Anyone can tell the difference between CST and PVST

[eigrpy]

Hi Anyone can tell the difference between CST and PVST? by default the switch should be in PVST mode instead of CST, right ? Thank you

[acampbell]

Hi,

Read this link:-
A big subect to answer in a few lines.

http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24248-147.html

Hope this helps
Regards
Alex

[Masoud Pourshabanian]

Hello,

As acampbell mentioned it is a big subject.

CST dates back to the bridges time. Bridges did not have VLAN feature. CST only runs one instance in entire bridge and switch, so there is only one root bridge in entire switch or bridge.

PVST runs one instance for each VLAN group so you may configure different root bridge for each VLAN group. In this way ,you can share traffic across two links by having different instances and root bridge. In CST, some links are always in block state and some in forwarding state. In PVST, a link is block for some VLANs but that link is in forwarding state for other VLANs so traffic can be shared among all links. Still CST runs on Native vlan even if you configure PVST.

Hope it helps,

Masoud

[eigrpy]

Thank you for your reply. Can you confirm the PVST is by default configuration in latest Cisco devices? 

[Masoud Pourshabanian · Accepted solution]

Hello,

please check the link below.

PVST+—This spanning-tree mode is based on the IEEE 802.1D standard and Cisco proprietary extensions. It is the default spanning-tree mode used on all Ethernet port-based VLANs. The PVST+ runs on each VLAN on the switch up to the maximum supported, ensuring that each has a loop-free path through the network.

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2960/software/release/12-2_53_se/configuration/guide/2960scg/swstp.html

Masoud

[Masoud Pourshabanian]

Adding to my previous comment.

PVST and PVST+ are the same in operation interms of instances; however, PVST+  improves PVST capabilities by allowing to transport PVST over 802.q and makes it compatible with other non-cisco devices.

PVST olny sends BPDU over ISL links.

Masoud