Cisco IE 4000, Software Configuration Manual

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Configuring MSTP
Information About Configuring MSTP
If all the legacy switches on the link are RSTP switches, they can process MSTP BPDUs as if they are RSTP BPDUs.
Therefore, MSTP switches send either a Version 0 configuration and TCN BPDUs or Version 3 MSTP BPDUs on a
boundary port. A boundary port connects to a LAN, the designated switch of which is either a single spanning-tree switch
or a switch with a different MST configuration.
RSTP
The RSTP takes advantage of point-to-point wiring and provides rapid convergence of the spanning tree.
Reconfiguration of the spanning tree can occur in less than 1 second (in contrast to 50 seconds with the default settings
in the IEEE 802.1D spanning tree).
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
Configuring STP, page 1 . 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 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 42 on page 339 provides a comparison of IEEE 802.1D and RSTP port states.
To be consistent with Cisco STP implementations, this guide defines the port state as blocking instead of discarding .
Designated ports start in the listening state.
Table 42 EEE 802.1D and RSTP Port States
Operational Status STP Port State
(IEEE 802.1D) 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