auto negotiating port speed and duplexing

[state]

Good afternoon friends. Can anyone point me to documentation on why it is a good idea, or not a good idea to statically set port speed and duplexing on interfaces connecting a router to a switch, a switch to a switch, or a switch to a PC/server. I've heard stories supporting both sides of this issue (some say statically set them, others say let auto negotiate determine it). I need best practice documentation. Not just what you've seen or what you've heard. Anything will be sufficient: white papers, TAC case notes, product release notes, anything. Thank you

[cgregg]

In the past I have seen network problems related to port mismatches with port speed and duplex which will cause network errors, dropped packets and retransmissions. I am including a good document link.

LINKs:

http://www.cisco.com/en/US/partner/tech/tk389/tk214/technologies_tech_note09186a0080094781.shtml#when

http://www.cisco.com/en/US/partner/products/hw/switches/ps700/products_tech_note09186a008015bfd6.shtml

Hope this helps,

Curt

[editorstudio]

for more reliable link and secure link state its recommended to statically set your port transfer mode to FULL/HALF and 10/100/1000 Mbps link speed .

attention to these notes :

-- A Port set to Full Duplex connected to another that set to half duplex should be checked for LATE COLLISION ERRORS.

-- A Port set to Half duplex connected to Full duplex port should be checked for FRAME CHECK SEQUENCE errors .

in this case you can use these CLI commands to set your port to work at your needs :

Switch>ena

Switch#conf t

Switch(config)#int fa 0/X

Switch(config-if)#sp 10 / 100 / au

Switch(config-if)#du ha / fu / au

Switch(config-if)# Ctrl+Z

in a router you can same command with little difference by your Router Series and IOS version .

Good Luck

Yours Truly

N.Baradaran

Senior Admin of Central Library - IUST university

[glen.grant]

On a big network it is next to impossible to hardcode everything , specially if you have people moving around all the time . Usually auto/auto works sufficently but there are still times when you have nic incompatibilities . If you have high priority servers and connections to other switches and routers those are the ones I would hardcode . If you a smaller network then maybe you could hardcode everything . When you are working on a network with 30000 nodes it's a little tough to hardcode everything . :-)

[geethajit]

I agree with cgregg. Sometimes it doesn't negotiate properly when it's left at 'auto neg' as a result of this u end up getting quite a lot of errors & re-transmit.

I had a scenario where the link util was under 5% but the user was complaining about the slow performance and upon investigation found out that because of auto neg ON and mis-match duplex it was generating quite a bit of errors and the re-transmission rate was quite high as well.

As a norm on the server as well as the switch end, we normally leave the port speed as FIXED and auto neg FALSE to avoid the above scenario.