Re: About ethernet autonegotiation.

speculor_cisco · ‎12-31-2010

Suppose I enter these commands:

Switch(config)#int fa1/1

Switch(config-if)#speed 100

Switch(config-if)#duplex auto

It is not clear if with this configuration the interface can do autonegotiation.

And if the interface can do autonegotiation, it is not clear if the autonegotiation is only for duplex setting

or also for speed setting.

Thanks.

Peter Paluch · ‎12-31-2010

Hi,

Yes, the interface will be able to perform autonegotiation but with the speed fixed to a preconfigured value, the only negotiable parameter is the duplex. The capability as advertised by this interface in FLP pulses will indicate only 100Mbps as a valid speed variant.

Best regards,

Peter

speculor_cisco · ‎12-31-2010

Hello Peter and thanks for your answer.

Do you know if, when autonegotiation fails but the interface has sensed a speed of 100,

the default value for duplex is half or full?

I have found different informations about this topic.

Peter Paluch · ‎12-31-2010

Hello,

To my experience, the default duplex setting used when the autonegotiation failed is half duplex. I don't know if that is a standard-required default setting or if that is vendor dependent. But the half-duplex is actually quite sensible default, considering that there still may be hubs in use, and hubs do not perform autonegotiation and are unable to provide full duplex communication. The fallback to half duplex when connected to a hub is therefore a logical thing to do.

Best regards,

Peter

Federico Ziliotto · ‎12-31-2010

Hi,

By manually defining a speed/duplex mode other than "auto" we disable auto-negotiation:
http://www.cisco.com/en/US/partner/tech/tk389/tk214/technologies_tech_note09186a0080094781.shtml

According to what you configured, the duplex mode should be auto-negotiated and the speed should be set to 100 Mbps.

Regards,

Fede

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Peter Paluch · ‎12-31-2010

Hi Federico,

I think it is very important to distinguish between disabled autonegotiation and an autonegotiation advertising only one capability set. I have seen both on Cisco Catalyst switches.

A disabled autonegotiation is common on older Catalyst switches (I know about 2950, 2960, 3550, 3560v1). If you set both speed and duplex statically on a port then the autonegotiation will be completely turned off on that port, i.e. no capability sets will be advertised in link pulses. As a result, the opposite device will determine the speed by sensing the line coding and levels (PAM-5, MLT-3, Manchester) but as no duplex capability is advertised, the opposite device will fall back to half duplex operation. The the static duplex setting was the full duplex, both devices would end up in duplex mismatch.

I was very surprised to learn (during a course where I made myself somewhat of an idiot because of this change ) that 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, by the way, makes the document you have referenced contain outdated and incorrect information.

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.

Best regards,

Peter

Federico Ziliotto · ‎12-31-2010

Thanks for expanding this Peter.

Yup, I agree that usually it is just easier to set both sides at the same time to either "static" or auto-negotiation.

Fede

speculor_cisco · ‎12-31-2010

In the document "Configuring and Troubleshooting Ethernet 10/100/1000Mb Half/Full Duplex Auto-Negotiation" you can find:

"

It is not possible to detect the correct duplex mode in the same method that the correct speed can be detected.

In this case, where the 1/1 port of switch B is configured for auto-negotiation and the 1/1 port of switch A is not, the 1/1 port of switch B is forced to select the default duplex mode. On Catalyst Ethernet ports, the default mode is auto-negotiate. If auto-negotiation fails, the default mode is half-duplex."

It seems that the port select the default duplex mode, the mode that appears in running-config at the begenning of the switch's life.

And as this mode is auto and auto fails, we have half-duplex.

But if the default mode was been full, not auto, is it possible that the port would have selected full?

Sorry but it is not possible that David Hucaby writes:

Note:

Multispeed links default to autonegotiate the duplex mode.

Ok, even if I do not understand why the 100 FE (dual speed 10/100) is not considered multispeed, the author says that 100 FE default to full duplex.

Is it possible that some models have 100 FE full duplex mode in running-config at the begenning of their life?

May be in this case, if I change to auto but auto fails, then we have full duplex mode.

A 10-Mbps Ethernet link (fixed speed) defaults to half duplex, whereas a 100-Mbps Fast Ethernet (dual speed 10/100) link defaults to full duplex.

speculor_cisco · ‎12-31-2010

I repeat the post for an error.

In the document "Configuring and Troubleshooting Ethernet 10/100/1000Mb Half/Full Duplex Auto-Negotiation" you can find:

"It is not possible to detect the correct duplex mode in the same method that the correct speed can be detected.
In this case, where the 1/1 port of switch B is configured for auto-negotiation and the 1/1 port of switch A is not, the 1/1 port of switch B is forced
to select the default duplex mode. On Catalyst Ethernet ports, the default mode is auto-negotiate. If auto-negotiation fails,
the default mode is half-duplex."

It seems that the port select the default duplex mode, the mode that appears in running-config at the begenning of the switch's life.
And as this mode is auto and auto fails, we have half-duplex.
But if the default mode was been full, not auto, is it possible that the port would have selected full?

Sorry but it is not possible that David Hucaby writes:

Note: A 10-Mbps Ethernet link (fixed speed) defaults to half duplex, whereas a 100-Mbps Fast Ethernet (dual speed 10/100) link defaults to full duplex.
Multispeed links default to autonegotiate the duplex mode.

Ok, even if I do not understand why the 100 FE (dual speed 10/100) is not considered multispeed, the author says that 100 FE default to full duplex.
Is it possible that some models have 100 FE full duplex mode in running-config at the begenning of their life?
May be in this case, if I change to auto but auto fails, then we have full duplex mode.

Peter Paluch · ‎12-31-2010

Hi,

It 
seems that the port select the default duplex mode, the mode that 
appears in running-config at the begenning of the switch's life.

I do not see it the same way. The default Ethernet duplex mode is not the IOS configuration default. Do not confuse these two things. The IOS default value is duplex auto. However, the Ethernet's physical controller duplex default is obviously - at least on Catalyst switches - duplex half. This "physical default" is the fallback value for duplex setting should the autonegotiation be deactivated or fail.

Note:
 A 10-Mbps Ethernet link (fixed speed) defaults to half duplex, whereas a
 100-Mbps Fast Ethernet (dual speed 10/100) link defaults to full 
duplex.

This does not conform to my practical experiences though I do not want to assert myself here. My experiences always have been that if the autonegotiation fails, the port falls back to half duplex. But that may really depend on particular Ethernet PHY vendor.

Best regards,

Peter

speculor_cisco · ‎12-31-2010

Hello Peter.

I have appreciated very much your argument and it seems really reasonable.

At this point I had a last curiosity about autonegotiation.

You know that, for example, while it is possible this configuration:

speed 100

duplex auto

it is not possible this other configuration:

speed auto

duplex full

I wondered why and if it made sense to wonder why.

So, this morning, I found a very interestring academic document about autonegotiation, where

you can find the details of the protocol.

My impression is that, while it is possible a second entry point in the finit state machine when I set the speed,

there is not an entry point other that the beginning of the machine when I set the duplex mode.

This would mean that it made sense to wonder why, and the answer is that it depends on the finite state machine.

May be you are thinking that it was obvious, but I think that it is always better to understand why something

works the way it works and that I need to order my stream of thought yet.

I promise you that this is the last question...of the year, but what do you thing about?

Thanks for your invaluable aid and HAPPY NEW YEAR.

Peter Paluch · ‎12-31-2010

Hello,

You know that, for example, while it is possible this configuration:

speed 100

duplex auto

it is not possible this other configuration:

speed auto

duplex full

That may have been a limitation of a particular switch platform and model but I've just tried that on a 2950 and I've had no problems:

Switch(config)#do show run int fa0/24
Building configuration...
Current configuration : 57 bytes
!
interface FastEthernet0/24
 switchport mode trunk
end
Switch(config)#int fa0/24
Switch(config-if)#speed auto
Switch(config-if)#duplex full
Switch(config-if)#! It does not complain 
Switch(config-if)#speed 100
Switch(config-if)#duplex auto
Switch(config-if)#! It does not complain neither

I've discovered that on Gigabit Ethernet interfaces, the duplex can be manually configured only to auto or full but not to half-duplex. Nevertheless, if autonegotiated or autonegotiation fails, even a GigE interface can fall back to half duplex operation. Confusing ... but that's the way it is.

Best regards,

Peter

Ajay Patel · ‎12-31-2010

Hi Speculor/Peter,

Appricate you argument on Ethernet Speed and Duplex Configuration.

As per my Exprience Speed and Duplex configuration vary based on the Cisco Platform,

There is No proper document which talk about Speed and Duplex exect configuration;

As I have Experienced; Port Speed and Duplex defualton Cisco comes autonegotiation which mean Port can perform autonegotiation with connected End device; which mean whenever ports comes UP they try to autonegotiation with the remote Node, if remote port support Speed 10, half Duplex then port fall back and will set port Speed 10, half Duplex.

autonegotiation also defend on the End device if End device autonegotiation disable or doesn't support then Port will force to act defult role (10Mbs Max, half duplex)

for example: Serve connected with switch where autonegotiation Disable or doesn't support then both end will run on defualt characteristic.

defualt characteristic:

10Mbs: Speed 10 Max. Half Duplex

10/100: Speed 10/100, Duplex Full

10/100/1000: Speed 10/100/100, Duplex Full

Coming back to Speculor Question:

Switch(config)#int fa1/1

Switch(config-if)#speed 100

Switch(config-if)#duplex auto

It is not clear if with this configuration the interface can do autonegotiation.

And if the interface can do autonegotiation, it is not clear if the autonegotiation is only for duplex setting

or also for speed setting

Yes; above configuration eligible for autonegotiation, Speed you are forcing to Act 100Mbs and Duplex leaving for autonegotiate with remote end.

if you force Duplex Full then also not mean you are stopping autonegotiation; only you are forcing them to act as executed but still autonegotiation will happen between the node and remote node;they will try to autonegotiate the same Speed and Duplex but they won't be able to do that, only they will thorugh Error message on consol (Duplex mismatch) and Speed set to 100 on remote end forcefully.

For Example: 1). Switch A configured Speed-100, Duplex-Auto, and Switch B configured Speed 100, Duplex Full

Result: Switch A port Will be on Speed 100, Dulpex-A-Full state

2). Switch A configured Speed 100, Duplex Full and connected with Server which has only capability to perform Speed 10 half Duplex

Result: Duplex mismatch Error on Consol, and there might be duplex mismatch can result in performance issues, intermittent connectivity, and loss of communication, Switch Will Forward the packet by 100Mbs Speed but server won't be able to handle Buffer for 100Mbs that will cause Slowness in Network.

Note: Idea configuration force configuration(SET Static) when connecting with Non-Cisco vendor & Disable autonegotiation

For Disabling autonegotiation on Cisco Platform need to Push below command:

Switch(config)# interface gigabitethernet 2/1/1

Switch (config-if)# no negotiation auto

Please follow below link for defualt characteristic:

Table 1—Autonegotiation Valid Configuration

Configuration NIC (Speed/Duplex)	Configuration Switch (Speed/Duplex)	Resulting NIC Speed/Duplex	Resulting Catalyst Speed/Duplex	Comments
AUTO	AUTO	1000 Mbps, Full-duplex	1000 Mbps, Full-duplex	Assuming maximum capability of Catalyst switch, and NIC is 1000 Mbps, full-duplex.
1000 Mbps, Full-duplex	AUTO	1000 Mbps, Full-duplex	1000 Mbps, Full-duplex	Link is established, but the switch does not see any autonegotiation information from NIC. Since Catalyst switches support only full-duplex operation with 1000 Mbps, they default to full-duplex, and this happens only when operating at 1000 Mbps.
AUTO	1000 Mbps, Full-duplex	1000 Mbps, Full-duplex	1000 Mbps, Full-duplex	Assuming maximum capability of NIC is 1000 Mbps, full-duplex.
1000 Mbps, Full-duplex	1000 Mbps, Full-duplex	1000 Mbps, Full-duplex	1000 Mbps, Full-duplex	Correct Manual Configuration
100 Mbps, Full-duplex	1000 Mbps, Full-duplex	No Link	No Link	Neither side establishes link, due to speed mismatch
100 Mbps, Full-duplex	AUTO	100 Mbps, Full-duplex	100 Mbps, Half-duplex	Duplex Mismatch ¹
AUTO	100 Mbps, Full-duplex	100 Mbps, Half-duplex	100 Mbps, Full-duplex	Duplex Mismatch ¹
100 Mbps, Full-duplex	100 Mbps, Full-duplex	100 Mbps, Full-duplex	100 Mbps, Full-duplex	Correct Manual Configuration²
100 Mbps, Half-duplex	AUTO	100 Mbps, Half-duplex	100 Mbps, Half-duplex	Link is established, but switch does not see any autonegotiation information from NIC and defaults to half-duplex when operating at 10/100 Mbps.
10 Mbps, Half-duplex	AUTO	10 Mbps, Half-duplex	10 Mbps, Half-duplex	Link is established, but switch does not see Fast Link Pulse (FLP) and defaults to 10 Mbps half-duplex.
10 Mbps, Half-duplex	100 Mbps, Half-duplex	No Link	No Link	Neither side establishes link, due to speed mismatch.
AUTO	100 Mbps, Half-duplex	100 Mbps, Half-duplex	100 Mbps, Half-duplex	Link is established, but NIC does not see any autonegotiation information and defaults to 100 Mbps, half-duplex.
AUTO	10 Mbps, Half-duplex	10 Mbps, Half-duplex	10 Mbps, Half-duplex	Link is established, but NIC does not see FLP and defaults to 10 Mbps, half-duplex.

Please let me know your input for better understanding.

Thank you

Ajay

speculor_cisco · ‎12-31-2010

Hello Ajay. In your post you wrote:

defualt characteristic:

10Mbs: Speed 10 Max. Half Duplex

10/100: Speed 10/100, Duplex Full

10/100/1000: Speed 10/100/100, Duplex Full

but I have not understood which is the subject of this default characteristic.

Thanks.

Ajay Patel · ‎12-31-2010

Hi Speculor,

I just wanted to strees 10Mbs link Duplex which is defualt Half Duplex.

10Mbs: Speed 10 Max. Half Duplex

10/100: Speed 10/100, Duplex Full

10/100/1000: Speed 10/100/100, Duplex Full

Thank you

Ajay