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Bronze

gigabit ethernet port and auto-negotiation

hi every body!

i understand that on gigabitethernet link, default duplex setting is full.

how about if sw1---------------sw2

f 10/100 f 100/1000

where sw1 has following configuration :

sw1# speed 100

duplex full

will sw2 default to full duplex?

thanks a lot!

3 ACCEPTED SOLUTIONS

Accepted Solutions
Hall of Fame Super Silver

Re: gigabit ethernet port and auto-negotiation

Hello Sarah,

it depends from what devices are at the two ends of link.

In a real world scenario the better choices are:

both ends manually configured the same

both ends auto

other choices lead to errors

Hope to help

Giuseppe

Hall of Fame Super Bronze

Re: gigabit ethernet port and auto-negotiation

sw2 which has gigabit port will default to full

The answer is, it depends. A device set to auto/auto will expect its neighbor device be set to auto/auto. When a device expects autonegotiation information from its remote device and it does not receive it, the safe mode is to set itself to half-duplex.

This wiki page contains useful information:

http://en.wikipedia.org/wiki/Autonegotiation

HTH,

__

Edison.

Hall of Fame Super Silver

Re: gigabit ethernet port and auto-negotiation

Hello Sarah,

fiber based GBICs are full duplex in their nature: they have two fibers one is TX and one is RX so with them there is nothing to be negotiated (well limited to speed and duplex ... there is also flowcontrol ...)

speed is fixed to 1000 Mbps

duplex is full

You don't need and you actually cannot set speed or duplex on a GE port using a GBIC (at least on a lan switch an execption could be the GE ports of NPE-G1 and NPE-G2 for 7206VXR that have at the same type the rj45 and the gbic slot in that case you need to define the media-type also)

on a switch with built-in RJ-45 port that is a 10/100/1000. I think there is also a GBIC with RJ45 output in that case the port is not capable of negotiation and behaves like fiber based GBICs is 1000 Mbps full.

Hope to help

Giuseppe

11 REPLIES
Hall of Fame Super Bronze

Re: gigabit ethernet port and auto-negotiation

It may default to full duplex, but you may also experience speed/duplex mismatch when one side is configured for speed/duplex while another side is configured as auto/auto.

The proper configuration is to match both ends w/ auto/auto or the same speed/duplex.

HTH,

__

Edison.

Hall of Fame Super Silver

Re: gigabit ethernet port and auto-negotiation

Hello Sarah,

you can get a duplex mismatch with the other side that sets itself to 100 half (this should be the default for speed 100 Mbps) because the interface configured for 100 full could not take part in duplex negotiation and the other side can only assumes its default.

Speed can be detected using FLP (fast link pulse) but duplex detection needs a cooperative other side.

Hope to help

Giuseppe

Bronze

Re: gigabit ethernet port and auto-negotiation

thank for your reply.

sw1 has both speed 100, duplex configured, sw2 which has gigabit port will default to full so there should not be duplex mismatch as sw1 is manually configured with speed, duplex setting.

Am i correct?

New Member

Re: gigabit ethernet port and auto-negotiation

hi,

your opinion is correct.

but in some case there is problem with auto configuration.

i think for the critical one, it's not bad to force configure speed and duplex on both sides.

cheers.

Hall of Fame Super Silver

Re: gigabit ethernet port and auto-negotiation

Hello Sarah,

it depends from what devices are at the two ends of link.

In a real world scenario the better choices are:

both ends manually configured the same

both ends auto

other choices lead to errors

Hope to help

Giuseppe

Bronze

Re: gigabit ethernet port and auto-negotiation

Thanks Giuseppe!

Hall of Fame Super Bronze

Re: gigabit ethernet port and auto-negotiation

sw2 which has gigabit port will default to full

The answer is, it depends. A device set to auto/auto will expect its neighbor device be set to auto/auto. When a device expects autonegotiation information from its remote device and it does not receive it, the safe mode is to set itself to half-duplex.

This wiki page contains useful information:

http://en.wikipedia.org/wiki/Autonegotiation

HTH,

__

Edison.

Bronze

Re: gigabit ethernet port and auto-negotiation

Thanks Edison!

Bronze

Re: gigabit ethernet port and auto-negotiation

just want to add some thing.

From cisco press book book , I learn

some gigabit port such as 1000 base T, can support triple speed i.e 10/100/1000. Here auto negotiation determines the duplex mode on the link.

All other ports such as 1000base sx,1000 base zx, 1000 base lx/lh must be set to full duplex mode so auto-negotiation is not possible.

Is my understanding correct?

thanks a lot!

Hall of Fame Super Silver

Re: gigabit ethernet port and auto-negotiation

Hello Sarah,

fiber based GBICs are full duplex in their nature: they have two fibers one is TX and one is RX so with them there is nothing to be negotiated (well limited to speed and duplex ... there is also flowcontrol ...)

speed is fixed to 1000 Mbps

duplex is full

You don't need and you actually cannot set speed or duplex on a GE port using a GBIC (at least on a lan switch an execption could be the GE ports of NPE-G1 and NPE-G2 for 7206VXR that have at the same type the rj45 and the gbic slot in that case you need to define the media-type also)

on a switch with built-in RJ-45 port that is a 10/100/1000. I think there is also a GBIC with RJ45 output in that case the port is not capable of negotiation and behaves like fiber based GBICs is 1000 Mbps full.

Hope to help

Giuseppe

Bronze

Re: gigabit ethernet port and auto-negotiation

Thanks a lot Giuseppe! I intended to ask few more questions before i read your reply. your reply has answered them all . Thanks a lot!

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