Cisco 3945 with SFP - full duplex possible?

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Sep 27th, 2010
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My provider (Deutsche Telekom) is only able to hand over a 1Gbps-Link with speed und full-duplex configured fix.

To force the router (cisco 3945) for 1000/full, I had to remove the 'media-type sfp'

SFP is MM-SX on Router-Interface Gi0/2

This configuration worked a while but shows many interface resets (more than 30 in one minute).

Telekom confirmed, their connection is working properly.

After switching the router off an on, I could no longer connect with telnet or ssh. Ping is also not successful but cdp shows the neighbor router!?!

Maybe this is a problem with SFP and duplex configuration.

Any ideas?

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Overall Rating: 5 (1 ratings)
Jean-Pierre Burlet Tue, 09/28/2010 - 04:24
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with Gigabit there is only full. But it could also be the software. A customer of us has had almost the same problems and by a software update the problem was repaired.

adjovikoffi Fri, 02/26/2016 - 01:28
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my problem is that my cisco router 3845-MB has its interface gig0/0 down. admistratively down. the ios version is the 12.4(20)T4 ,do i upgrade the version? if yes which one doesnt have a such bug.

thanks to reply

mbrunton77 Tue, 09/28/2010 - 07:48
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have you tried the command speed nonegotiate under the interface?

kluehspiesp Tue, 09/28/2010 - 09:14
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Software-Update to 151-2.T1 did not solve the issue, either :-(

Leo Laohoo Tue, 09/28/2010 - 15:20
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A GLC-SX-MM will always negotiate to full 1G.

kluehspiesp Wed, 09/29/2010 - 08:05
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After replacing the 3945 with a 3845 the Link is up and running - without any 'interface resets'

dahirton Thu, 11/25/2010 - 12:47
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Hi, I'm facing a similar issue with a 3845 with GLC-SX-MM module trying to connect at 100Mbps with a telco device. Documentations says 'media-type sfp' defaults to 1Gig, and only changing to 'media-type rj45' would allow me to set 100Mbps instead. Any clue what should I do in order to allow speed compatibility with this telco device ? Follows the current interface config:

Interface GigabitEthernet0/0

description ---

bandwidth 100000

no ip address

duplex auto

speed 1000

media-type sfp

negotiation auto

kluehspiesp Thu, 11/25/2010 - 23:35
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with IOS version 15.0(1)M4 the Gig interface can be left on speed/duplex autosensing mode. Its default is to connect with 1000/full, again.

IOS versions before connected with half-duplex.

The new software fixed the Bug CSCtj35792

kluehspiesp Thu, 11/25/2010 - 23:36
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the command 'no negotiation auto' should fix your issue

on a 3845

Tom Randstrom Fri, 11/26/2010 - 07:48
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In order to connect to a 100BaseFX link, you will need to install a GLC-GE-100FX SFP module in your 3945 switch.  This 100M SFP module is designed to be installed in a GE SFP slot.

If, in the future, you want to support GE on the 3945 switch's port, you will have to change the SFP to a compatible GE module.

Hope this helps!


kluehspiesp Tue, 11/30/2010 - 02:45
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the request was to connect a 1000/full link.

After the software update it works without any problems

Tarun Vyas Sat, 09/24/2011 - 03:41
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Did Router 3945 SFP Port support SFP-10G-LR, How to define it ?

loizosko Tue, 08/13/2013 - 20:00
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i had the same problem. 3925 running 151-3T. we got a new isp link that required glc-sx-mm. the link was not coming up.

i tried different gbics.

no command to nonegotiate or specify speed/duplex

i upgraded ios to 153.2T and interface came right up.

still don't have those commands but it works.

there must be a bug or something in some of the ios.


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