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Re: How many routed hops would be considered too many to leave an AS

  • From: Alan Hannan
  • Date: Tue May 15 06:10:47 2001 
  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 ([email protected])
 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