ZyXEL Communications IES-708-22A - Manual (Page 170)

Chapter 23 Spanning Tree Protocol
IES-708-22A User’s Guide
170
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 80 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
50 on page 170 ). A device port is not allowed to go directly from blocking state to forwarding
state so as to eliminate transient loops.
Table 50 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.