AutoNegotiate on 10/100/1000 ports

[sglynn]

GigabitEthernet or more specifically 1000BaseT specification requires autonegotiate.

So, if an interface is configured:

interface GigabitEthernet7/43

switchport

switchport access vlan 254

switchport mode access

speed 1000

duplex full

Auto-Negotiation is ENABLED, but the advertised capability is limited 1000

Now, what if the interface is configured

interface GigabitEthernet7/43

switchport

switchport access vlan 254

switchport mode access

speed 100

duplex full

Does this disable auto-negotiate in a 10/100 backward compatibility way, Or, because its a GigE port does it still autonegotiate but with 100 as the advertised capability.

To put the question another way, what defines the auto-negotiate behaviour, the interface type, or the configuration of the interface.

[Peter Paluch]

Hello,

I am not sure that there is a simple answer to your question. The presence of autonegotiation with both speed and duplex set statically depends on the Ethernet controller that drives the interface, and may be different across various Catalyst models. I have personally seen that:

• 2950, 2960, 3550, 3560 Catalysts deactivate the autonegotiation and auto-MDIX (if supported) if both speed and duplex are set statically
• 3560V2 Catalysts keep the autonegotiation activated but advertise only a single alternative

So the default behavior varies with the switch platform.

I have done these tests on FastEthernet ports but I can briefly test it tomorrow in our lab on GigE as well.

Best regards,

Peter

[hulbertj17613]

Concur with Peter.  The Ethernet PHY assembly on different switches can behave differently.  The one thing to keep in mind, is once you hard set the duplex, you pretty much won't be advertising anymore. 

[sglynn]

Thanks Peter,

Makes sense on FastE since autonegotiate in the spec was optional, which caused us all to shy away from autoneg.

In GigE, they made it mandatory to do auto neg, but allow you to change the capabilities advertised, so I suspect a lab test on GigE will go the second route you mention, i.e. autoneg activated but advertise only a single alternative.

unfortunately, I do not have a couple of switches with GigE interfaces to be able to test with so would be very interested in the results if you do manage to test it.

Thanks!

sglynn

[Peter Paluch · Accepted solution]

Hello sglynn,

What I was able to lab up:

1. On 2960, 3560 and 3560V2, the GigE port can be configured both with speed N and speed auto N. However, the first option deactivates the autonegotiation while the second option leaves it active but advertises only the capability that was specified explicitly.
2. On 2960, the same behavior was observed on FastEthernet ports.
3. On 3560 and 3560V2, on FastEthernet ports, using speed N is identical to using speed auto N - the autonegotiation remains active all the time, regardlessly of the auto keyword.

The 3560V2 and 2960 were running IOS 12.2(58)SE1, the 3560 was running 12.2(55)SE3.

This all leaves but one result: it is SO CONFUSING to remember all this that I would never rely on a speed/duplex configuration performed on a single link end only

Best regards,

Peter

[hulbertj17613]

For the first example, you would need to change "speed 1000" to "speed auto 1000", but you would also want to leave the duplex setting to "auto" and not full.  You could even have "speed auto 1000 10" and that would advertise during negotiation that you wanted to link at either 1000, or 10, but not 100.

The second example, your switchport would not be sending or listening for FLP's (Fast Link Pulse) and would set it's NIC to operate in a Full Duplex mode.  This effectively disables auto-negotiation on your port. 

[Martin Koch]

What do you think of this table:

http://www.cisco.com/en/US/products/hw/switches/ps708/products_tech_note09186a00800a7af0.shtml#auto_neg_valid

If I interpret it right it shall work proper as if it is a manual configured 100mbit interface.

But like Peter said, different types different behavoiors and a short lab test is always a good idea :-)

[sglynn]

Thank you so much!

While confusing it helps to understand that the behaviour is unpredictable

Very much appreciate this clarification.

Sg