Fujitsu BX600 SB9 manual Spanning Tree Protocol, Running ST P 802.1D with PVST+ on Vlan Trunks

White Paper ⏐ Issue: October 2006 ⏐ Integration of BX600 SB9 Switches in Cisco Networks Page 11 / 47

2.4 Spanning Tree Protocol

2.4.1Introduction

When the only standard for spanning tree protocols in LANs was STP, as specified in 802.1D, Cisco developed a number of proprietary protocol enhancements. Some of these were adopted into the RSTP standard but others were not. Cisco therefore also modified their RSTP implementation to be compatible with their enhanced STP. Table 3 shows all current STP implementations.

STP 802.1D	STP as specified in 802.1D. Slow convergence, does not	SB9:	conforms to the standard
	support multiple instances for VLAN trunks.	Cisco: supported only on access ports
			not on trunks.
RSTP 802.1w	Rapid STP as specified in 802.1w. Fast convergence, does not	SB9:	conforms to the standard
	support multiple instances for VLAN trunks.	Cisco: supported only on access ports
			not on trunks.
MSTP 802.1s	Multiple Instance STP as specified in 802.1s. Fast convergence,	SB9:	conforms to the standard
	support multiple instances for VLAN trunks	Cisco: conforms to the standard but not
			common in Cisco environments
PVST+	STP as specified in 802.1D with the following enhancements:	Cisco: proprietary solution
	∙ port-fast feature	SB9:	not supported yet
	∙ uplink-fast feature
	∙ backbone-fast features
	∙ spanning tree for each VLAN
	Fast convergence, compatible to 802.1D even on VLAN trunks.
PVST	Like PVST+ but supporting only ISL trunks	Cisco: proprietary solution
RAPID-PVST	RSTP as specified in 802.1w with the following enhancements:	Cisco: proprietary solution
	∙ spanning tree for each VLAN	SB9:	not supported yet
	Fast convergence, compatible to 802.1D even on VLAN trunks.

Table 3 : Spanning tree protocol implementations

When connecting switches without VLAN trunks, PVST+ and STP are compatible with RSTP and RAPID-PVST respectively without any problems. Other combinations are discussed in the following section.

Running ST P 802.1D with PVST+ on VLAN Trunks

When running STP over VLAN trunks, MSTP is the only STP protocol implemented by Cisco that completely complies with the IEEE standard. This is unfortunately not usually used in datacenter networks, where PVST+ and RAPID-PVST are more common.

Unlike 802.1D, in which only one STP instance is used to control the STP state of the trunk, PVST+ runs one STP instance per VLAN, sends BPDUs and maintains one STP state per VLAN on a trunk. In addition to this major deviation from the standard, Cisco added a number of minor changes, such as the port-fast, uplink-fast and backbone-fast features, which have only local effects and do not limit their interoperability.

PVST+ is also compatible to STP as specified in 802.1D when there is a native VLAN on the trunk. Figure 3 shows a scenario in which two Cisco switches are running PVST+ and an SB9 is running STP as specified in 802.1D.

Figure 3 : Combining PVST+ and 802.1D

Switch A is configured as root bridge, while switch B will take over the root role when A fails. Since switch A sends untagged BPDUs from VLAN 1 to Po1, the SB9 uses Po1 as root port. Po2 of SB9 will take on port role “alternate” and will be in the state “discarding” and will not send any BPDUs at this port. Switch B will therefore also set its port Po2 to “designated” and “forwarding”. The SB9 takes all decisions as indicated by the BPDUs in VLAN 1, and all other BPDUs will be ignored. It is