Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring RSTP (Rev. 03)
Rapid Spanning Tree Protocol (RSTP)
© 2008 Foundry Networks, Inc.
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In a stable topology with consistent port roles throughout the network, the RSTP ensures that the
immediate transition of every root port and designated port to the forwarding state and the alternate
and backup ports are all kept in the discarding state (equivalent to the blocking state in STP). The
operation of the forwarding and learning processes are controlled by the port state.
Rapid Recovery and Convergence
If a device, a port or a LAN fails, RSTP provides for a rapid recovery of network connectivity. As
shown in
Table 2
, edge ports, new root ports, and ports connected through point-to-point links
converge rapidly.
Table 2: The RSTP Rapid Convergence
Port Type
Description
Edge ports
When a port is configured as an edge port on an RSTP switch by using the
rapid-spanning-tree edge-port
command in Interface Configuration mode, it
immediately transitions to the forwarding state. Note that only ports that
connect to a single end station should be configured as edge ports.
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 the user connects 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.
Figure 1
shows an example for proposal-and-agreement handshaking for rapid convergence. In this
example, switches A and B are connected through a point-to-point link, and all the ports are in
blocking state. The priority of switch A is higher (smaller numerical value) than that of switch B.
The proposal and agreement handshaking proceeds as follows:
Switch A proposes itself as the designated switch by sending a proposal message (a configuration
Bridge Protocol Data Unit (BPDU) with the proposal flag set).
Switch B reacts to Switch A proposal message as follows:
•
It selects the port on which the proposal message was received as its new root port.
•
It forces all non-edge ports to the blocking state.
•
It sends an agreement message to switch A (a BPDU with the agreement flag set) through
its new root port.
Switch A reacts to Switch B agreement message by immediately transitioning its designated port to
the forwarding state. No loops in the network are formed because Switch B has blocked all of its
non-edge ports and because switches A and B are connected by a point-to-point link.
When Switch C is connected to Switch B, RSTP launches the exchange of a similar set of
handshaking messages. Switch C selects the port connected to Switch B as its root port, and both
ends immediately transition to the forwarding state. The same handshaking process is repeated for
each switch that joins the active topology, progressing from the root toward the leaves of the
spanning tree as the network converges.