Extreme Networks ExtremeWare XOS Guide Manual page 158

Spanning Tree Protocol (STP)
6 To complete the topology change:
• Bridge D moves the port that received the agree message into the forwarding state.
• Bridge F confirms that its receiving port (the port that received the "propose" message) is the root
port, and immediately replies with an "agree" message to bridge E to unblock the proposing port.
Figure 22: Completing the topology change
A
A , 0
F
A , 5
Figure 23 displays the new topology.
Figure 23: Final network configuration
A
A , 0
F
A , 5
Compatibility With STP (802.1d)
RSTP interoperates with legacy STP protocols; however, the rapid convergence benefits are lost when
interacting with legacy STP bridges.
Each RSTP bridge contains a port protocol migration state machine to ensure that the ports in the STPD
operate in the correct, configured mode. The state machine is a protocol entity within each bridge
configured to run in 802.1w mode. For example, a compatibility issue occurs if you configure 802.1w
mode and the bridge receives an 802.1d BPDU on a port. The receiving port starts the protocol
migration timer and remains in 802.1d mode until the bridge stops receiving 802.1d BPDUs. Each time
the bridge receives an 802.1d BPDU, the timer restarts. When the port migration timer expires, no more
802.1d BPDUs have been received and the bridge returns to its configured setting, 802.1w mode.
158
B C
A , 1 A , 2
Root Designated
port port
E D
A , 4 A , 3
B C
A , 1 A , 2
Root Designated
port port
E D
A , 4 A , 3
EX_055g
EX_055h
ExtremeWare XOS 10.1 Concepts Guide