Receiving Bridge Behavior; Rapid Reconvergence - Extreme Networks ExtremeWare XOS Guide Manual

Spanning Tree Protocol

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 must be communicated. When an RSTP bridge detects a topology change, that bridge 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
the following is true:
Bridge A is the root bridge.
●
Bridge D contains an alternate port in the blocking state.
●
All other ports in the network are in the forwarding state.
●
Figure 34: Initial network configuration
A
A , 0
F
A , 1
Designated
port
ExtremeWare XOS 11.1 Concepts Guide
Figure 34, with six bridges (bridge A through bridge F) where
B
A , 1
E
A , 2
Root Blocked
port port
C
A , 2
D
A , 3
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