3500 spanning tree

[Patrick McHenry]

This is output from a 3548 switch.

Does this output mean I am running CST?
When I try to configure the spanning tree mode , I have no options.
Is PVST and PVST+ not an option for the 3548s?

Also, I have read that CST is not compatable w/ ISL. Is this correct?

Thanks, Pat.

LearningCtr#sh span

Spanning tree 1 is executing the IEEE compatible Spanning Tree protocol
  Bridge Identifier has priority 49152, address 0004.dd40.5400
  Configured hello time 2, max age 20, forward delay 15
  Current root has priority 8192, address 0015.2c03.1001
  Root port is 48, cost of root path is 3004
  Topology change flag not set, detected flag not set, changes 1
  Times:  hold 1, topology change 35, notification 2
          hello 2, max age 20, forward delay 15
  Timers: hello 0, topology change 0, notification 0
  Fast uplink switchover is enabled

Interface Gi0/1 (port 40) in Spanning tree 1 is BLOCKING
   Port path cost 3004, Port priority 128
   Designated root has priority 8192, address 0015.2c03.1001
   Designated bridge has priority 16384, address 00d0.0356.1c01
   Designated port is 3, path cost 2
   Timers: message age 3, forward delay 0, hold 0
   BPDU: sent 1, received 216

Interface Gi0/2 (port 48) in Spanning tree 1 is FORWARDING
   Port path cost 3004, Port priority 128
   Designated root has priority 8192, address 0015.2c03.1001
   Designated bridge has priority 8192, address 0015.2c03.1001
   Designated port is 14, path cost 0
   Timers: message age 0, forward delay 0, hold 0
   BPDU: sent 1, received 217

[Antonio Knox · Accepted solution]

Do this command

show spanning-tree summary

You should get a better idea of what's running.  PVST should be on by default.

[Patrick McHenry]

Thanks, Antonio.

I guess it is running pvst.

What about my other question. About CST and ISL. I know know that I am running PVST but, are both ISL and DOT1q compatable?

I've heard that CST is only compatable with DOT1q, PVST is only compatable with ISL and PVST can do both. Is this correct?

Thanks, Pat.

[Antonio Knox]

Dot1q and ISL are trunking encapsulations for VTP.  They cannot share a trunk, meaning you can't run both on a trunk at the same time.

As for CST with ISL, there should be no reason why they wouldn't work together, and I'm not privy to any issues of compatibility between the other protocols you mentioned as well.

Message was edited by: Antonio Knox

[Patrick McHenry]

Thanks, Antonio.

IF I am running PVST on the 3500 with one trunk using 802.1q on both sides of the link, and functioning, can I savely assume it is PVST+?

Does Cisco support the use of PVST?

Found this on Wiki:

PVST works only with ISL (Cisco's proprietary protocol for VLAN encapsulation) due to its embedded Spanning tree ID. Due to high penetration of the IEEE 802.1Q VLAN trunking standard and PVST's dependence on ISL, Cisco defined a different PVST+ standard for 802.1Q encapsulation. PVST+ can tunnel across an MSTP Region.^[12]

^{Thanks, Pat.}

[Antonio Knox]

Yes, you are running PVST+ if your encapsulation is 802.1q.  Here is a good read on the differences between PVST & PVST+

http://cciethebeginning.wordpress.com/2008/10/15/differences-between-pvst-and-pvst/

[Patrick McHenry]

Thanks, Antonio!

I'll read up.