ZyXEL Communications IES-612-51A manual 176

Chapter 22 Spanning Tree Protocol

Figure 82 STP Root Ports and Designated Ports

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

In RSTP, the devices send BPDUs every Hello Time. If an RSTP-aware device does not get a Hello BPDU after three Hello Times pass (or the Max Age), the device assumes that the link to the neighboring bridge is down. This device then initiates negotiations with other devices to reconfigure the network to re-establish a valid network topology.

In STP, once a stable network topology has been established, all devices listen for Hello BPDUs transmitted from the root bridge. If an STP-aware device does not get a Hello BPDU after a predefined interval (Max Age), the device assumes that the link to the root bridge is down. This device then initiates negotiations with other devices to reconfigure the network to re-establish a valid network topology.

RSTP assigns three port states to eliminate packet looping while STP assigns five (see Table 49 on page 176). A device port is not allowed to go directly from blocking state to forwarding state so as to eliminate transient loops.

Table 49 RSTP Port States

RSTP PORT STATE	STP PORT STATE	DESCRIPTION
Discarding	Disabled	RSTP or STP is disabled (default).
Discarding	Blocking	In RSTP, BPDUs are discarded.
		In STP, only configuration and management
		BPDUs are received and processed.
Discarding	Listening	In RSTP, BPDUs are discarded.
		In STP, all BPDUs are received and processed.
Learning	Learning	All BPDUs are received and processed.
		Information frames are submitted to the learning
		process but not forwarded.
Forwarding	Forwarding	All BPDUs are received and processed. All
		information frames are received and forwarded.

See the IEEE 802.1w standard for more information on RSTP. See the IEEE 802.1D standard for more information on STP.

176
	IES-612-51A User’s Guide