Foundry Configuration Guide for the FESX, FSX, and FWSX
7 - 34 © Foundry Networks, Inc. December 2005
Convergence in a Simple Topology
The examples in this section illustrate how 802.1W convergence occurs in a simple Layer 2 topology at start-up.
NOTE: The remaining examples assume that the appropriate handshake mechanisms occur as port roles and
states change.
Convergence at Start Up
In Figure 7.14, two bridges Switch 2 and Switch 3 are powered up. There are point-to-point connections between
Port3/Switch 2 and Port3/Switch 3.
Figure 7.14 Convergence Between Two Bridges
At power up, all ports on Switch 2 and Switch 3 assume Designated port roles and are at discarding states before
they receive any RST BPDU.
Port3/Switch 2, with a Designated role, transmits an RST BPDU with a proposal flag to Port3/Switch 3. A ports
with a Designated role sends the proposal flag in its RST BPDU when they are ready to move to a forwarding
state.
Port3/Switch 3, which starts with a role of Designated port, receives the RST BPDU and finds that it is superior to
what it can transmit; therefore, Port3/Switch 3 assumes a new port role, that of a Root port. Port3/Switch 3
transmits an RST BPDU with an agreed flag back to Switch 2 and immediately goes into a forwarding state.
Port3/Switch 2 receives the RST BPDU from Port3/Switch 3 and immediately goes into a forwarding state.
Now 802.1W has fully converged between the two bridges, with Port3/Switch 3 as an operational root port in
forwarding state and Port3/Switch 2 as an operational Designated port in forwarding state.
Port3
Designated
port
Port3
Root port
Bridge priority = 2000
Bridge priority = 1500
Switch 2
Switch 3
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