Cisco Systems ONS 15310-CL, ONS 15310-MA Port Roles and the Active Topology, Is Port Included

Chapter 7 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 determining 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 7-3. 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 through 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 together in a loopback by a point-to-point link or when a switch has two or more connections 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 port with the alternate or backup port role is excluded from the active topology.

In a stable topology with consistent port roles throughout the network, the RSTP ensures that every root port and designated port immediately transition to the forwarding state while all alternate and backup ports are always in the discarding state (equivalent to blocking in IEEE 802.1D). The port state controls the operation of the forwarding and learning processes. Table 7-3provides a comparison of

IEEE 802.1D and RSTP port states.

Table 7-3	Port State Comparison
				Is Port Included in the
Operational Status		STP Port State	RSTP Port State	Active Topology?
Enabled		Blocking	Discarding	No
Enabled		Listening	Discarding	No
Enabled		Learning	Learning	Yes
Enabled		Forwarding	Forwarding	Yes
Disabled		Disabled	Discarding	No

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-numberspanning-disabledon 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, Table 7-3describes the port state as blocking instead of discarding. Designated ports start in the listening state.

Cisco ONS 15310-CL, ONS 15310-MA, and ONS 15310-MA SDH Ethernet Card Software Feature and Configuration Guide, R9.1 and R9.2

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