ZyXEL Communications AAM1212, Руководство пользователя

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AAM1212 User’s Guide
128 Chapter 14 Spanning Tree Protocol
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 62 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
STATE STP PORT
STATE
DESCRIPTION
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.