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White Paper Issue: October 2006 Integration of BX600 SB9 Switches in Cisco Networks Page 13 / 47
Rapid Spanning Tree
The standard IEEE 802.1w (RSTP) defines only BPDUs in the native VLAN as implemented by the SB9. Cisco also enhanced
RSTP to RAPID-PVST which is compatible to RSTP in a number of ways. Figure 5 shows this scenario.
Figure 5 : Combining RAPID-PVST and 802.1w
All RSTP features are functioning for the native (in this example VLAN1). Since the SB9 implements the standard, and does not
know about tagged BPDUs, RAPID-PVST has the same restrictions as PVST+.
There is an additional problem due to the fact that RSTP generates a Topology Change Notification (TCN) only when changing
a port to the state “designated”. If the Po1 link in Figure 5 fails, port Po1 of switch A will go down and will not generate a TCN as
specified in 802.1w. SB9 will change the role of port Po2 to root port and its state to “forwarding” and will generate a TCN as
specified in 802.1w on the native VLAN. This has the effect that the Cisco switches will flush their MAC address tables of
VLAN 1 but not for the other VLANs.