Dell BigIron RX Series, Configuration Manual

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374 BigIron RX Series Configuration Guide
53-1002253-01
Convergence in a simple topology
13
NOTE
The rapid convergence will not occur on ports connected to shared media devices, such as hubs. To
take advantage of the rapid convergence provided by RSTP, make sure to explicitly configure all
point-to-point links in a topology.
Convergence at start up
In Figure 53 , 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 53
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 port 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 RSTP 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.
Next, Switch 1 is powered up ( Figure 54 ).
Port3
Designated
port
Port3
Root port
Bridge priority = 1500
Routing Switch 2
Routing Switch 3
Bridge priority = 2000