RE: BGP Config

Totally slipped my mind about supplementing the as path along with the
prefix. Time to shake off that rustiness lol. I was able to workaround
the issue by configuring:

router bgp 40037

 neighbor 12.118.128.245 route-map PREFERATT in

 neighbor 69.38.143.241 advertise-map ADVERTISE non-exist-map NON-EXIST

 neighbor 69.38.167.137 advertise-map ADVERTISE non-exist-map NON-EXIST

ip as-path access-list 10 permit ^$

ip as-path access-list 100 permit ^7018_

ip prefix-list ATTPREFIX seq 5 permit 1.12.0.0/14

ip prefix-list LNET seq 5 permit 12.230.83.0/24

ip prefix-list NET4 seq 5 permit 4.0.0.0/8 le 24

route-map NON-EXIST permit 10

 match ip address prefix-list ATTPREFIX

 match as-path 100

route-map ADVERTISE permit 10

 match ip address prefix-list LNET

route-map PREFERATT permit 10

 match ip address prefix-list NET4

 set local-preference 300

route-map PREFERATT permit 1000

Now it shows up on an external BGP router:

*> 12.230.83.0/24 38.103.72.158 2000 0 174 7018
40037 i

What I still don't understand is why the path shown above is not chosen
as best by default since I have configured the prepending as previously
mentioned. When I had initially cleared the BGP sessions it showed up:

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

*> 38.103.72.158 2000 100 0 174
7018 40037 i <-- (AT&T)

And then disappeared a second later:

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

I still am trying to figure out this weird behavior. Why would the
shortest path/AT&T route withdraw from the table in the first place but
when I configure conditional advertisement it remains since it is the
only path?

Timothy Chin

CCIE #23866

________________________________

From: Mirco Orlandi [mailto:mirco.orlandi_at_gmail.com]
Sent: Wednesday, January 05, 2011 6:23 AM
To: Timothy Chin
Cc: Tyson Scott; Shaughn Smith; ccielab_at_groupstudy.com
Subject: Re: BGP Config

Watch the NON-EXIST map at the end of the document posted by Tyson.
You can match prefix xxx advertised by AS yyy (at&t)

route-map NON-EXIST permit 10
 match ip address prefix xxx
 match as-path 1

ip as-path access-list 1 permit ^yyy

-m.

On Wed, Jan 5, 2011 at 10:49 AM, Timothy Chin <tim_at_1csol.com> wrote:

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_example
0918
6a0080094309.shtml
<http://www.cisco.com/en/US/tech/tk365/technologies_configuration_exampl
e0918%0A6a0080094309.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 - 16:18:06 ART