Rstp Operation - Extreme Networks ExtremeWare XOS Guide Manual

Table 47: Derived timers
Timer
TCN
Topology change
Message age
Hold
Recent backup
Recent root
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 an edge port receives a BPDU, it enters
an inconsistency state. An inconsistency state puts the edge port into the blocking state and starts the
message age timer. Every time the edge port receives a BPDU, the message age timer restarts. The edge
port remains in the blocking state until no further BPDUs are received and the message age timer
expires.
ExtremeWare XOS 11.1 Concepts Guide
Description
The root port uses the topology change notification (TCN) timer when it detects a
change in the network topology. The TCN timer stops when the topology change
timer expires or upon receipt of a topology change acknowledgement. The default
value is the same as the value for the bridge hello timer.
The topology change timer determines the total time it takes the forwarding ports to
send configuration BPDUs. The default value for the topology change timer depends
upon the mode of the port:
• 802.1D mode—The sum of the forward delay timer value (default value is 15
seconds; range of 4 to 30 seconds) and the maximum age timer value (default
value is 20 seconds; range of 6 to 40 seconds).
• 802.1w mode—Double the hello timer value (default value is 4 seconds)
A port uses the message age timer to time out receiving BPDUs. When a port
receives a superior or equal BPDU, the timer restarts. When the timer expires, the
port becomes a designated port and a configuration update occurs. If the bridge
operates in 1w mode and receives an inferior BPDU, the timer expires early. The
default value is the same as the STPD bridge max age parameter.
A port uses the hold timer to restrict the rate that successive BPDUs can be sent.
The default value is the same as the value for the bridge hello timer.
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).
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.
Rapid Spanning Tree Protocol
311