Spanning Tree Basics
15-12 Configuring Spanning Tree
Identifying Designated, Alternate, and Backup Port Roles
Ports in a Spanning Tree configuration are assigned one of four roles: root, designated, alternate,
or backup. Figure 15-6 presents an overview of Spanning Tree port roles.
Figure 15-6 Spanning Tree Port Role Overview
All ports which act as edge ports take on the designated port role. If the bridge has been elected
root, all ports connected to ports on other bridges are also designated ports.
On non-root bridges, Spanning Tree identifies ports which provide a path to the root bridge and
selects the best path among these as the root port as described in “Paths to Root” on page 15-10
(Figure 15-6, call out 2). There may be only a single port providing a path to root, in which case
that is the root port and the remaining ports are designated. If there are other ports providing a
path to root, these ports are selected as alternate paths. Should the root port become disabled, one
of the alternate ports will be selected as the new root port. (Figure 15-6, call out 1)
A port which is not a designated port, but is connected to another port on the same bridge
(Figure 15-6, call out 5) or connected to a shared LAN on which this bridge already provides a
1 Port 1, Bridge A, Alternate Port 5 Port 5, Bridge A, Backup Port
2 Port 2, Bridge A, Root Port 6 Port 6, Bridge A, Backup Port
3 Port 3, Bridge A, Designated Port 7 Port 7, Bridge A, Designated Port
4 Port 4, Bridge A, Designated Port 8 Port 1, Bridge B, Root Port
Blocked Port
Cost 10 Cost 5
ROOT Bridge
Bridge A
Bridge B
KEY:
Network
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