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19.1.4 Configuration components for Spanning Tree topologies
Designing a Spanning Tree topology involves roles for bridges and ports, as well as
parameters used to determine these roles. The following explains usage for these
configuration components and topology designs.
(1) Bridge role
The table below describes bridge roles. Spanning Tree topology design starts with
determining the root bridge.
Table 19-5 Bridge role
Bridge role
Root bridge
Designated bridge
(2) Port role
The table below describes port roles. Ports on designated bridges have three types of roles.
For root bridges, all ports are designated ports.
Table 19-6 Port role
Port role
Root port
Designated port
Non-designated port
(3) Bridge ID
Each parameter that identifies a switch in a topology is called a bridge ID. The switch that
has the lowest bridge ID has the highest priority, and is selected as the root bridge.
Bridge IDs consist of a bridge priority (16 bits) and the bridge MAC address (48 bits). The
lowest 12 bits of a bridge priority is the extended system ID. For an extended system ID, 0
is set for Single Spanning Tree or Multiple Spanning Tree, and the VLAN ID is set for
PVST+. The following figure shows a bridge ID.
Figure 19-2 Bridge ID
Overview
The switch at the logical center of a built topology. There can only be one within
a topology.
A switch other than the root bridge for forwarding frames from the root bridge.
Overview
A port for a communication path from a designated bridge to the root bridge.
This port allows communication.
A port, other than the root port, for which communication is possible. It allows
communication downstream from the root bridge to other ports in the topology.
A port other than a root port or designated port, for which communication is not
possible. It serves as an alternate path when a fault occurs.
19 Spanning Tree Protocol
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