From: Michelle T (mtruman@xxxxxxxxxxxxxxx)
Date: Fri Apr 27 2001 - 13:52:09 GMT-3
The real answer is /24 unless you are talking about a couple of very
specific ISP's who have much more restrictive peer filtering policies, none
of which are AT&T, Sprint, nor UUNET.
They all have the more relaxed /24 policy.
-----Original Message-----
From: Murphy, Brennan [mailto:Brennan_Murphy@NAI.com]
Sent: Friday, April 27, 2001 11:46 AM
To: 'Michelle T'; 'ccielab@groupstudy.com'
Subject: RE: real world BGP question
I guess that is my real question: what is the longest prefix that
is exchanged among/between major carriers.
The real world example here is what if you had 4 server farms answering
to one DNS name: ftp.foo.com You have Round Robin DNS running
round trip times to match a user with their nearest server farm....
so it sends back the closest/fastest IP. The question is, how big do those
subnets for the server farms have to be in order to be maximally
advertised throughout the internet?
So, I've seen two answers in this thread /20-21 or /24. I wonder
where I could find the real answer? Maybe Halabi has a link in the
back of his book to an organization that maintains info such as
this.
Any more input is greatly appreciated. Thanks to all who have responded.
I figured this question was a relavant BGP question relating our studies
to an actual scenario.
-----Original Message-----
From: Michelle T [mailto:mtruman@mn.mediaone.net]
Sent: Friday, April 27, 2001 12:06 PM
To: Murphy, Brennan; ccielab@groupstudy.com
Subject: RE: real world BGP question
/24 is the longest prefix you will see accepted by nearly any carrier out
there. Many will only accept /20 or /21. All perform aggregation to some
degree, though exception routing is allowed to send the /24's (/23, /22,
etc) out to the ISP peers when the customer is multi-homed two diverse
carriers.
I can tell you that I work for a Tier 1 ISP and we accept longer prefixes
for many customers who are multi-homed just to us. They use the various
subnets as a simple method of controlling inbound traffic distribution, to
enact policy, etc...
Many times we see multi-homed (dual-ISP) customers advertise an aggregate
/16 or longer and also advertise /24's for the same reaason (policy,
distribution, etc).
Michelle Truman
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
Murphy, Brennan
Sent: Friday, April 27, 2001 10:28 AM
To: 'ccielab@groupstudy.com'
Subject: real world BGP question
What is the smallest subnet that major carriers will exchange with one
another? /24..../26.../27?? I know that the real issue is the size of
the route table.
I'm just wondering about the reallity of scenarios that Habali describes
where an institution advertises an aggregate with specific subnets.
I know that when you're multi-homed to a carrier, that carrier will
sometimes
take your /26 and /27 nets to help route inbound traffic but that carrier
will not advertise those nets to its neighbors.....at least thats what I've
heard.
Anyone have any real world experience with this? Or is there a URL
I could read up on?
Thanks,
BM
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