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Steve Graham

Spanning tree root

All, this subject is definately my weakest area of knowledge, so bare with my questions.

Right now our environment has one core layer 3 switch (Cat 6513 SUP2/MSFC2) that hosts my entire layer 3 environment. (I know this is scarey and a HUGE SPOF, I am new to the organization and am working diligently to build a robust core with two 6509's, but that's another question for another day). My question is this. I ran a "sh spanning-tree root" from my core and it shows a floor switch as the root for spanning tree. This floor switch is directly connected to the core via a dot1q trunk. My instinct tells me that I want my core to be my root for spanning tree. What I need to know is the justification for moving this. It's really knowledge transfer more than "how to's" that I'm looking for.


Prashanth Krishnappa
Cisco Employee

From our best practice document

"Influence and know where Root functionality and blocked ports reside, and document them on the topology diagram. The blocked ports are where STP troubleshooting begins - what made them change from blocking to forwarding is often the key part of root cause analysis. Choose the distribution and core layers as the location of root/secondary Root, since these are considered the most stable parts of the network. Check for optimal L3 and HSRP overlay with L2 data-forwarding paths."

Georg Pauwen
VIP Master


the only reason I could think of why this might have been done deliberately is that the VLAN for which the floor switch is the root has the majority of its ports on that switch; or the VLAN might even exist ONLY on that switch. If that is the case, having the core switch as the root for that VLAN would negatively impact the traffic flow.

If that is not the case, it is probably a configuration mistake...



Even if this switch has many ports, it is not a good idea to have an access layer switch to be STP root.

Probably, in your case, the switch won the election with default STP configuration and hence the root.