Extreme Networks ExtremeWare XOS 11.3, Manual

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Spanning Tree Protocol
ExtremeWare XOS 11.3 Concepts Guide
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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.
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.
●
Is now a root port and no other ports have a recent role assignment that contradicts with its root
port role.
●
Is a designated port and attaches to another bridge by a point-to-point link and receives an “agree”
message from the other bridge port.
●
Is an edge port.
An edge port is a port connected to a non-STP device and is in the forwarding state.
The following sections provide more information about RSTP behavior.
Root Port Rapid Behavior
In Figure 38 , the diagram on the left displays the initial network topology with a single bridge having
the following:
●
Two ports are connected to a shared LAN segment.
●
One port is the designated port.
●
One port is the backup port.
The diagram on the right displays a new bridge that:
●
Is connected to the LAN segment.
●
Has a superior STP bridge priority.
●
Becomes the root bridge and sends a BPDU to the LAN that is received by both ports on the old
bridge.