Dell N3200-ON Networking N-Series Switches User Guide version 6.8.0 - Page 838

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 21-6. RSTP-PV and IEEE Spanning Tree Interoperability RSTP‐PV IEEE BPDUs for VLAN1 (untagged) IEEE BPDUs for VLAN1 (untagged) SSTP BPDUs for VLAN1 (untagged) SSTP BPDUs for other VLANs (tagged) IEEE 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 RSTPPV 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 838 Spanning Tree Protocol