Port Roles And The Active Topology; Table 6-3 Port State Comparison - Cisco ONS 15454 Software Feature And Configuration Manual

Port Roles and the Active Topology

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
Switch" section on page
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•
•
•
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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.
IEEE 802.1D and RSTP port states.
Table 6-3
Operational Status
Enabled
Enabled
Enabled
Enabled
Disabled
STP edge ports are bridge ports that do not need STP enabled, where loop protection is not needed out
Caution
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,
of discarding. Designated ports start in the listening state.
Cisco ONS 15454 SONET/SDH ML-Series Multilayer Ethernet Card Software Feature and Configuration Guide, R4.0
6-10
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 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.
Port State Comparison
STP Port State
Blocking
Listening
Learning
Forwarding
Disabled
Chapter 6
Table 6-3 provides a comparison of
Is Port Included in the
RSTP Port State Active Topology?
Discarding No
Discarding No
Learning Yes
Forwarding Yes
Discarding No
Table 6-3 describes the port state as blocking instead
Configuring STP and RSTP
"Election of the Root
January 2004