STP. Part 3. PVST+

STP. Part 1. STP (802.1D-1998)
STP. Part 2. RSTP (802.1w)
STP. Part 3. PVST+
STP. Part 4. MSTP
STP. Part 5. Bridge Assurance

Nowadays, Cisco uses PVST+, Rapid PVST+ and MSTP:

SW2(config)#spanning-tree mode ?
  mst         Multiple spanning tree mode
  pvst        Per-Vlan spanning tree mode
  rapid-pvst  Per-Vlan rapid spanning tree mode

At first, there was a Per-Vlan Spanning Tree (PVST), then it was modified to PVST+.

To the question “what is difference between PVST and PVST+” Scott Morris said:
“…keep in mind that these “enhancement” were a pissing contest between Cisco and IEEE.

For a long time, IEEE believed there should be one common spanning tree (CST) and obviously Cisco had come out with ISL trunking and PVST allowing for multiple instances.

When the 802.1Q standard was derived, they mandated a single spanning tree, which if you followed the spec would **** the operation of PVST. So Cisco “improvised” by merely changing the destination address to a different L2 multicast address.

The good thing about that is that in case you had a mixed environment, now any non-Cisco switch receiving a PVST+ BPDU would simply flood it out all available ports for that vlan instead of killing it if it were using the original IEEE multicast address and not in the Native vlan.

Sometimes the history of “why” makes it easier to remember the details of “how”. So PVST doesn’t “not support” 802.1Q, it’s the 802.1Q won’t support PVST.”

IEEE STP
CiscoPress CCNA Self-Study (ICND Exam): Extending Switched Networks with Virtual LANs:
“The 802.1Q standard defines one unique Spanning Tree instance to be used by all VLANs in the network. STP runs on the Native VLAN so that it can communicate with both 802.1Q and non-802.1Q compatible switches. This single instance of STP is often referred to as Common Spanning Tree (CST).”

IEEE BPDUs are sent untagged to the destination MAC address 01:80:C2:00:00:00.

PVST+
CiscoPress CCNA Self-Study (ICND Exam): Extending Switched Networks with Virtual LANs:
“Because a trunk link carries traffic for more than one broadcast domain and switches are typically connected together via trunk links, it is possible to define multiple Spanning Tree topologies for a given network. With PVST+, a root bridge and STP topology can be defined for each VLAN. This is accomplished by exchanging BPDUs for each VLAN operating on the switches.”

PVST+ BPDUs have additional TLV field with the source VLAN number.

PVST+ BPDUs on Native VLAN are sent untagged.
PVST+ BPDUs on other VLANs are send tagged with corresponding VLAN tag.

This TLV field used for detecting inconsistency in Native VLAN configuration.

PVST+ BPDUs’ destination MAC address is 01:00:0C:CC:CC:CD. This address is also called Shared Spanning Tree Protocol (SSTP)-Address.

In addition to PVST+ BPDUs, PVST+ sends regular IEEE STP BPDUs to IEEE reserved multicast MAC address 01:80:C2:00:00:00. These BPDUs contain same information as PVST+ BPDUs on Native VLAN. This makes PVST+ interoperble/compatible with IEEE STP.

PVST+ enabled device on Access port sends regular IEEE STP BPDUs.

There are couple document describing PVST+ interoperability with examples:
– PVST+ Explained
– Troubleshooting Spanning Tree PVID- and Type-Inconsistencies