Spanning Tree Protocol
11-9
transparent bridges the bridged LAN can contain paths on which packets
may loop and multiply, lowering its effective overall bandwidth.
Figure 11-4
Packet looping through redundant bridges
To solve this problem, the IEEE 802.1d Bridging Standard describes the
Spanning Tree Protocol (STP), an algorithm which seeks to establish a
unique path between each pair of LANs in the bridged network by
selectively blocking redundant bridge ports. The resulting loop-free tree
spans
the network: each LAN can access any other LAN. The STP
continuously monitors the tree and, in the event of a failure, acts to
establish a new spanning tree by reactivating blocked ports.
The STP is executed collectively by the bridges themselves by means of
special messages called Configuration Bridge Protocol Data Units
(CBPDUs) continuously sent between them. Using these messages, the
bridges inform the other bridges of their unique
bridge-ID
, and they
calculate the
shortest (lowest cost) path
from themselves to other
locally-connected bridges. Using this information, the STP determines the
following:
Root Bridge
The
root-bridge
is selected among all bridges on the LANs as the one with
the lowest bridge ID. The network manager can influence the algorithm’s
choice of root bridge through the management parameter
bridge-priority
. The bridge-priority is appended as the most significant
part of a bridge ID; the lower its value, the more likely the bridge is to
become the root.
Root Port
Once the root bridge has been determined, each bridge selects the port
with the shortest path from it to the root bridge to be the
root-port
.
abtthgde.book Page 9 Tuesday, June 23, 1998 10:29 AM
