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New Member

Disconnection of uplinks between 2950C & 3550C

I've a LAN composed of 10 2950-C & one core Switch 3550C. Some switches have a double fiber uplink, some other only one.

Several time per day (3/4 time) one link is dropped, but it's not always the same one. By disconnecting quickly the uplink's fiber connector the link is restalished and in most cases this link will not be dropped anymore during the day. If checked the System Message Log available in CMS and only LINE and LINEPROTO are shown. No trace of a Loopback or something like this.

I've also noticed that on some switched both uplinks are active and on some other only one is one, even if both links are connected. Switches configuration is always the same one except for the 3550 which has been declared as root switch.

Help & recommandation most welcome


Re: Disconnection of uplinks between 2950C & 3550C

Keep in mind that bridging loops might be the case of your link flapping. By this I mean that Spanning Tree, which is enabled by default, might be having Topology Changes that are causing your ports to go through the different states, thus interupting traffic (Blocking, Listening, Learning, Forwarding (Disabled is one too but not for this discussion)).. I would check your Spanning tree and your topology and ensure that your 3550C is the Root Bridge.

Here's a good link on configuration of spanning tree on the Catalysts.

Spanning tree design Doc:

Spanning Tree Topology Changes:

Hope this helps you,


New Member

Re: Disconnection of uplinks between 2950C & 3550C

Hi Don,

Thanks very much for your help. It was quite usefull and it seems to work now. I've setup the root priority to 8192 on the root switch, and enable PortFast to all ports connected to workstations and servers.

I'll see what happend over the weekend and see the results on monday.

By the way do you have usefull links about VLAN ? I'm starting to study how to setup few Vlans but still a bit confused about differences between VTP, VMPS and VLAN Maps as lots of explainations are for Catalyst switches running CatOS and mine have IOS.

Once again, thanks for your usefull help


New Member

Re: Disconnection of uplinks between 2950C & 3550C

I've made the following modifications:

Main root switch configured with 8192 priority

Secondary switch configured (default config)

UDLD Agressive mode on all optical ports (uplink for 2950C and all ports for 3550C)

100TX ports connected to a terminal are PortFast enable,

From CMS tool I can see all links (from all 2950 switches, 1 link goes to 3550 main root switch, 1 link goes to secondary 3550 switch), but from time to time (between hours & days) some links fall down and cannot find any reason to explain this problem. Should I configure manualy the cost of every uplink port ? (i.e. cost = 0 to main root switch & cost = 19 to secondary switch).

Any idea ?

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