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autonegotiation changed on C3750V2 ?

Hello all,

Has Cisco changed something on the way autonegotiation works in the new IOS ?? I feel like a dummie: i hooked up

two switches with a cross cable, configured one port in static 100 full duplex, the other one in autonegotiation and the

autoneg one negotiates to full duplex ? doh ? shouldn't it fallback to half duplex ? whats even more: if i change the static one to

half duplex, the autoneg one follows to half duplex ? seems like they are still both running autoneg

Switch A: 15.0(1)SE1, WS-C3750V2-48PS

Switch B: 12.2(53)SE, WS-C3750V2-48PS

Between the switches on port fa1/0/48 a CROSSED cable.

Port config on A:

interface FastEthernet1/0/48

no switchport

ip address

no mdix auto

no cdp enable


Por config on B:

interface FastEthernet1/0/48

no switchport

ip add

carrier-delay msec 0

speed 100

duplex full

no mdix auto

no cdp enable



SWITCH A/ Fa1/0/48 routed     a-full  a-100 10/100BaseTX

SWITCH B: Fa1/0/48 routed       full    100 10/100BaseTX


Aren't there any certainties any more.....


autonegotiation changed on C3750V2 ?

Ha...added "power inline never" to both ports and guess what:

Fa1/0/48  routed a-half a-100 10/100BaseTX

Since when does power influence autonegotiation ??

Super Bronze

Re: autonegotiation changed on C3750V2 ?


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Pre-standard Cisco PoE also uses FLP for power detection.  This FLP power detection method might be "confused" as speed/duplex autonegotiation.

Cisco Employee

autonegotiation changed on C3750V2 ?

I was able to test this with 2 3750-E switches.  In this scenario I  am using port 1/0/48 on both switches.  The switches are called switch A  and switch B

When I set the duplex to full and speed to 100 on  switch B switch A's interface shows 100 Full

Here is the config and show  command for switch B

Switch-C#sh run in gi1/0/48
Building configuration...

Current  configuration : 63 bytes
interface GigabitEthernet1/0/48
speed  100
duplex full

Switch-B#sh int gi1/0/48

  Keepalive  set (10 sec)
  Full-duplex,  100Mb/s, media type is 10/100/1000BaseTX
  input  flow-control is off, output flow-control is unsupported
  ARP type:  ARPA, ARP Timeout 04:00:00
  Last input 00:00:08, output 00:00:00,  output hang never

Switch A  config and show command

Switch-A#sh run int gi1/0/48
Building  configuration...

Current configuration : 63 bytes
interface  GigabitEthernet1/0/48
spanning-tree portfast


Switch-A#sh  interfaces gi1/0/48

   Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
Full-duplex, 100Mb/s, media  type is 10/100/1000BaseTX
  input flow-control is  off, output flow-control is unsupported
  ARP type: ARPA, ARP  Timeout 04:00:00

When auto-negotiation is disabled on one switchport the switch at the  other end of the link can use the Parallel detection function to  determine the speed. Put simply the switch does not negotiate the speed  with the other switch but it can still work it out.

However what  it cannot work out without auto-negotiation is the duplex. Now as far  as i know 10/100 ethernet interfaces default to half-duplex and gigabit  interfaces default to full duplex. So you could interprete this to mean  the port would set itself to 100 Full because of it's default setting is  gigabit.

Follow this discussion for further explanation:

Hope that helps.


Please rate useful posts.

Cisco Employee

autonegotiation changed on C3750V2 ?

No sure about 3750v2 but Catalyst 3560v2 behave differently: if you configure both speed and  duplex statically on a port, the autonegotiation  will not be  deactivated but will advertise only a single capability including the  preconfigured speed and duplex .This is actually more intuitive  and natural behavior but obviously, this is not supported on older  switches. Probably it depends on the PHY controllers used in Catalyst  switches and they seem to be different between various Catalyst  platforms.

This all makes the  autonegotiation and all exceptions around it a most cumbersome topic -  so my recommendation usually is that either both ends of a link are set  to autonegotiation with no static settings, or both speed and duplex are  set statically on both link ends. The behavior of autonegotiation on  Catalyst switches is not well documented and is obviously platform and  version dependent, making it quite hard to remember.


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