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Spanning-Tree queries

Just a few questions concerning spanning-tree

Question 1

When I use the show spanning-tree command, how do I know if the switch is using CST or PVST+.

Currently I am getting

Switch5#show spanning-tree vlan 1


  Spanning tree enabled protocol ieee

  Root ID    Priority    24577

             Address     a8b1.d419.a780

             This bridge is the root

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

I know rapid spanning tree shows as rstp and mst shows as mst. But how do I differentiate CST and PVST+

Question 2

The fourth tie breaker in determining the designated ports is the port ID. But when would this ever come into play? The bridge ID is the tie breaker BEFORE the port ID. But the bridge ID is composed of the priority PLUS the MAC address. Everything has a unique MAC address so the port tie breaker would never be needed....

Question 3

Only the root port ever sends out BPDUs correct? Naturally to start with every switch does because every switch thinks its the root bridge but once the election is complete only the root is sending BPDUs...

Question 4

With an indirect link failure... once the MAX AGE time expires, does the switch in question send out a TCN... or does it just discard the best BPDU packet that it had before.


Spanning-Tree queries

A1. 802.1D is ieee standard, PVST+ is not.

A2. Imagine two switches connected to each other via 2 links. The same BID will be sent from root to non-root switch on both ports. That's where port-id would come into play.

A3. In RSTP all switches generate BPDUs. Root BPDUs are not just relayed. The link says:

BPDU are sent every hello-time, and not simply relayed anymore. With 802.1D, a non-root bridge only generates BPDUs when it receives one on the root port. In fact, a bridge relays BPDUs more than it actually generates them. This is not the case with 802.1w. A bridge now sends a BPDU with its current information every seconds (2 by default), even if it does not receive any from the root bridge.

A4. I am not sure about Max Age expiration. I think TCNs are sent as soon as a failure is detected.


Cisco Employee

Spanning-Tree queries

Hi Steven,

In addition to the nice reply posted by rais, let me add a few things.

A1. Cisco switches never implement pure STP/RSTP. They always run PVST/PVST+ or RPVST/RPVST+. If a Cisco switch says ieee then it really means PVST/PVST+, and if it says rstp then it really means RPVST/RPVST+.

A4. With an indirect failure, it depends whether the indirect failure was caused by a physical port going down somewhere, in which case the TCN would be sent immediately, or just by the STP software somewhere suddenly hanging, in which case the TCN would be sent after MaxAge seconds.

Note that this goes only for STP. RSTP sends TCNs only if a port becomes Forwarding, and it does not send TCNs if a previously Forwarding port becomes non-forwarding for some reason. Also, the failure of the neighboring switch is detected after 3xHello. The MaxAge timer is less relevant in RSTP.

Best regards,



Spanning-Tree queries


Thanks for your input.