Dell PowerConnect B-RX Configuration Manual page 442

13
Convergence in a complex RSTP topology
Next Switch 2 sends RST BPDUs with a proposal flag to Port3/Switch 4. Port3 becomes the Root
port for the bridge; all other ports are given a Designated port role with discarding states.
Port3/Switch 4 sends an RST BPDU with an agreed flag to Switch 2 to confirm that it is the new
Root port. The port then goes into a forwarding state.
Now Port4/Switch 4 receives an RST BPDU that is superior to what it can transmit. The port is then
given an Alternate port role, and remains in discarding state.
Likewise, Port5/Switch 4 receives an RST BPDU that is superior to what it can transmit. The port is
also given an Alternate port role, and remains in discarding state.
Port2/Switch 2 transmits an RST BPDU with a proposal flag to Port2/Switch 1. Port2/Switch 1
becomes the Root port. All other ports on Switch 1 are given Designated port roles with discarding
states.
Port2/Switch 1 sends an RST BPDU with an agreed flag to Port2/Switch 2 and Port2/Switch 1 goes
into a forwarding state.
Port3/Switch 1 receives an RST BPDUs that is inferior to what it can transmit; therefore, the port
retains its Designated port role and goes into forwarding state only after the forward delay timer
expires twice on that port while it is still in a Designated role.
Port3/Switch 2 sends an RST BPDU to Port3/Switch 3 that contains a proposal flag. Port3/Switch
3 becomes the Root port, while all other ports on Switch 3 are given Designated port roles and go
into discarding states. Port3/Switch 3 sends an RST BPDU with an agreed flag to Port3/Switch 2
and Port3/Switch 3 goes into a forwarding state.
Now, Port2/Switch 3 receives an RST BPDUs that is superior to what it can transmit so that port is
given an Alternate port state.
Port4/Switch 3 receives an RST BPDU that is inferior to what it can transmit; therefore, the port
retains its Designated port role.
Ports on all the bridges in the topology with Designated port roles that received RST BPDUs with
agreed flags go into forwarding states instantly. However, Designated ports that did not receive RST
BPDUs with agreed flags must wait until the forward delay timer expires twice on those port. Only
then will these port move into forwarding states.
The entire RSTP topology converges in less than 300 msec and the essential connectivity is
established between the designated ports and their connected root ports.
370 BigIron RX Series Configuration Guide
53-1001810-01