NETGEAR M4200 Software Administration Manual page 509

If you enable PVSTP or PVRSTP on a switch, all other spanning tree modes on the switch
become disabled. The difference between Multiple Spanning Tree Protocol (MSTP) and
PVSTP or PVRSTP lies primarily in the way that the protocol maps spanning tree instances
to VLANs: PVSTP or PVRSTP creates a spanning tree instance for each VLAN, whereas
MSTP maps one or more VLANs to each Multiple Spanning Tree (MST) instance.
If a switch that runs PVRSTP receives PVSTP Bridge Protocol Data Units (BPDUs), the
switch falls back from PVRSTP to PVSTP after its migration-delay timer expires.
A switch that runs PVSTP or PVRSTP transmits IEEE spanning tree BPDUs along with
Shared Spanning Tree Protocol (SSTP) BPDUs. The SSTP BPDU are transmitted as
untagged packets on an access or native VLAN and transmitted as tagged packets on other
VLANs. If a switch that runs PVSTP or PVRSTP receives IEEE spanning tree BPDUs, the
switch include them in an access VLAN instance or native VLAN instance.
The Per VLAN Spanning Tree (PVST) behavior is as follows:
• An access port sends IEEE spanning tree BPDUs.
• A trunk port sends IEEE spanning tree BPDUs and SSTP BPDUs on the native VLAN.
For other VLANs, the trunk port transmits SSTP BPDUs as tagged packets with the
respective VLAN. If the trunk port receives IEEE spanning tree BPDUs, the received
BPDUs are processed by the instance that is mapped to the native VLAN. The SSTP
BPDUs are processed by instances to which the respective VLANs are mapped.
If a switch that is running an IEEE standard spanning tree protocol (such as STP, RSTP, or
MSTP) receives SSTP BPDUs, the switch does not treat them as standard BPDUs. Instead,
the incoming SSTP BPDUs are flooded to all the ports of the corresponding VLAN. As a
comparison, incoming STP BPDUs are multicasted over the region.
A switch that runs an IEEE standard spanning tree protocol uses its Common and Internal
Spanning Tree (CIST) to communicate with a switch that runs PVSTP or PVRSTP. On the
other hand, a switch that runs PVSTP or PVRSTP uses an access VLAN instance or native
VLAN instance to communicate with a switch that runs an IEEE standard spanning tree
protocol.
PVRSTP embeds support for the FastUplink feature to speed up the selection of a new root
and the FastBackbone feature to speed up the selection of indirect ports. You do not need to
configure these features for RVRSTP. However, for PVSTP, you need to configure these
features:
• FastUplink. This feature allows for a quick selection of a port with the lowest cost after
the root port fails. That is, FastUplink reduces the converge time after a link fails. This
feature is similar to Cisco's UplinkFast feature. If the primary link fails, FastUplink creates
an alternate path immediately, speeding up the transition from the failed primary link to
the backup link.
• FastBackbone. This feature allows for faster convergence time when an indirect link to
the root fails. If a root port or blocked port receives an inferior BPDU from the designated
switch, the switch acts as if an indirect link to the root failed. To speed up the
convergence time, the switch expires the maximum age timer immediately and forces the
port through the Listening and Learning states.
Managed Switches
STP
509