ZyXEL IES-612-51A User Manual download pdf (Page 176)

Chapter 22 Spanning Tree Protocol

IES-612-51A User’s Guide

176

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.

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

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.

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ZyXEL IES-612-51A User Manual Page 176