MST with PVST+

STP was implemented to provide a loop-free connectivity at layer 2 network and Cisco has its own implementation standard which is known as Rapid PVST+, Which is default on most the newer versions including Nexus.

MST regions are connected to other domains - Per VLAN Spanning Tree Plus (PVST+) or rapid-PVST+, switches that run PVST+ (or rapid) cannot process MST-type Bridge Protocol Data Units (BPDUs). For this reason, there must be a backward-compatibility mechanism that runs so that these two domains can interact with each other seamlessly. This is what PVST simulation addresses and achieves.

This simulation must be run only on boundary ports - these are ports that are directly connected to the PVST+ domain switches. The receipt of a Shared Spanning Tree Protocol (SSTP) BPDU on the port of a switch that runs MST causes the PVST simulation mechanism to trigger.

However, to prevent an accidental connection to a switch that does not run MST as the default STP mode, we may want to disable this automatic feature.

If we disable PVST simulation, the MST-enabled port moves to the blocking state once it detects it is connected to a Rapid PVST+-enabled port. This port remains in the inconsistent state until the port stops receiving BPDUs, and then the port resumes the normal STP transition process.

VLAN 1's BPDUs are used to represent the entire PVST+ region, and IST (MST instance 0) repesents the MST region side using PVST Simulation.

For PVST simulation to work without failures, these two conditions must be met:

• ·      If the root bridge for CIST is within a non-MST region, the spanning-tree priority of VLANs 2 and above within that domain must be better (lesser) than that of VLAN 1.
• ·      If the root bridge for CIST is within a MST region, VLANs 2 and above defined in the non-MST domains must have their spanning-tree priorities worse (greater) than that of the CIST root.

If these conditions are not met, the boundary port is put into a PVST simulation inconsistent state until the problem is corrected.

PVST simulation is run on boundary ports and works in two ways:

• ·      If the MST region has the root bridge for CIST, PVST simulation is required in order to replicate instance 0 information, and create one BPDU for every VLAN that is allowed across the trunk and tag it with the appropriate VLAN information.
• ·      If the root bridge for CIST is outside of the MST region, then PVST simulation is required to process VLAN 1 information only. The other BPDUs (VLANs 2 and above) are used for consistency checks and information from these VLANs is never copied as root bridge information.