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New Member

Strange speed/duplex mismatch issue

Hi all,

I have a strange issue with speed and duplex which has me stumped. Noddy diagram below.

noddy_diag.jpg

The 2811 on the left is the PE which I have no visibility of. The 2911 on the right is the CE which I have full control of, but only via the internet connection, which makes trouble shooting somewhat difficult as I have no out-of-band management. This is a REMOTE site in another country with no operational users at site yet. I do have a guy at site helping with cabling and so on. The link between the two routers is a single Cat5e Copper cable.

My Router interface (Gi0/0) is currently set to auto/auto and has negotiated to 10/half. When I set the duplex to full (with a reload in x of course) the link drops and does not recover until the router has reloaded. Same symptoms with a straight-thru and a crossover cable.

I am told by the ISP that their interface (Fa0/0) is set to 100/full, in fact they sent me a snippet of their config which shows speed 100, duplex full and when I execute a show CDP neighbor detail it does report that their end is full duplex.

Q1 - how on Earth can my interface negotiate to 10meg when their interface is hard set to 100meg?? Is this possible? I didn't think so??

Q2 - why does the link drop when I hard set my interface to full duplex (straight-thru OR crossover cable)?

To get my interface to match their's exactly (so they tell me) I used TFTP to configure speed 100 and duplex full on my interface. Again, the link dropped.

As a further test, the guy at site connected an unnamanged switch between the 2 routers and low and behold my interface negotiated to 100/full. I'm told by the site contact that the switch-to-PE connection was showing 100/full also (according to the LEDs on the switch).

The only odd thing I noticed is that they have "no keepalives" configured on their interface. I very much doubt this will cause symptoms like this but thought I'd mention it anywayl.

Any ideas? IOS bug?

Many thanks in advance,

Matt.

Everyone's tags (3)
8 REPLIES

Re: Strange speed/duplex mismatch issue

What is the wan speed?

Sent from Cisco Technical Support iPad App

New Member

Re: Strange speed/duplex mismatch issue

16Mb (8x E1 in a multilink bundle).

Cisco Employee

Strange speed/duplex mismatch issue

Hi Matt,

This is strange indeed...

Q1 - how on Earth can my interface negotiate to 10meg when their  interface is hard set to 100meg?? Is this possible? I didn't think so??

I do not believe your interface could actually negotiate 10Mbps operation if the opposite device was running at 100Mbps. Even the interface simply sensed the speed without autonegotiation, it would not end up with a wrong speed and still be up/up - the link coding is different, the signal levels are different... physically, this is impossible.

Q2 - why does the link drop when I hard set my interface to full duplex (straight-thru OR crossover cable)?

On many interfaces, setting both duplex and speed manually deactivates both the autocross and the autonegotiation features. Perhaps this influences the device on the opposite side of the link.

In any case, this looks like there is yet another device (perhaps a metallic/fiber converter?) along the link between your ISP and your router. Your Ethernet connection seems to be terminated on that device instead of being terminated directly at your ISP's router. What is the technology that interconnects your router and the ISP, anyway?

Best regards,

Peter

New Member

Strange speed/duplex mismatch issue

Hi Peter,

The connection between my router and the ISP router is a standard layer 3 Ethernet connection. The ISP has sent me their config (minus passwords, etc) and I can see that they have their interface that faces my router hard set to 100/full. So I really can't understand how my end is negotiating to 10/half??? Like you say, surely this is impossible!?!?!? We have our interface in a VRF but again, I don't see how this can cause issues.

They have x.x.x.225/29 on their interface. I have x.x.x.226/29 on mine with a static default going out via their .225 interface. It works, apart from being 10/half when it should be 100/full.

At layer 2 the routers are neighbours as when I enable CDP, I can see their router. At layer 1 I am told (I'm based in UK and the site is Italy so I have to rely on a chap at site) that it is a direct router to router connection.

Confused, confused, confused

Cheers,

Matt.

Cisco Employee

Strange speed/duplex mismatch issue

Matt,

I am reluctant to believe that the connection is an Ethernet connection without any media converters or signal amplifiers. They can not be using copper media on the entire cable span between your router and the ISP PE without using repeaters each 100m, and if there is a copper/fiber converter somewhere along then that might be the culprit we are looking for.

How exactly does the Ethernet handoff look like? The CE is connected to a TP cable that goes to... what or where? Just a cable from the wall? Or some switch supplied by the ISP? Any other box? You see, I can not imagine a TP cable simply coming out of the building wall and being plugged right into your CE. There has to be something. Can you try to find that out?

Best regards,

Peter

New Member

Strange speed/duplex mismatch issue

Peter,

My apologies - I did not make this clear - the PE router is onsite, pretty much next to our router.

Regards,

Matt.

Cisco Employee

Strange speed/duplex mismatch issue

Hi Matt,

Now I am slack-jawed

The only thing I would personally recommend is to actually ask your ISP to remove all static speed and duplex setting from their router towards yours (setting speed/duplex statically does not make any sense anyway). There may be issues with this uni-lateral setting of speed and duplex, and I would like to see what the devices will do if both left in total freedom about the autonegotiation.

Best regards,

Peter

New Member

Strange speed/duplex mismatch issue

Yes, I think you're right. I'll do as you suggest and get them to configure their end as auto/auto and see what happens.

Cheers,

Matt.

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