[c-nsp] BGP Multihomed Selective/Conditional Advertisement

Sat Oct 25 13:19:14 EDT 2008

So what you are saying is that if I use communities to have Cogent prepend the pefix few times, Cogent's routers will ignore the multiple ASN in the path, but when they export it to their peers that path should be longer than the path through ATT because of Cogent's extra ASN in there, correct?

But if there could be many Cogent peers closer to Cogent than ATT where with let's say 3 perepended ASNs they  still prefere Cogent istead of ATT. So based on the peering on the internet I would create a sort of load-balancing on my links? 

Thank you,

________________________________

From: Ryan Otis [mailto:Ryan.Otis at WebTrends.com]
Sent: Fri 10/24/2008 6:29 PM
To: Kacprzynski, Tomasz
Subject: RE: [c-nsp] BGP Multihomed Selective/Conditional Advertisement

If you want the Cogent to act as a failover link, use AS prepending
instead of the no-export community. 174:3003 should be enough to cause
nearly all other ISPs to use the AT&T link, leaving only Cogent using
the Cogent link unless the AT&T link fails.

Another option is sending them the community string to set local
preference, but that can get trickier because they don't publish the
default local pref of their peers versus their customers.  174:70 is too
low, and would cause all routes to take AT&T, including Cogent
themselves, 174:120 might be something to try.

Verify everything with Looking glasses.

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of
tkacprzynski at SpencerStuart.com
Sent: Friday, October 24, 2008 4:18 PM
To: avayner at cisco.com; cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] BGP Multihomed Selective/Conditional Advertisement

Arie,
Thank you for your response. In my situation, where everything is
normal, I am actually sending their specific communities for them not to
advertise my route to their peers. My only problem is how to change that
automatically when my default route from ATT goes away (ATT circuit does
down and I'm in a failover situation)?

Thank you,

-----Original Message-----
From: Arie Vayner (avayner) [mailto:avayner at cisco.com]
Sent: Fri 10/24/2008 6:03 PM
To: Kacprzynski, Tomasz; cisco-nsp at puck.nether.net
Subject: RE: [c-nsp] BGP Multihomed Selective/Conditional Advertisement

Tom,

Instead of not advertising a certain prefix, there is another
alternative using BGP communities which are recognized by your upstream
providers.

Take a look for what Cogent supports for example (better ask them for
the official list...):
http://www.onesc.net/communities/as174/

You could play with the local pref communities or the no-export ones

Its not the full answer, but just another idea... Let me know if you are
still stuck...

Arie

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of
tkacprzynski at spencerstuart.com
Sent: Friday, October 24, 2008 23:07 PM
To: cisco-nsp at puck.nether.net
Subject: [c-nsp] BGP Multihomed Selective/Conditional Advertisement

I have been trying to figure out how to do this and maybe someone will
be able to help me out.

I have two ISP connections ISP ATT and ISP Cogent.

(ISP Cogent)    (ISP ATT)
     |                       |
  RO ------------------- R1

ATT would be used for primarily internet and access to our webservers.

Cogent would be primarily used to access Cognet's network that use VPN
for incoming connections only. I do not want to have other networks
besides Cogent's network using this path to access our webserver.

I would like to have each other act as a backup for one another. For
instance if ATT fails I want everyone on the internet use Cogent to
access me. If Cogent fails I want everyone on the internet and the VPN
connections on Cogent's network to use ATT.

So basically what I was thinking to setup is to accept a default router
from ATT and Cogent. Lower the local preference of Cogent and that way I
would accomplish using ATT as primary internet access.

The tricky part is with Cogent and using then to only access their local
networks. Looking through communities I found out Cogent's communities
that would not export my route to their peers and keep it internal
within their AS. This works fine but the problem now is how do I
failover if ATT fails? How do I automatically change my not-export
community I'm sending to Cogent to start adverting the route to its
peers?

I looked at conditional advertisement, I was able to basically send the
route map with not-export communities to Cogent if the default route
from ATT is present. The problem with this is that once the default
router disappears it doesn't advertise anything to Cogent, none of my
routes are advertised to Cogent.

I'm not sure if I could do this sort of a double condition such as

if ATT's default route is present send out to Cogent a route map with
prefixes to not-export my routes if ATT's default route is not present
sent to Cogent a route map without any communities on my routes

Basically I'm trying to figure out how I can have multihoming, but with
the constrains that I want 1 ISP to be used for internet and the other
to only access their AS, but still have the capability to automatically
failover in case one of the circuits dies.

Thank you for any input or help.

Tom Kacprzynski
Network Engineer

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