. Here are some examples for AT&T.

But again, BGP is kind. The road is wide open.

ommunity Received AT&T IP Backbone Function
None, 7018:100 Local Preference of 100 (Default) Assigned - Used for
Primary Routes
7018:90 Local Preference of 90 Assigned - Used for Customer Backup Routes
(INTRA - AT&T)
7018:80 Local Preference of 80 Assigned - Used for Routes Equal to Peer
Routes
7018:70 Local Preference of 70 Assigned - Used for Customer Provided Backup
(INTER-AT&T + OTHER ISP)
7018:20 (Default) Assign community 7018:2000 to routes. Community
7018:2000 routes are announced to peers and customers. This community needs
to be present on more specific routes from within AT&T-owned address blocks.
This community need not appear on routes for customer-owned addresses and
for addresses owned by a customer's other provider, as these routes will
normally be advertised to peers and customers. No harm is done if community
7018:20 appears on such routes.
7018:25 Assign community 7018:2500 to routes. Community 7018:2500 routes are
announced only to other customers, not to peers. This is appropriate when
customers do not want AT&T to provide global Internet transit service for
this route.
7018:21 Assign community of 7018:2010 to routes. Community 7018:2010 routes
are to be used within the AT&T IP Backbone, but not advertised to peers or
customers. Typically the customer will simultaneously announce a shorter
prefix covering this route, with the shorter prefix being announced to peers
and/or customers. Prefix lengths on such routes will frequently be longer
than /24.

http://www.onesc.net/communities/as7018/

Regards,
 
Tyson Scott - CCIE #13513 R&S, Security, and SP
Managing Partner / Sr. Instructor - IPexpert, Inc.
Mailto: tscott_at_ipexpert.com

-----Original Message-----
From: Timothy Chin [mailto:tim_at_1csol.com]
Sent: Wednesday, January 05, 2011 3:20 PM
To: Tyson Scott; Shaughn Smith
Cc: ccielab_at_groupstudy.com
Subject: RE: BGP Config

I was thinking about this before but the only problem is that if I were to
use a non-exist map it wouldn't work because the prefix in a non-exist map
will exist in the BGP table even if AT&T is down because I the backup ISP is
also advertising all prefixes.

Maybe I can configure a route-map to filter a specific prefix from the
backup ISP so that I will only receive that prefix from AT&T and then use a
non-exist map for that particular prefix in order to conditionally advertise
the network to the backup ISP if the BGP peering session to AT&T goes down.
Any thoughts?

Timothy Chin

CCIE #23866

-----Original Message-----
From: Tyson Scott [mailto:tscott_at_ipexpert.com]
Sent: Wednesday, January 05, 2011 4:20 AM
To: Timothy Chin; 'Shaughn Smith'
Cc: ccielab_at_groupstudy.com
Subject: RE: BGP Config

Most specific route will always win. Selectively advertise the more
specific only when BGP to AT&T is down.

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example0918
6a0080094309.shtml

Regards,
 
Tyson Scott - CCIE #13513 R&S, Security, and SP
Managing Partner / Sr. Instructor - IPexpert, Inc.
Mailto: tscott_at_ipexpert.com

-----Original Message-----
From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of
Timothy Chin
Sent: Wednesday, January 05, 2011 2:01 PM
To: Shaughn Smith
Cc: ccielab_at_groupstudy.com
Subject: RE: BGP Config

It's weird because it does show up when I clear the BGP session and then
goes away. Here is how it looks from an external BGP router:

* 12.230.83.0/24 38.103.72.158 1001 0 174 26878
40037 40037 40037 40037 40037 40037 I <-- (Other ISP)

*>i 38.125.93.3 0 100 0
19080 3549 7018 40037 i <-- (AT&T)

Then right after:

*> 12.230.83.0/24 38.103.72.158 1001 0 174 26878
40037 40037 40037 40037 40037 40037 i

BGP routing table entry for 12.230.83.0/24, version 45052045

Paths: (1 available, best #1, table default)

Multipath: eBGP

  Advertised to update-groups:

     3

  174 26878 40037 40037 40037 40037 40037 40037, (received & used)

    38.103.72.158 from 38.103.72.158 (66.28.1.176)

      Origin IGP, metric 1001, localpref 100, valid, external, best

      Community: 11424265 11425277

The pre-pending is valid and AT&T would be the shortest AS path but the
AT&T route disappears for some reason. So the only way through AT&T
would then be the aggregate prefix that AT&T advertises. I don't know
why the prefix disappears. AT&T should still be advertising the longest
match no matter what but it has me perplexed. Any ideas?

Timothy Chin

CCIE #23866

________________________________

From: Shaughn Smith [mailto:maniac.smg_at_gmail.com]
Sent: Wednesday, January 05, 2011 3:15 AM
To: Timothy Chin
Cc: ccielab_at_groupstudy.com
Subject: Re: BGP Config

I am not sure why/how AT&T are doing things on their side but by them
advertising the aggregate shouldnt affect the prepending.

Have you used some looking glass servers to check if your prepending is
still valid ?

CCIE # 23962

On Wed, Jan 5, 2011 at 10:12 AM, Timothy Chin <tim_at_1csol.com> wrote:

I have done the pre-pending already as a precaution but it doesn't have
any effect on AT&Ts side because they are aggregating the prefix. I was
thinking possibly something similar to BGP conditional advertisement?

Timothy Chin

CCIE #23866

________________________________

From: Shaughn Smith [mailto:maniac.smg_at_gmail.com]
Sent: Wednesday, January 05, 2011 2:54 AM
To: Timothy Chin
Cc: ccielab_at_groupstudy.com
Subject: Re: BGP Config

You could try AS Path pre-pending on the low bandwidth link, or you
could ask AT&T to not aggregate and advertise the correct prefix and ask
the other ISP to advertise the aggregate.

CCIE # 23962 (SP)

On Wed, Jan 5, 2011 at 9:48 AM, Timothy Chin <tim_at_1csol.com> wrote:

I've been working on a BGP configuration between one site multihomed to
2 ISPs. One ISP is basically a backup with a low bandwidth connection
and the other ISP is AT&T. AT&T assigned the Class C and I am
advertising the network. The problem is that AT&T is aggregating the
block and since I am advertising the specific class C all incoming
traffic is coming through the low bandwidth connection because of the
longest match. I hate to say my BGP is a little rusty but how would I be
able to conditionally advertise the longest match to the low bandwidth
ISP so that all traffic comes through AT&T via the aggregate?

Timothy Chin

CCIE #23866

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Received on Wed Jan 05 2011 - 15:55:51 ART