ZyXEL Communications MSAP2000 manual STP Root Ports and Designated Ports, Rstp Port States

MSAP2000 AAMS User’s Guide

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

Figure 45 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 following table). A device port is not allowed to go directly from blocking state to forwarding state so as to eliminate transient loops.

Table 34 RSTP Port States

RSTP PORT	STP PORT	DESCRIPTION
STATE	STATE
Discarding	Disabled	STP is disabled (default).
Discarding	Blocking	Only configuration and management BPDUs are received and
		processed.
Discarding	Listening	All BPDUs are received and processed

101