You won't achieve anything by advertising MED to different providers.
AS path prepending would work, if the backup provider does not override
that with Local Pref. If your back provider prefers the routes (for
your networks) learned through your primary provider (**not the direct
advertisement through you**) then it will advertise this preferred route
to all its neighbors. This way nobody on the Internet (except your
backup provider) will get this less preferred advertisement --> hence
no return traffic on that link.
This is also good for your backup provider because it won't be carrying
any of your traffic as far as your primary path is working. There
might be some other factors that I'm ignoring due to which the backup
provider might not agree to make this exception for your netowrk
advertisements.
Regards,
Ahmer Ghazi
-----Original Message-----
From: Stephane Perez [mailto:perezs@nortelnetworks.com]
Sent: Wednesday, January 23, 2002 1:56 PM
To: Charles Sprickman; cisco-nsp@puck.nether.net
Subject: RE: [nsp] basement multihoming
Hi,
Some solutions for the backup purpose could be:
Use a higher MED value for the routes advertised to the backup BGP
speaker
Use AS-Prepending for the routes of the other ISP advertised to the
"normal" preferred ISP. And the same for the routes advertised to the
backup link. By default all the BGP speakers consider the AS-Path in
order to calculate the local-preference and then the metric.
I hope this will help
regards
------------------------------------------------------------------------
------------------------------------------------------------------------
------------------------
PEREZ Stephane
Carrier IP Specialist, Shasta GTS
Nortel Networks
ATS IP Core Infrastructure
25, allee Pierre Ziller
06560 Valbonne, FRANCE
Phone: +33 4 92 96 18 05 Mail :
mailto:perezs@nortelnetworks.com
Fax : +33 4 92 96 16 68 www : http://www.nortelnetworks.com
Esn : 296-1805 Intranet :
http://hector.europe.nortel.com
ICQ : 120356046
-----Original Message-----
From: Charles Sprickman [ mailto:spork@inch.com]
Sent: Tuesday, January 22, 2002 9:37 PM
To: cisco-nsp@puck.nether.net
Subject: [nsp] basement multihoming
Hi,
We currently have our own AS and a single upstream. Since we'd been
planning for quite some time to get a second upstream, we run BGP with
our
existing upstream. We get a full table from them and announce our
netblocks to them.
We have made an agreement with another provider in the co-lo space to
back
each other up... I'm wondering what my options are here; it's strictly
for backup purposes. I've considered announcing routes to both
upstreams,
padding heavily to the backup, and doing a floating static default. Is
there really any point in taking two full views for this simple
configuration?
We connect to both upstreams via FE, and both hit a switch before the
router. In a situation where an upstream's router is down, but their
switch is up, would there be any way to detect that in my simplistic
floating static scenario?
Thanks,
Charles
| Charles Sprickman | Internet Channel
| INCH System Administration Team | (212)243-5200
| spork@inch.com | access@inch.com
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