[37523] in North American Network Operators' Group
Re: How many routed hops would be considered too many to leave an AS
daemon@ATHENA.MIT.EDU (Alan Hannan)
Tue May 15 06:09:49 2001
Date: Mon, 14 May 2001 23:09:42 -0700
From: Alan Hannan <alan@routingloop.com>
To: Thomas Gainer <TGainer@e-xpedient.com>
Cc: "'nanog@nanog.org'" <nanog@nanog.org>
Message-ID: <20010514230942.A39411@routingloop.com>
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In-Reply-To: <E6F85CA58D2A834E99B1683C05BC7987D60EE0@mail.corp.com>; from TGainer@e-xpedient.com on Mon, May 14, 2001 at 05:59:35PM -0400
Errors-To: owner-nanog-outgoing@merit.edu
Subject: Re: How many routed hops would be considered too many to leave an AS
Douglas Adams, had he done routing, would say 42. Sadly he's
no longer with us.
You won't get a concensus or a summary, but if I post
you're sure to get many opinions with people correcting
or disagreeing, or ranting about nonsequiter things
in a pedantic way, so I'll try to help, or at least incite
a flurry of vindicative responses.
I believe the answer is A/ It Depends, and B/ Usually 6 to 8
routed hops for each AS.
In general, a network operator must be considerate of the
lowest-common-denominator traffic on one's network.
With regards to hop-count, many would say that Windows 95 (98?)
is the LCD, as it emits IP packets (by default) with a TTL
of 32 (Thirty-Two). I believe Win2k (maybe 98) has a default
TTL of 128. I assume that WinME and WinXP are at 128 or more
as well.
It is fairly reasonable to consider that most (90+%) traffic
goes through at most 4 ASes:
(no AS) Client ->
AS 1. Upstream AS of "client"
AS 2. "Tier 1" Upstream to #1
AS 3. "Tier 1" Peer and Upstream to #4
AS 4. Upstream AS of "server"
-> Server (no AS)
Since there are 4 ASes, and 32 hops, each AS should get
something close to 8 hops. Add a few at each end for
aggregation, and you can get give each AS 6 hops, with
some leftover for end-network traffic.
MIN_HOPS_PER_AS = ( LCD_TTL - (END_TTL * 2) ) / MAX_REG_AS_HOPS)
or, MIN_HOPS_PER_AS = ( ( 32 - (2 * 2)) / 4 )
MIN_HOPS_PER_AS = ( ( 32 - 4) / 4 )
MIN_HOPS_PER_AS = ( 28 / 4 )
MIN_HOPS_PER_AS = 7
It is interesting to note that more IP elements, without more meshing,
will create more hops. For example, a SONET network without significant
meshing will have more IP hops than a fully meshed IP/ATM network. There
are many many things of more significant importance than how many IP/L3
hops a network has, however. Certain ATM and MPLS networks create
logical meshes which decrease (IP L3) hop counts in certain
circumstances. Pros/Cons all the way around.
-alan
Thus spake Thomas Gainer (TGainer@e-xpedient.com)
on or about Mon, May 14, 2001 at 05:59:35PM -0400:
>
> Kind of a newbie questions, but I would like to know what the consensus is.
>
> Thanks,
>
> Thomas Gainer
