I'm preparing for the CCNA and I have a little trouble understanding the outputs for STP.

By default a CISCO switch use PVST (Per VLAN Spanning Tree) but if we use the command show spanning-tree with the default configuration the output is:

Switch#show spanning-tree


Spanning tree enabled protocol IEEE

[output omitted]

I don't understand why it said IEEE, as far i know the PVST is a CISCO proprietary protocol.

If I'm not wrong the PVST is a modification from the IEEE 802.1D Common Spanning tree protocol(CSTP) then why in the output said "enabled protocol IEEE"?

Thanks for the help!

  • 1
    PVST is a Per VLAN implementation of IEEE 802.1d, so a single VLAN is just the IEEE standard. More than one VLAN using the IEEE standard per VLAN is PVST. – Ron Maupin Sep 16 '15 at 18:45

one needs to look at it from a historic perspective.

1) The original STP was designed by Radia Perlman working for DECnet. Then IBM & other vendors implemented their own versions of STP.
2) Then IEEE created their standard (IEEE 802.1D) for STP.
3) STP, as defined in 8012.1D standard was implemented as 802.1Q CST (Common Spanning Tree) for all VLANs by IEEE & as PVST by Cisco on a per-VLAN basis. Later, PVST+ was created to interoperate with 802.1Q trunks.

But all these implementations, whether they're Cisco's PVST(+) or IEEE's CST, implemented the same mechanism (BPDU Structure/ STP timers/ Topology changes) as defined in the standard 802.1D. I guess that's why Cisco prefers to call their version (PVST+) as "IEEE", although RSTP is also IEEE (802.1w). Maybe, they don't want to change their code now to avoid creating any confuson, so they just prefer to write IEEE for PVST+ (Using old timers etc.) & RSTP for 802.1w (Using new tmers & synchronizations etc.). That's how I've understood it so far. Would love to hear some expert opinion though....maybe people who were around when the standards were created.

make use of this discussion Cisco Learning Network

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