Cisco 2960 model

[13jobsp90]

I have a Cisco 2960 model switch which was a refurbished one, I have put up the basic configurations just like we do have in other access layer switches. I have a configured a trunk port also when I connected this switch to another uplink switch, the whole network got in loop.I havent connected any end devices to this new switch only trunk port is connected to another switch.What must be the reason for this flapping? All the configurations are looking good.Any suggestions?

[Flavio Miranda]

@13jobsp90 

Loop and Flap are different. The problem can be related to spanning tree. When you connect switch to another switch, the spanning tree can be recalculated and a New root bridge election can happen and the network can crash.

 Before connect the New switch, make sure the corrent switch Will keep its role in the spanning tree

Use show spanning-tree to verify

Run this command to set the root

spanning-tree vlan vlan-id root primary

Run this command on the corrent switch for all vlans

[13jobsp90]

but how do i know the newly yet to connect switch will make issue or not?

[Flavio Miranda]

For spanning-tree you can run the command "spanning-tree vlanvlan-idroot primary" on the switch you have installed.

This will present new switches to try to become root for spanning tree. Another config you also need to check when connecting new switches on the network is VTP. Keep newly connected switch always in VTP transparent mode.

[ammahend]

Also share the port config, status and logs