Port Roles
RSTP uses information from BPDUs to assign port roles for each LAN
segment. Port roles are not user-configurable. Port role assignments are determined
based on the following criteria:
- A unique bridge identifier (MAC address) associated with
each bridge
- The path cost associated with each bridge port
- A port identifier associated with each bridge port
RSTP assigns one of the following port roles to bridge ports in
the network, as described in the following table.
RSTP Port Roles
Port Role |
Description |
Root |
Provides the shortest (lowest)
path cost to the root bridge. Each bridge has only one root port;
the root bridge does not have a root port. If a bridge has two or
more ports with the same path cost, the port with the best port
identifier (lowest MAC address) becomes the root port. |
Designated |
Provides the shortest path
connection to the root bridge for the attached LAN segment. To
prevent loops in the network, there is only one designated port on
each LAN segment. To select the designated port, all bridges that
are connected to a particular segment listen to each other‘s BPDUs
and agree on the bridge sending the best BPDU. The corresponding
port on that bridge becomes the designated port. If there are two or
more ports connected to the LAN, the port with the best port
identifier becomes the designated port. |
Alternate |
Provides an alternate path to
the root bridge and the root port. |
Backup |
Supports the designated port
on the same attached LAN segment. Backup ports exist only when the
bridge is connected as a self-loop or to a shared-media
segment. |
Disabled |
A port in the disabled state
does not participate in RSTP; however, it will forward traffic and
learn new MAC source addresses. |
|
When RSTP stabilizes:
RSTP makes the distinction between the alternate and backup port
roles to describe the rapid transition of the alternate port to the forwarding state
if the root port fails.
To prevent a port from becoming an alternate or backup port, use the
command:
configure stpd stpd_name ports active-role enable port .
To revert to the default that allows a port to be elected to any STP
port role, use the command:
configure stpd stpd_name ports active-role disable port
To view the active-role status, use the command: show stpd
ports.