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New Member

The behaviour of auto-negotation

Could you tell me if a pc or a switch will negotiate the speed and duplex periodically when connection does not change if both ends configured to be auto? What could cause a pc negotiate the speed and duplex? The CPU utilization of my switch goes up to 76% when the state of any port change? At this moment, the netowrk experiences intermittent connectivity problem. Could you advise me how to solve the problem? Thanks. The switch version is

switch> sh ver

WS-C4006 Software, Version NmpSW: 5.5(1)

Copyright (c) 1995-2000 by Cisco Systems, Inc.

NMP S/W compiled on Jun 8 2000, 22:26:00

GSP S/W compiled on Jun 08 2000, 20:55:48

System Bootstrap Version: 5.4(1)

Hardware Version: 1.5 Model: WS-C4006 Serial #: JAB041704LH

Mod Port Model Serial # Versions

--- ---- ---------- -------------------- ---------------------------------

1 2 WS-X4013 JAB041704LH Hw : 1.5

Gsp: 5.5(1.0)

Nmp: 5.5(1)

2 48 WS-X4148-RJ JAE042115GL Hw : 2.3

3 48 WS-X4148-RJ JAE0420136S Hw : 2.3

4 48 WS-X4148-RJ JAE0420135W Hw : 2.3

5 48 WS-X4148-RJ JAE050803D6 Hw : 2.3

6 48 WS-X4148-RJ JAE053800HH Hw : 3.0


Module Total Used Free Total Used Free Total Used Free

2002 Jan 07 16:11:14 AEST +10:00 %SPANTREE-6-PORTFWD: Port 4/48 state in

vlan 1 changed to forwarding

1 65536K 31618K 33918K 16384K 10299K 6085K 480K 263K 217K

Uptime is 2 days, 14 hours, 40 minutes

New Member

Re: The behaviour of auto-negotation

A general rule of thumb is to have spanning tree running on ports that ONLY connect to other switching/trunking devices. By default (I think), a switch port will run FULL STP and try to negotiate a set of other protcols when the link on a port goes live (such as PAGp). For a connected PC/Printer, these protocols are not required, especially as STP will recompute every time the port status changes, which during a Windows PC boot-up, can happen several times !! STP is slow at the best of times, because of the five STP states it has to process. If you enable a feature called Spanning Tree PortFast on ports that connecting to PC/Printers (non-switch/trunk devices) you should see a better performance.


I think the command (on CatOS) is "set spanntree portfast". Do that for ports that connect to PC/Printers.


Hope this helps

New Member

Re: The behaviour of auto-negotation

The later versions of CatOS include a marco command called SET PORT HOST, which disabled several things, including spanning tree, etherchannel autoconfig and trunking autoconfig. It's a very handy command, to run on the ports with end stations attached (i.e. no links to other networks/segments).

Cisco Employee

Re: The behaviour of auto-negotation

Linkup or change in link status will cause auto-negotiate to restart. Keep in mind, Operating Systems will put NIC cards into sleep mode and brink down link on NICs.

If you experiencing interimittant connectivity, review the following 2 links and make sure the ports are configured as host ports.

In addition, I would strongly recommed upgrading your software to 5.5.11.

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