ZyXEL Communications ES3500 Series Table, VALUE, RANGE, RANGE, ALLOWED, LINK SPEED, PORT STATE, RECOMMENDED, RECOMMENDED, DESCRIPTION, STP Path Costs, Table 32 STP Port States

Chapter 13 Spanning Tree Protocol

Table 31	STP Path Costs
		LINK SPEED	RECOMMENDED	RECOMMENDED	ALLOWED
			VALUE	RANGE	RANGE
Path Cost		1Gbps	4	3 to 10	1 to 65535
Path Cost		10Gbps	2	1 to 5	1 to 65535

On each bridge, the bridge communicates with the root through the root port. The root port is the port on this Switch with the lowest path cost to the root (the root path cost). If there is no root port, then this Switch has been accepted as the root bridge of the spanning tree network.

For each LAN segment, a designated bridge is selected. This bridge has the lowest cost to the root among the bridges connected to the LAN.

13.1.2 How STP Works

After a bridge determines the lowest cost-spanning tree with STP, it enables the root port and the ports that are the designated ports for connected LANs, and disables all other ports that participate in STP. Network packets are therefore only forwarded between enabled ports, eliminating any possible network loops.

STP-aware switches exchange Bridge Protocol Data Units (BPDUs) periodically. When the bridged LAN topology changes, a new spanning tree is constructed.

Once a stable network topology has been established, all bridges listen for Hello BPDUs (Bridge Protocol Data Units) transmitted from the root bridge. If a bridge does not get a Hello BPDU after a predefined interval (Max Age), the bridge assumes that the link to the root bridge is down. This bridge then initiates negotiations with other bridges to reconfigure the network to re-establish a valid network topology.

13.1.3 STP Port States

STP assigns five port states to eliminate packet looping. A bridge port is not allowed to go directly from blocking state to forwarding state so as to eliminate transient loops.

Table 32 STP Port States

PORT STATE	DESCRIPTION
Disabled	STP is disabled (default).
Blocking	Only configuration and management BPDUs are received and processed.
Listening	All BPDUs are received and processed.
	Note: The listening state does not exist in RSTP.
Learning	All BPDUs are received and processed. Information frames are submitted to the
	learning process but not forwarded.
Forwarding	All BPDUs are received and processed. All information frames are received and
	forwarded.

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ES3500 Series User’s Guide