Port Roles And The Active Topology; Rapid Convergence - Cisco 3845 - Security Bundle Router Software Manual

Chapter 14 Configuring MSTP

Port Roles and the Active Topology

The RSTP provides rapid convergence of the spanning tree by assigning port roles and by learning 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
and BPDUs" section on page
•
•
•
•
•
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 802.1D). The port state controls the
operation of the forwarding and learning processes.
and RSTP port states.
Table 14-2
Operational Status
Enabled
Enabled
Enabled
Enabled
Disabled
To be consistent with Cisco STP implementations, this guide documents the port state as blocking
instead of discarding. Designated ports start in the listening state.

Rapid Convergence

The RSTP provides for rapid recovery of connectivity following the failure of a switch, a switch port, or
a LAN. It provides rapid convergence for edge ports, new root ports, and ports connected through
point-to-point links as follows:
•
OL-23400-01
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 (IEEE
802.1D)
Blocking
Listening
Learning
Forwarding
Disabled
Edge ports—If you configure a port as an edge port on an RSTP switch by using the spanning-tree
portfast interface configuration command, the edge port immediately transitions to the forwarding
state. An edge port is the same as a Port Fast-enabled port, and you should enable it only on ports
that connect to a single end station.
13-2. Then the RSTP assigns one of these port roles to individual ports.
Table 14-2
RSTP Port State
Discarding
Discarding
Learning
Forwarding
Discarding
Cisco ME 3800X and 3600X Switch Software Configuration Guide
Understanding RSTP
"Spanning-Tree Topology
provides a comparison of IEEE 802.1D
Is Port Included in the
Active Topology?
No
No
Yes
Yes
No
14-9