3Com 9100 manual Domains, Stabilization, Reconfiguration

90CHAPTER 6: SPANNING TREE PROTOCOL (STP)

	The Root Bridge generates BPDUs on all ports at a regular interval known
	as the Hello Time. All other bridges in the network have a Root Port. This
	is the port that costs the least in getting to the Root Bridge, and it is used
	for receiving the BPDUs initiated by the Root Bridge.
	Stabilization
	After all bridges on the network have determined the configuration of
	their ports, each bridge only forwards traffic between the Root Port and
	the ports that are the Designated Bridge Ports for each network segment
	to which they are attached. All other ports are blocked, which means that
	they are prevented from forwarding traffic.
	Reconfiguration
	In the event of a network failure (such as a segment going down) the STP
	system reconfigures the network to adjust for the changes. If the
	topology of the network changes, the Root Bridge sends out an SNMP
	trap.
Spanning Tree	The switch can be partitioned into multiple virtual bridges. Each virtual
Domains	bridge can run an independent Spanning Tree instance. Each Spanning
	Tree instance is called a Spanning Tree Domain (STPD). Each STPD has its
	own Root Bridge and active path. Once the STPD is created, one or more
	VLANs can be assigned to it.
	A port can belong to only one STPD. If a port is a member of multiple
	VLANs, then all those VLANs must belong to the same STPD.
	The key points to remember when configuring VLANs and STP are the
	following:
	■ Each VLAN forms an independent broadcast domain.
	■ STP blocks paths to create a loop-free environment.
	■ When STP blocks a path, no data can be transmitted or received on
	the blocked port.
	■ Within any given STPD, all VLANs belonging to it use the same
	spanning tree.