11-10
C
HAPTER
11: E
THERNET
LAN
S
AND
B
RIDGES
IN
THE
7600 C
ARD
Designated Bridge
on a LAN
Among all bridges on a LAN, the bridge with the shortest path to the root
bridge (through its root port) is selected as the
designated-bridge
on that
LAN.
Spanning Tree port
selection
Each bridge selects its root port and any of its ports (LANs) on which it is
the designated bridge to be in the spanning tree. All other ports are
blocked to data traffic (but not to CBPDUs). Thus, traffic on a LAN flows
only through its designated bridge.
Re-configuring the
Spanning Tree
When a change occurs in the network, such as a new bridge or link or a
device failure, the STP acts to establish a new Spanning Tree
configuration. Until the new Spanning Tree configuration stabilizes, it is
necessary to restrict data packet transmissions in order to avoid
temporary loops. For this purpose the 801.2 standard calls for two
intermediate bridge port
states
: the
listening
state followed by the
learning
state. In the listening state is a wait state in which a port neither
learns station addresses nor forwards data packets. In the learning state a
port learns station addresses but still does not forward data packets. The
management parameter
forward-delay
controls
the amount of time that
a bridge port remains in the listening or the learning state.
Port States
Summing up, a bridge port can be in one of the following port states:
Q
Listening
- When the Spanning Tree begins re-configuring after a
network change, all ports are placed in the listening state. Data
packets received on a port are discarded. After the forward-delay
time, the bridge port enters the learning state, unless set to the
blocking state by the STP.
Q
Learning - Data packets are received for the purpose of learning
station addresses only. After the forward-delay time, the bridge port
enters the forwarding state, unless set to the blocking state by the STP.
Q
Blocking - The bridge continues to send and receive CBPDUs on that
port but does not receive data packets.
Q
Forwarding - The bridge performs standard bridging functions.
Disabling the STP
The STP can be disabled and enabled by management either for the
entire bridge or for specific ports. When disabled, the port or bridge does
not participate in the Spanning Tree algorithm but continues to receive
and transmit data packets.
abtthgde.book Page 10 Tuesday, June 23, 1998 10:29 AM
