STP default mode

[ddujma]

Hi dear friends,

This keeps on puzzling me.

If I am correct, the default STP mode on cisco switches is PVST+. But when I issue show spanning-tree i get the output that says IEEE protocol is used.

Spanning tree enabled protocol ieee.

If i'm correct, PVST+ is CISCO proprietary protocol, why isn't there stated PVST, as the case is when you use Rapid PVST:

Spanning tree enabled protocol rstp.

Did i got something wrong? The output was from Packet Tracer cisco switch 2960.

Thanks in advance.

[Big Evil]

Take a look here mate.

http://blog.ine.com/2008/07/17/pvst-explained/

[ddujma]

Thanks friend,

Great material!
It was a bit tough reading material but I extracted the info I needed :)

So, by default cisco switch would use CST (Common Spanning Tree Protocol) - a single STP instance for all VLAN's in the network in order to give a switch a compatibility to 802.1Q and non-802.1Q switches.

It is a bit non-flexible, but it gives a switch working configuration right when it comes out of a box, so for example you could attach some non-cisco, but IEEE compliant switch to a cisco switch access port and it works just fine.

Wow (ok, i am a bit geeky now :D), it also help PVST+ to interoperate with MST (Mono Spanning Tree) on 802.1Q trunks so you can have different regions in network all conjoined by one single protocol - CST.

To conclude CST is a IEEE standard protocol used by default and thus my 2960 switch gave me following output:

Spanning tree enabled protocol ieee

I hope i got it all right.
Your article was a big help,
Cheers.

[Big Evil]

No problem mate, glad it helped. Yes maybe a little ott, but you read it and understood it.