Spanning Tree Path Cost Question

[brentz]

We have a Cisco 9407 as our Core switch.  The Spanning Tree Path Cost is set to short as it is on the rest of the switches in our network.  We connected a 9200 via an ether channel configuration.  The 9200 Spanning Tree Patch Cost was set to long.  After the connection was established and the port channel came up on both switches the network stopped working.  We could not ping anything on the network from the 9200 switch.  We lost network connectivity to switches in the same building and remote buildings.  From the 9400 we could ping any switch we tried.  Some switches could be pinged from Computers some could not.  We disconnected the 9200, disabled the ports and the ether channel on the 9400 switch,  After checking several devices we ended up rebooting the 9400 switch.  Once it was back up all network connectivity was restored.  I have a case open with support but have not heard anything at this point.  Is there a chance that the different path cost settings could have caused the 9400 to stop working?

Thanks

[balaji.bandi]

You need to provide more information, what IOS XE code running on Both Cat 9400 and 9200 ?

is the Cat 9400 running SVL or Only 1 Switch ?

If SVL how is your connection to Cat 9200 - provide the config and STP config.

are you running any VTP ? if no VTP using I suggest using VTP Transparent.

Before reboot Cat 9400 have you collected the Logs ?

Make sure you always setup a root bridge priority on Cat 9400 for all VLAN.

you can also enable root guard on the config, so accidentally other switches can not become the Root bridges.

STP is evil if you not follow some rules and if you have STP loops  :

STP election process :

Spanning Tree Protocol (STP) – 
As IEEE STP is used to make a loop-free network by monitoring the network to track all the links and shut down the redundant ones. These are some important terms related to Spanning Tree Protocol: 

• Bridge Priority Data Unit (BPDU) – It contains the Bridge ID, Sender’s Bridge ID, Cost to the Root Bridge, Timer values on Root Bridge. All switches exchange BPDU in order to elect root bridge. The switch with the lowest Bridge ID become the root bridge.
• Bridge ID – It is an 8-byte field that is a combination of bridge priority (2 bytes) and Base Mac address (6 bytes) of a device. If there is a tie on bridge priority then the Base Mac address is considered.
• Bridge Priority – It is a priority, which is assigned to every switch, 32768 by default.
• Root Bridge – The root bridge is the bridge with the lowest Bridge ID. All the decisions like which ports are the root ports (the port with the best path to the root bridge) are made from the perspective of the root bridge.
• Path cost – A switch may encounter one or more switches in the path to the root bridge. All the paths are analyzed and the path with the lowest cost will be selected. 

[brentz]

The 9400 is a single switch.  It is running 16.09.04  The 9200 is running 17.06.XX. I noticed in this version the Path cost defaults to Long.  We have other 9200/9500 running 17.03.XX and the path cost is set to short.( it is the default in that version)

VTP is disabled on all switches in the network.  The 9400 is set to be the root bridge for all layer 3 interfaces on the switch.

We looked at the logs and really did not see anything that showed any value.  the other item is that the 9400 has been up for 2 years without a reload.  

Thanks

[balaji.bandi]

path cost long and short only changes from 16bit to 32-bit. so for consistency personally I use all same.

also, what STP are you using MST or Rapid PVST+  (I suggest this for faster convergence).

is this VLANs are flat network for all switches ( I mean all switches have same VLAN ?) or each access switch have different VLAN segment

as I mentioned you need to provide the config for us to understand the issue and suggest anything we can.

if you already working on a TAC case, they can guide you better as the subject matter of experts

[paul driver · Accepted solution]

Hello

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@brentz wrote:

Is there a chance that the different path cost settings could have caused the 9400 to stop working?

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Yes  - having differing stp path method can cause inconsistency in the stp estate, as such ALL switch's need to be running either the short.or long path method , otherwise stp port costing would vary for the same size interface.

[ramarchant]

Is the changing from one path cost method to another disruptive to the STP operation?

[brentz]

We installed version 16.03.069 (the default for Spanning Tree Path Cost is set to short) and the switch installation went great.  No issues.  When you go to version 16.0.6.xx the path cost goes to long by default