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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.