BigIron RX Series Configuration Guide 33353-1001810-01PVST/PVST+ compatibility 12Overview of PVST and PVST+Per VLAN Spanning Tree (PVST) is a Cisco proprietary protocol that allows a Cisco device to havemultiple spanning trees. The Cisco device can interoperate with spanning trees on other PVSTdevices but cannot interoperate with IEEE 802.1Q devices. An IEEE 802.1Q device has all its portsrunning a single spanning tree. PVST+ is an extension of PVST that allows a Cisco device to alsointeroperate with devices that are running a single spanning tree (IEEE 802.1Q).The PVST+ support allows the device to interoperate with PVST spanning trees and the IEEE802.1Q spanning tree at the same time.IEEE 802.1Q and PVST regions cannot interoperate directly but can interoperate indirectly throughPVST+ regions. PVST BPDUs are tunneled through 802.1Q regions, while PVST BPDUs for VLAN 1(the IEEE 802.1Q VLAN) are processed by PVST+ regions. Figure 31 shows the interaction of IEEE802.1Q, PVST, and PVST+ regions.FIGURE 31 Interaction of IEEE 802.1Q, PVST, and PVST+ regionsVLAN tags and dual modeTo support the IEEE 802.1Q (Common Spanning Tree) portion of PVST+, a port must be a memberof VLAN 1. Cisco devices always use VLAN 1 to support the IEEE 802.1Q portion of PVST+.For the port to also support the other VLANs (the PVST+ VLANs) in tagged mode. The port must bea dual-mode port.The untagged frames are supported on the port’s native VLAN. By default, the native VLAN is thesame as the device’s default VLAN1, which by default is VLAN 1. Thus, to support IEEE 802.1Q in atypical configuration, the port must be able to send and receive untagged frames for VLAN 1 andtagged frames for the other VLANs.1. Cisco PVST/PVST+ documentation refers to the Default VLAN as the Default Native VLAN.PVST+Region IEEE 802.1Q Region PVST+RegionPVST RegionPVST BPDUs(over ISL trunks)PVST BPDUs(over ISL trunks)Do notconnectdual modeportdual modeport802.1D BPDUs 802.1D BPDUsPVST BPDUs tunneled throughthe IEEE 802.1Q region
