internet down

[Nishan]

i have three stack cisco switches 2960 and i added a stand alone  cisco 2960 and i took two uplink from those stack switches which is port channel suddenly my whole internet goes down and when i remove the uplink i got internet back. 

[balaji.bandi]

Make sure the new switch not become root bridge for the VLANs

the new switch should be as below :

1. NO VTP Servers

2. spanning priority high than stack

3. you can also configure trunk root gaurd

still issue post below information :

1. show spann summary

2. stackswitch connected to other switch port config

3. new switch uplink port-configuration

on the Stack switch you see the Logs - what is the reason all the ports went dow.

[MHM Cisco World]

The only reason thar uplink of access SW effect Core SW internet access is L2 loop strom.

This make CPU high utilize and make packet drop.

Share the config of PO in stack Core SW and in access SW

MHM

[Nishan]

dear mhm,

the core switch is nexus with vpc and already stack siwthces are connected. and now i added a stand alone switch to this vpc and the uplink i configuerd as portchannel. here the nexus switch with vpc will a single logical switch to third device am i right

[MHM Cisco World]

Nop' it two SW' vPC NSK have two control then it see by other L3 or L2 device as two.

With NSK it give more indications that there is l2 loop.

Check the NSK stp when connect the access SW.

I think it BLK status.

Also what mode you run in NSK and stack' is it MST or PVST+?

MHM

[Richard Pidcock]

A diagram of your topology would be helpful.  I think the most likely cause of your issue is spanning tree related.  When you connect your new switch spanning tree must start blocking on another port somewhere in your topology causing your issue.  

1.  Ensure your core switches have a lower STP priority so they are guaranteed to remain as root.

2.  Evaluate STP status prior to connecting your switch.  

3.  Evaluate STP status after connecting your new switch.