multiple Cisco 3550 Switches on network fail

Posted on 2004-09-24
Medium Priority
Last Modified: 2010-04-17
I have several 3550 24 port 10/100s that were in boxes. I pulled one out and plugged it into my wall ethernet port and then plugged some computers into it. Everything worked great without having to setup anything on the switch but defaults. Then I pulled another 3550 out and plugged it into another network jack in another building. Then both switches quit operating. That is to say, all computers connected to them couldn't get out to the network. The only connectivity they had was the ability to ping the other 3550 switch I had on the network, and vice versa.

Both switches are plugged into regular wall jacks with standard cables (not crossover).

 I am sure this must be a simple solution, but I can't seem to find it. Can anyone help me out?
Question by:igood
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Expert Comment

ID: 12148304
this is stricltly a guess:
what is the IP of the switches? Since you used defaults it may be that you have the same Ip on both machines thus causing a conflict and block the ports.


Expert Comment

ID: 12158131
Well, this is not your question but let me add my personal opinion is that 3550 is not a box to plug into wall just like a hub, although there may be times when you experience no problems at all, on some other times, things may get complicated.

Here is a few suggestions/ideas regarding your problem further analysis is a bit difficult without knowing what is behind your wall plugs :)

1. Normally you seem to be connecting a switch to another (one behind the wall, one in your hand), i.e. stacking 2 switches, and this is normally requires a cross-over cable.

2. One probable cause is that, the port of the switch behing the wall that your are connecting into is a workgroup/access port rather than a uplink port used for stacking and uplinking. Workgroup ports my have limitations on the number of mac addresses they would serve simultaneously (due to hardware limitations) and things might get complicated if you go over this limit. These ports also might have some administrative limitations that might disable the port in the case of frequent mac address changes, use of multiple macs etc. System admins normally do not want you to plug switched to the wall.

Under normal conditions, you have a VLAN structure (which is the logical structure of your net) behind the wall and the new switched must be connected to the existing network via their uplink connections and VLAN structure must extent to the switches in order to provide proper communication.


Author Comment

ID: 12159219
I understand that it isn't the right way of doing things. What puzzles me, however, is how everything works fine unless I plug another 3550 into a jack anywhere on my network. What is the underlying cause of the failure, and is it something that can be fixed?
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Expert Comment

ID: 12177003
What are the default IP addresses on both of the switches?  I tend to agree with MarkDozier - IP conflict.

Author Comment

ID: 12178750
A CCNP and I worked the issue out. It was because VTP control was set to "Server" on the switches. While I don't know much about VTP, setting it from "Server" to "Transparent" resolved the problem.
LVL 28

Expert Comment

ID: 12194017
FYI for everyone, a switch IP conflict will not cause this problem. The IP on the switch is for switch management only and a conflict would affect your access to that but not hosts (unless the conflict is with a host). Devices plugged into it will work with duplicate IP, no IP etc.
LVL 13

Expert Comment

ID: 12223146
VTP, Virtual Trunk Protocol, it’s for VLAN’s, and VLAN management purposes. It can be set for server, client, or transparent mode. If you have a stack of switches and you configure one as a server, and the rest as clients, any VLAN’s you configure on the master will propagate to all the clients. Saving you from having to configure VLAN’s on every switch in the network. If you set a switch to transparent, it operates alone and nether receives, or transmits VLAN information to other switches, in other words it operates independently.

As to why they shut down when more than one master was detected, I don’t have a clue as I never would have expected that behavior. What I would have expected is them to ignore each other and switch into transparent mode until the problem was resolved.      

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