ZyXEL Communications IES-612-51A User Manual

Chapter 22 Spanning Tree Protocol

IES-612-51A User’s Guide

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 

). A device port is not allowed to go directly from blocking state to forwarding 

state so as to eliminate transient loops.

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

Table 49   RSTP Port States

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.