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List:       isp-bgp
Subject:    Re:[isp-bgp] isp-bgp digest: December 07, 2004
From:       "Ejay Hire" <ejayhire () hotmail ! com>
Date:       2004-12-10 18:27:55
Message-ID: BAY2-F205DD411023E63CA4ED68FCEA80 () phx ! gbl
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>We must be careful to distinguishbetween load-sharing and load-balancing
-snip-
This is absolutely correct.  When discussing multihoming to multiple isp's, 
you always do load sharing.  I don't usually make the distinction, since it 
confuses people, but this is technically correct.

>When multi-homed to two AS'es, 1. Arrange to receive full routing updates 
>from both.
>2. Then weed out ip prefixes that belong to the specific AS'es only. You'll 
>need a pencil and paper
and the internet for this.
--snip--
>3. Take the balance list of prefixes that is left, and divide it..
--snip--

While it mighthave been possible at one point in the past to do this 
manually, it's unrealistic in today's world of 150k routes.  I load share 
ds-3's between 3 different providers.  Using this method would take weeks

Instead, you setup the session, watch mrtg for an hour or so, and adjust the 
traffic as necessary.
my current configuration is to prepend routes recieved from the 56 marietta 
peering point, accept MCI routes unobfuscated, and prepend the swamp from 
at&t.  that puts our 3 ds-3's at 25, 20, and 15mbps respectively, which is 
where we want it.

>Referring to Ejay's post, AS prepending is only possible on the advertising 
>router,
--snip--
This is incorrect.  As-prepending can be done inbound or outbound on ebgp 
connections on cisco routers.  I do it all the time.  I prefer localpref to 
weight, as it is as-wide.  weight is only within 1 router, and 
misconfiguration can cause routing loops.

>You will have to update the access-lists manually and regularly ....
We adjust our traffic flows approximately once a month, usually in response 
to business changes as opposed to route flows.

-Ejay

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