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Root bridge promotion/demotion

Barry Fields
Barry Fields used Ask the Experts™
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I have several issues in my layer2 topology. This is a mixed switch environment (Dell and Cisco). Presently one of my Dell switches is the root bridge, and I want to promote the Cisco switch ( where my servers are attached) to the root bridge. I also have some protocol mismatches;
1. the cisco switches have LLDP, CDP and fast spanning tree running.
2. the dell switches have only spanning tree running.

I have ethernet ports flapping (going up and down) on every switch. What would be the best approach to clear these issues.
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nociSoftware Engineer
Distinguished Expert 2018

Commented:
Spanning tree & Fast Spanning tree are compatible. Fast works on interface change, regular updates in intervals. (30 seconds).... same BDPU's are user / vlan.
If all switches are Fast convergence of the network state will be quicker (within seconds) in stead of on 45 second average.

First find out why the ports are flapping.    Which ports and what is connected. Are both sides on those ports configured the same way.
Portflaps are mostly with either broken hardware or mis configured devices.    (auto is often not a wise choice, port-channels preferably fixed / static config).
Ensure VLAN's are consistently defined &  connected.
Don JohnstonInstructor
Top Expert 2015

Commented:
Presently one of my Dell switches is the root bridge, and I want to promote the Cisco switch ( where my servers are attached) to the root bridge.

Here's the problem:  Cisco's STP implementation is propiretary PVSTP (or PVRSTP).  As in "Per VLAN".  IIRC, Dell only supports IEEE STP's (common or mono spanning-tree)  Which means Dell and Cisco don't play well together.  

In multi-vendor environments, I use 802.1s (MSTP).  

https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24248-147.html
IT manager
Commented:
I solved this issue by changing the root bridge. I was getting network topology change issues every second once I changed the root bridge i have experienced only one topology change message in 15 hours