Prio.nbr value in stp output

phoeneous

In a show spanning-tree output, what does the "128" value represent under Prio.nbr column? I know the value to the right of the period is the port number but I dont know what the value on the left is besides being a priority. But how is it calculated and whats the difference between the Prio.nbr column on the left and the Prio.nbr column on the right? Thanks.

kryolla

is port 7 connected to port 25 on the other end

miller811

Port Priority – This parameter can be used to assign a higher (or lower) priority to a port. In the event that traffic is re-routed, this parameter gives the port forwarding preference over lower priority ports within a VLAN or on the switch or routing switch (when no VLANs are configured for the system). Ports are re-routed based on their priority. The highest value is routed first. Possible values: 0 – 255. Default is 128. This value overrides the system-wide STP priority.

captobvious

miller811 wrote: »

Port Priority – This parameter can be used to assign a higher (or lower) priority to a port. In the event that traffic is re-routed, this parameter gives the port forwarding preference over lower priority ports within a VLAN or on the switch or routing switch (when no VLANs are configured for the system). Ports are re-routed based on their priority. The highest value is routed first. Possible values: 0 – 255. Default is 128. This value overrides the system-wide STP priority.

+1 Beat me to it.

I don't type as fast! =P

kryolla

port priority is based on the upstream switch so you have port 7 connected to port 25 and port 8 connected to port 26, since the priority are the same the lower port on the upstream switch which is 25 local port 7 is forwarding as in your example and port 8 is blocking. Now on the upstream switch lower the priority on port 26 and port 8 will be forwarding. This priority is different than the root bridge election priority. The right side says designated which is upstream from you, so the upstream bridge id, cost, bridge priority and ports. Since the upstream bridge id is different than yours and the root bridge there is atleast 3 switches in this topology and you are 2 hops away from the root bridge.

edit:captainobvious that links is for etherchannel and not spanning tree

phoeneous

kryolla wrote: »

This priority is different than the root bridge election priority.

And thats where I was getting confused. Thanks for clearing it up.