Pvst and ISL

kpjungle

Hi,

Im reviewing the BCMSN guide, and it states that PVST (without +) can only run trunks with ISL. However on my home lab i have dot1q trunks running with pvst. Is it actually using pvst+ then?

Thanks ppl.

Mrock4

Currently the IOS only allows you to enter "spanning-tree pvst"- therefore you are not able to specify whether to use PVST or PVST+. That being said, your Dot1q trunks are running PVST+. If you created ISL trunks, PVST would run over them. Essentially there is no difference in the config, but the logic behind the config varies based on trunk type. Make sense?

Plazma

PVST was created first as ISL was the only trunking protocol available at the time. PVST+ is Cisco's PVST with support for 802.1Q. Both use the same STP algorithms and configuration its just since ISL is "dying" , you often see PVST+ thrown everywhere in regards to trunking.

kpjungle

Thanks guys. Appreciate the clarification.

tech-airman

kpjungle wrote:

Hi,

Im reviewing the BCMSN guide, and it states that PVST (without +) can only run trunks with ISL. However on my home lab i have dot1q trunks running with pvst. Is it actually using pvst+ then?

Thanks ppl.

kpjungle,

Here's some official documentation from Cisco...

Per VLAN Spanning Tree (PVST) wrote:

Per-VLAN Spanning Tree (PVST) maintains a spanning tree instance for each VLAN configured in the network. It uses ISL Trunking and allows a VLAN trunk to be forwarding for some VLANs while blocking for other VLANs. Since PVST treats each VLAN as a separate network, it has the ability to load balance traffic (at layer-2) by forwarding some VLANs on one trunk and other Vlans on another trunk without causing a Spanning Tree loop.

Per VLAN Spanning Tree Plus (PVST+) wrote:

Per VLAN Spanning Tree Plus (PVST+) provides the same functionality as PVST using 802.1Q trunking technology rather than ISL. PVST+ is an enhancement to the 802.1Q specification and is not supported on non-Cisco devices.

The indirect point that Cisco wants to make is how PVST and PVST+ are better than CST. What is CST?

Common Spanning Tree (CST) wrote:

Common Spanning-Tree (CST) assumes one spanning-tree instance for the entire bridged network, regardless of the number of VLANs. This implementation reduces CPU load since only one Spanning Tree instance is maintained for the entire network. This implementation can be used when only one Layer 2 topology is needed in the network

So in summary (indirect BSCI joke attempt) with CST, the root bridge is the same for all VLANs. The weakness in this design is if the CST root bridge device goes down, there's going to be a new STP re-election for a new root bridge which may become a major disruption for all other switches and hosts in the LAN, since all VLANs are affected. However, PVST and PVST+ both reduce the STP re-election for root bridge disruption because the unaffected VLANs are left alone in their current stable and converged state and only the affected VLANs are involved in the PVST or PVST+ root bridge re-election. Does this help understand PVST and PVST+ better?

Source:

1. Per VLAN Spanning Tree (PVST) - http://www.cisco.com/en/US/tech/tk389/tk621/tk846/tsd_technology_support_sub-protocol_home.html
2. Per VLAN Spanning Tree Plus (PVST+) - http://www.cisco.com/en/US/tech/tk389/tk621/tk847/tsd_technology_support_sub-protocol_home.html
3. Common Spanning Tree (CST) - http://www.cisco.com/en/US/tech/tk389/tk621/tk868/tsd_technology_support_sub-protocol_home.html

Mrock4

I think he was more of referring to the operation of an ISL or 802.1Q trunk in relation to PVST/PVST+, not so much what PVST/PVST+ are..but good links.