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Cisco ONS 15310-CL and Cisco ONS 15310-MA Ethernet Card Software Feature and Configuration Guide R8.5
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Chapter 6 Configuring STP and RSTP on the ML-Series Card
RSTP Features
Port Roles and the Active Topology
The RSTP provides rapid convergence of the spanning tree by assigning port roles and by de termining
the active topology. The RSTP builds upon the IEEE 802.1D STP to select the switch with the highest
switch priority (lowest numerical priority value) as the root switch as described in the “Election of the
Root Switch” section on page 6-3. Then the RSTP assigns one of these port roles to individual ports:
• Root port—Provides the best path (lowest cost) when the switch forwards packets to the root switch.
• Designated port—Connects to the designated switch, which incurs the lowest path cost when
forwarding packets from that LAN to the root switch. The port th rough which the designated switch
is attached to the LAN is called the designated port.
• Alternate port—Offers an alternate path toward the root switch to that provided by the current root
port.
• Backup port—Acts as a backup for the path provided by a designated port toward the leaves of the
spanning tree. A backup port can exist only when two ports are connected toge ther in a loopback by
a point-to-point link or when a switch has two or more co nnections to a shared LAN segment.
• Disabled port—Has no role within the operation of the spanning tree.
A port with the root or a designated port role is included in the active topology. A p ort with the alternate
or backup port role is excluded from the active topology.
In a stable topology with consistent port roles throughout the net work, the RSTP ensures that every root
port and designated port immediately transition to the forwar ding state while all alternate and backup
ports are always in the discarding state (equivalent to blocking in IEEE 802.1D). The port state co ntrols
the operation of the forwarding and learning processes. Ta ble 6-3 provides a comparison of
IEEE 802.1D and RSTP port states.
Caution STP edge ports are bridge ports that do not need STP enabled, where loop protection is not needed out
of that port or an STP neighbor does not exist out of that port. For RSTP, it is important to disable STP
on edge ports, which are typically front-side Ethernet ports, using the command bridge
bridge-group-number spanning-disabled on the appropriate interface. If RSTP is not disabled on edge
ports, convergence times will be excessive for packets traversing those ports.
Note To be consistent with Cisco STP implementations, Tabl e 6- 3 describes the port state as blocking instead
of discarding. Designated ports start in the listening state.
Table 6-3 Port State Comparison
Operational Status STP Port State RSTP Port State Is Port Included in the
Active Topology?
Enabled Blocking Discarding No
Enabled Listening Discarding No
Enabled Learning Learning Yes
Enabled Forwarding Forwarding Yes
Disabled Disabled Discarding No