Rapid Convergence - Cisco WS-SUP32-GE-3B - Supervisor Engine 32 Software Configuration Manual

Chapter 17 Configuring STP and MST
Table 17-3
Operational Status
Enabled
Disabled
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 connected through
point-to-point links as follows:
•
•
•
OL-11439-03
Port State Comparison (continued)
STP Port State
(IEEE 802.1D)
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.
Root ports—If the RSTP selects a new root port, it blocks the old root port and immediately
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 with the other port by using the
proposal-agreement handshake to ensure a loop-free topology.
As shown in Figure 17-8, 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 blocking state, and sends an
agreement message (a BPDU with the agreement flag set) through its new root port.
After receiving switch B's agreement message, switch A also immediately transitions its designated
port to the forwarding state. No loops in the network are formed because switch B blocked all of its
nonedge ports and because there is a point-to-point link between switches A and B.
When switch C is connected to switch B, a similar set of handshaking messages are exchanged.
Switch C selects the port connected to switch B as its root port, and both ends immediately transition
to the forwarding state. With each iteration of this handshaking process, one more switch joins the
active topology. As the network converges, this proposal-agreement handshaking progresses from
the root toward the leaves of the spanning tree.
The switch learns the link type from the port duplex mode: a full-duplex port is considered to have
a point-to-point connection and a half-duplex port is considered to have a shared connection. You
can override the default setting that is controlled by the duplex setting by using the spanning-tree
link-type interface configuration command.
Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY
Understanding How IEEE 802.1w RSTP Works
Is Port Included in the
RSTP Port State Active Topology?
Forwarding Yes
Discarding No
17-13