[c-nsp] Routing-induced latency

[christian.macnevin at uk.bnpparibas.com]

Thanks a lot for the helpful suggestions, it was an isolated question 
intended to settle an argument, not
intended as a general architectural foray - it's in hand ;)






Internet
cchurch at netcogov.com

02/02/2006 15:41

To
Christian MACNEVIN, cisco-nsp
cc

Subject
RE: [c-nsp] Routing-induced latency






What is the current latency?  What latency are those people asking you
looking for?  By 'campus', I assume all that separate any two end hosts
are a few L2 and L3 devices, and all high speed links
(Ethernet/FDDI,etc) covering at most 5 miles of distance?  If so, you
shouldn't see light-load latency of more than 10ms.  If it is much more
than that, it's possible something's deeply queuing and/or process
switching in the middle of the two hosts.  VoIP and Video can cope with
100ms.  Are any of your links approaching capacity?  If someone is
complaining about latency, it sounds like an issue that can be dealt
with via QOS.  Or possibly packets are being lost due to queue drops or
frame errors.  They may be mistaking lost packets for latency.


Chuck 

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of
christian.macnevin at uk.bnpparibas.com
Sent: Thursday, February 02, 2006 7:32 AM
To: cisco-nsp at puck.nether.net
Subject: [c-nsp] Routing-induced latency

Hi all,

In a campus environment I'm working on, I'm consistently being asked to 
'reduce latency' between devices. So here's my question.
In (usually Catalyst) hardware, should you expect to see any latency 
induced by routing? By which I mean, if all routes are present
in the table, and you're running MLS CEF, I would expect that the 
switching is being done in hardware, and we should be introducing
zero latency. The older kit in here is not running CEF, so will be fast 
switching, I presume, which may sitll incur processor overhead.

Is that assumption correct? That in the absence of a converging network,

there should in theory be no latency induced by switching
between two VLANs on the same switch as opposed to switching between two

hosts on the same VLAN?

Cheers
Christian


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