Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

Network Expansion Problems

I have recently been required to connect up a new building to our infrastructure via a fibre link.

The existing "backbone" of the network is a stack of 2950T switches. This has been like this for over a year.

A new 3750 has been purchased and linked to one of the spare gigabit ports on one of the 2950T via copper.

Using an SX SFP module, this new switch is then connected to the fibre link to the new building. At the other end of the link, resides a stack of 2950G switches, where it is connected to an SX GBIC module. The stacking is facilitated by use of GBIC SuperStack modules.

The problem is, when everything was initially connected, it was fine for a couple of days and then the network showed signs of a layer 2 loop. Everything ground to a halt. Had to disconnect the fibre link to bring everything up again.

I then upgraded the software on all relevant switches to get them all to the same release, and plugged the fibre back in and everything was fine. For a week and a half. The network went down again.

I have now had to disconnect the fibre again, much to the annoyance of the users in the new building.

Does anyone have any ideas as to why this may be happening, and/or any possible routes that I can take to try and fix it??


New Member

Re: Network Expansion Problems


Have you checked the gig interfaces that connect the two buildings for a "Spanning-tree Portfast" setting. If you see this setting on the interfaces, you might try to remove it and see how that works.

Also, check to see which switch is set as the root bridge and check if VTP is enable and conflicting with the new switch somehow.

When the network went down at the new building, was the link light on or off on the the switches?


New Member

Re: Network Expansion Problems

The problem could be with the GBIC itself. How far is the distance between buildings? Each GBIC is rated for different distances much like different cable for Ethernet is rated for different distances. I would guess that since the GBIC connection continues to fail, there may be an issue with excessive errors that cause the port with the GBIC to shut down, thus failing your connection. You could do a check on the Router interface to see if the port connecting to your switch stack is being disabled. Or you could run a cable test to verify the integrity of the connection. Also, make sure that the connection is not getting bumped accidently or is not coming lose over time as this would cause problems. Good luck.


CreatePlease to create content