100Mb vs 1000 Base T question.

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Questions, I have 3550 switch, unfortunately its the one that has 10 1000mb slots and 2 10/100/1000 slots. I just purchased a copper GBIC converter that converts 1000mb slot into RJ45 connection. Unfortunately when I try to hook up 100mb fasteth0/0 port on my 2621XM Cisco router using straight through cable to the GBIC converter on the 3550 switch I don't see the port status showing up. As the matter of fact the light doesn't even show up on the 3550 switch, neither does the light show up on the router's end. I have a feeling that I might be using the wrong cable. When connecting 1000 mb gbic on the 3550 switch to 100mb ethernet on 2621xm do I have to use some modified straight through cable and if so what are the pinouts for that?

I have this problem too.
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Anand Narayana Tue, 05/29/2007 - 02:20


is the port status is administratively down? make it up if it is shutdown, also try connection with the cross cable.

mohammedmahmoud Tue, 05/29/2007 - 02:38


1000BaseT can use CAT5/CAT5e/CAT6 copper cabling with four twisted pairs.

As suggested by Anand, check if your interface is not adminshut, if thus do "no shut" this is most probably the problem, and you must use straight cable between a switch and a router.

HTH, please do rate all helpful replies,

Mohammed Mahmoud.

glen.grant Tue, 05/29/2007 - 03:32

Somebody correct me if I am wrong but those copper gbics just convert to gigabit copper only it will not run 10/100 so if you are trying to hook a 100 meg port to it , it is not going to come up as you have a speed mismatch. You don't specifically say what the gbic model number is but this is probably what you are running in to . You cannot connect a 100 meg port to a gigabit only gbic .

mohammedmahmoud Tue, 05/29/2007 - 03:44

Hi Glen,

You are correct, the default for the speed and duplex are Auto, thus the interfaces should come up with 100/Full in case his interface supports 10/100/1000, and it would never come up if the interface supports only 1000, as speed mismatch prevents the interface from being operational.


Mohammed Mahmoud.

Ok, to answer the first question, I did make sure that no shut command is executed on the router port and the gigabit switch port (that's the first thing I checked). The light on the port didn't come up.

Answer to the second question is, well I am using 1000Base-T GBIC WS-G5483 model gbic.

If this is a speed mismatch and 100mb can't communicate with 1000mb, how come 1000Base-T Gbic is not backwards compatible? Why can't I just adjust both sides to have the same speed 100mb on both sides and do not use the full capacity of 1000mb?

If this is something that is way out of my hands and there is nothing I can do can anyone recommend me, how can I hook up 1000mb port on a switch to 100mb port on the router?

I tried Gbic converter that didn't work isn't there any other media converters that allow me to hook up 1000mb Gbic ports on the switch to a regular 100mb Rj45 port on a router?

The reason why I am trying to make this whole thing work is because I am studying for my CCIE and the lab requirement has 2 multilayer 3550 switches and they all hooked up to 6 or 7 different routers if not more. Unfortunately I have only 2 10/100/1000 mb ports on each switch, which makes the total of 4 ports. All the other ports on the switch are 1000mb ports and they do not look like typical RJ45 ports, hence I can't connect them directly to a router.

Are there any other possible media converters that I can purchase that would allow me to make both devices talking to each other?

Any help would be greatly appreciated. Thank You.

Anand Narayana Tue, 05/29/2007 - 20:58


WS-G5483 doesn't support 10/100/1000, it supports only 1000 Mbps speed.that is the reason that if you connect a 100 Mbps to the WS-G5483 module, it doesn't comes up.

mohammedmahmoud Tue, 05/29/2007 - 23:17


The problem doesn't reside in the converter, it resides in that your switch has 10 GBIC-based Gigabit Ethernet ports and 2 10/100/1000 ports, and thus the 10 GBIC-based Gigabit Ethernet ports won't be able to work with 100M even if you connected a converter, you can never connect an interface that supports only 1000M to an interface that can't support 1000M.

HTH, please do rate all helpful replies,

Mohammed Mahmoud.


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