Per VPLS instance, a preferred STP variant can be configured. The STP variants supported are:
While the 7450 ESS, 7750 SR, or 7950 XRS initially use the mode configured for the VPLS, it dynamically falls back (on a per-SAP basis) to STP (IEEE 802.1D-1998) based on the detection of a BPDU of a different format. A trap or log entry is generated for every change in spanning tree variant.
Some older 802.1w compliant RSTP implementations may have problems with some of the features added in the 802.1D-2004 standard. Interworking with these older systems is improved with the comp-dot1w mode. The differences between the RSTP mode and the comp-dot1w mode are:
The RSTP mode implements the improved convergence over shared media feature; for example, RSTP transitions from discarding to forwarding in 4 seconds when operating over shared media. The comp-dot1w mode does not implement this 802.1D-2004 improvement and transitions conform to 802.1w in 30 seconds (both modes implement fast convergence over point-to-point links).
In the RSTP mode, the transmitted BPDUs contain the port's designated priority vector (DPV) (conforms to 802.1D-2004). Older implementations may be confused by the DPV in a BPDU and may fail to recognize an agreement BPDU correctly. This would result in a slow transition to a forwarding state (30 seconds). For this reason, in the comp-dot1w mode, these BPDUs contain the port's port priority vector (conforms to 802.1w).
The 7450 ESS, 7750 SR, and 7950 XRS support two BPDU encapsulation formats, and can dynamically switch between the following supported formats (on a per-SAP basis):
IEEE 802.1D STP
Cisco PVST