Fujitsu BX600 SB9 manual Combining RAPID-PVST and 802.1w after failure of Po1

White Paper ⏐ Issue: October 2006 ⏐ Integration of BX600 SB9 Switches in Cisco Networks Page 14 / 47

Figure 6 : Combining RAPID-PVST and 802.1w after failure of Po1

Figure 6 shows this scenario. When server 1 now wants to send data to server 2, switch B will send it to switch A via Po3 (as indicated by the MAC address table), which has no connection to the SB9 and will drop the packet. This will not change until either the MAC address table entry times out (after ~300 seconds) or the server SB9 sends a packet that has been seen by switch B, whichever happens first.

This scenario shows that RSTP and RAPID-PVST are not compatible in this respect. A worst-case failover time of 300 sec will not be acceptable.

Running RAPID-PVST on VLAN Trunks while disabling STP at the SB9

When RAPID-PVST is running at the Cisco switches and STP is disabled at the SB9 we have almost the same scenario as above, where the Cisco switches were running STP and STP was disabled at the SB9. Figure 7 shows this scenario.

Cisco A

priority 0 for all vlans

Designated port

forwarding Po1

Po1

SB9

STP disabled

Designated port	Root port
	forwarding
forwarding
Po3		Po3

Po2

Po2

Cisco B

priority 4096 for all vlans

Alternate

discarding

On all trunks:

VLAN 1 native

VLAN 10 tagged

VLAN 20 tagged

Figure 7 : RAPID-PVST while STP is disabled at SB9

When the Po1 link fails, the Po2 of switch B will stop receiving BPDUs. After three times the “hello” interval, the switch will change the state of port Po2 to “learning” and will then follow the normal state machine so that the convergence time is the same as with 802.1D.

Since the RSTP cannot operate with the proposal/agreement mechanism on this link, root changes will also be relatively slow within all the VLANs that are running on the trunks to the SB9.