RE: BGP Config

I am no longer being able to send it to the group?

Marcin Zgola
Internetwork Lead
CCIE #18676
Netrix, LLC
http://www.netrixllc.com
Ph. 847-964-5300
Fax.: 847-964-5350

-----Original Message-----
From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of Mirco Orlandi
Sent: Wednesday, January 05, 2011 5:35 PM
To: Timothy Chin
Cc: Tyson Scott; Shaughn Smith; ccielab_at_groupstudy.com
Subject: Re: BGP Config

Timothy,
for some reason, maybe AS174 is setting higher local-preference to prefixes
received from AS26878.

-m.

On Wed, Jan 5, 2011 at 10:18 PM, Timothy Chin <tim_at_1csol.com> wrote:

> 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 dont 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_example0918
>
6a0080094309.shtml<http://www.cisco.com/en/US/tech/tk365/technologies_configu
ration_example0918%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 Thu Jan 06 2011 - 01:45:12 ART