Extreme Networks ExtremeWare XOS 10.1, Manual

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154 ExtremeWare XOS 10.1 Concepts Guide
Spanning Tree Protocol (STP)
Designated Port Rapid Behavior
When a port becomes a new designated port, or the STP priority changes on an existing designated
port, the port becomes an unsynced designated port. In order for an unsynced designated port to rapidly
move into the forwarding state, the port must propose a confirmation of its role on the attached LAN
segment, unless the port is an edge port. Upon receiving an “agree” message, the port immediately
enters the forwarding state.
If the receiving bridge does not agree and it has a superior STP priority, the receiving bridge replies
with its own BPDU. Otherwise, the receiving bridge keeps silent and the proposing port enters the
forwarding state and starts the forward delay timer.
The link between the new designated port and the LAN segment must be a point-to-point link. If there
is a multi-access link, the “propose” message is sent to multiple recipients. If only one of the recipients
agrees with the proposal, it is possible for the port to erroneously enter the forwarding state after
receiving a single “agree” message.
Receiving Bridge Behavior
The receiving bridge must decide whether or not to accept a proposal from a port. Upon receiving a
proposal for a root port, the receiving bridge:
• Processes the BPDU and computes the new STP topology
• Synchronizes all of the designated ports if the receiving port is the root port of the new topology
• Puts all unsynced, designated ports into the blocking state
• Sends down further “propose” messages
• Sends back an “agree” message through the root port
If the receiving bridge receives a proposal for a designated port, the bridge replies with its own BPDU.
If the proposal is for an alternate or backup port, the bridge keeps silent.
Propagating Topology Change Information
When a change occurs in the topology of the network, such events are communicated through the
network.
In an RSTP environment, only non-edge ports entering the forwarding state cause a topology change. A
loss of network connectivity is not considered a topology change; however, a gain in network
connectivity needs to be communicated. When an RSTP bridge detects a topology change, it starts the
topology change timer, sets the topology change flag on its BPDUs, floods all of the forwarding ports in
the network (including the root ports), and flushes the learned MAC address entries.
Rapid Reconvergence
This section describes the RSTP rapid behavior following a topology change. In this example, the bridge
priorities are assigned based on the order of their alphabetical letters; bridge A has a higher priority
than bridge F.
Suppose we have a network, as shown in Figure 16, with six bridges (bridge A through bridge F) where
the following is true:
• Bridge A is the root bridge.
• Bridge D contains an alternate port in the blocking state.