Rapid Convergence - Cisco 7604 Configuration Manual

Chapter 19 Configuring Standard-Compliant IEEE MST

Rapid Convergence

The RSTP provides for rapid recovery of connectivity following the failure of a router, a router port, or
a LAN. It provides rapid convergence for edge ports, new root ports, and ports connected through
point-to-point links as follows:
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Edge ports—If you configure a port as an edge port on an RSTP router 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 19-4, router A is connected to router B through a point-to-point link, and all of
the ports are in the blocking state. Assume that the priority of router A is a smaller numerical value
than the priority of router B. Router A sends a proposal message (a configuration BPDU with the
proposal flag set) to router B, proposing itself as the designated router.
After receiving the proposal message, router 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 router B's agreement message, router A also immediately transitions its designated
port to the forwarding state. No loops in the network are formed because router B blocked all of its
nonedge ports and because there is a point-to-point link between routers A and B.
When router C is connected to router B, a similar set of handshaking messages are exchanged.
Router C selects the port connected to router B as its root port, and both ends immediately transition
to the forwarding state. With each iteration of this handshaking process, one more router joins the
active topology. As the network converges, this proposal-agreement handshaking progresses from
the root toward the leaves of the spanning tree.
The router 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.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
Understanding RSTP
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