340
Configuring MSTP
Information About Configuring MSTP
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:
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 4 on page 9, 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; 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.
To Next Page
Loading...
Need help?
General
