Owner's Manual

550 | Chapter 18. STP
NETGEAR 8800 User Manual
The protocol migration timer is neither user-configurable nor derived; it has a set value of 3
seconds. The timer starts when a port transitions from STP (802.1D) mode to RSTP (802.1w)
mode and vice-versa. This timer must expire before further mode transitions can occur.
RSTP Operation
In an RSTP environment, a point-to-point link LAN segment has two bridges. A switch that
considers itself the unique, designated bridge for the attached LAN segment sends a
“propose” message to the other bridge to request a confirmation of its role. The other bridge
on that LAN segment replies with an “agree” message if it agrees with the proposal. The
receiving bridge immediately moves its designated port into the forwarding state.
Before a bridge replies with an “agree” message, it reverts all of its designated ports into the
blocking state. This introduces a temporary partition into the network. The bridge then sends
another “propose” message on all of its designated ports for further confirmation. Because all
of the connections are blocked, the bridge immediately sends an “agree” message to unblock
the proposing port without having to wait for further confirmations to come back or without the
worry of temporary loops.
Beginning with the root bridge, each bridge in the network engages in the exchange of
“propose” and “agree” messages until they reach the edge ports. Edge ports connect to
non-STP devices and do not participate in RSTP. Their role does not need to be confirmed. If
you have an 802.1D-2004 compliant edge port, the bridge detection mechanism causes the
edge port to transition to a non-edge port upon receiving a BPDU. If the former edge port
does not receive a subsequent BPDU during a pre-determined interval, the port attempts to
become an edge port.
RSTP attempts to transition root ports and designated ports to the forwarding state and
alternate ports and backup ports to the blocking state as rapidly as possible.
A port transitions to the forwarding state if any of the following is true. The port:
• Has been in either a root or designated port role long enough that the spanning tree
information supporting this role assignment has reached all of the bridges in the network.
Note: RSTP is backward compatible with STP, so if a port does not move to
the forwarding state with any of the RSTP rapid transition rules, a forward delay
timer starts and STP behavior takes over.
Recent backup The timer starts when a port leaves the backup role. When this timer is running,
the port cannot become a root port. The default value is double the hello time
(4
seconds).
Recent root The timer starts when a port leaves the root port role. When this timer is running,
another port cannot become a root port unless the associated port is put into the
blocking state. The default value is the same as the forward delay time.
Table 55. Derived Timers (Continued)
Timer Description