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Configuring the Spanning Tree Protocol 729

Interoperability Between STP-PV and RSTP-PV Modes

STP-PV is derived from 802.1D and RSTP-PV is derived from 802.1w. The

fallback mechanism is the same as between a standard 802.1D switch and a

standard 802.1w switch. When a lower protocol version BPDU is received on

a switch that runs a higher protocol version, the latter falls back to the lower

version after its migration delay timer expires.

For example, an RSTP-PV switch, when connected to STP-PV switch, falls

back to the STP-PV protocol after the migration delay timer expires.

Interoperability With IEEE Spanning Tree Protocols

When a switch configured with RSTP-PV receives IEEE standard RSTP

BPDUs on a port, it responds with two versions of BPDUs on the port: SSTP

formatted BPDUs and IEEE standard STP BPDUs. The IEEE standard

BPDUs are processed by the peer switch running MSTP/RSTP, and the SSTP

format BPDUs are flooded across the MSTP/RSTP domain.

Figure 22-6. RSTP-PV and IEEE Spanning Tree Interoperability

Common Spanning Tree

There are differences between the ways that MSTP and RSTP-PV map

spanning tree instances to VLANs: RSTP-PV creates a spanning tree instance

for each VLAN, and MSTP maps one or more VLANs to each MST instance.

Where an RSTP-PV region is connected to an MSTP region, the set of RSTP-

PV instances does not generally match the set of MST instances. Therefore,

the RSTP-PV region and the MSTP region communicate with each other on a

single common spanning tree instance.

For the MSTP region, the MSTP instance communicates to the RSTP-PV

region using the CIST. For the RSTP-PV region, switches use the VLAN 1

RSTP-PV instance as the common spanning tree. On the link between the

RSTP-PV IEEE

IEEE BPDUs for VLAN1 (untagged)

SSTP BPDUs for VLAN1 (untagged)

SSTP BPDUs for other VLANs (tagged)

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