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Cisco Catalyst Blade Switch 3130 for Dell Software Configuration Guide
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Chapter18 Configuring MSTP
Understanding RSTP
•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 con ne cte d in a l o opbac k 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 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 stat e w hile a ll a ltern at e a nd bac kup
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. Table 18-2 provides a comp aris on 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.
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 conn ec te d thr ou gh
point-to-point links as follows:
•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, a nd you sh ould e nab le it on ly on port s
that connect to a single end station.
•Root ports—If the RSTP selects a new root port, it blocks the ol d roo t port and i mmed iat ely
transitions the new root port to the forwarding state.
•Point-to-point links—If you connect a port to another port through a point-to-point link and the local
port becomes a designated port, it negotiates a rapid transition w ith the o th er p ort by us ing the
proposal-agreement handshake to ensure a loop-free topology.
As shown in Figure 18-4, Switch A is connected to Switch B through a point-to-point link, and all
of the ports are in the blocking state. Assume that the priority of Switch A is a smaller numerical
value than the priority of Switch B. Switch A sends a proposal message (a configuration BPDU with
the proposal flag set) to Switch B, proposing itself as the designated switch.
After receiving the proposal message, Switch B selects as its new root port the port from which the
proposal message was received, forces all nonedge ports to the bloc king stat e, an d s en ds an
agreement message (a BPDU with the agreement flag set) through its new root port.
Table18-2 Port State Comparison
Operational Status STP Port State
(IEEE802.1D) 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