Specifications

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Cisco MWR 2941 Mobile Wireless Edge Router Release 3.5 Software Configuration Guide, Cisco IOS Release 15.1(3)MR
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Chapter 10 Configuring MSTP
Understanding RSTP
The sending switch sets the proposal flag in the RSTP BPDU to propose itself as the designated switch
on that LAN. The port role in the proposal message is always set to the designated port.
The sending switch sets the agreement flag in the RSTP BPDU to accept the previous proposal. The port
role in the agreement message is always set to the root port.
The RSTP does not have a separate topology change notification (TCN) BPDU. It uses the topology
change (TC) flag to show the topology changes. However, for interoperability with 802.1D switches, the
RSTP switch processes and generates TCN BPDUs.
The learning and forwarding flags are set according to the state of the sending port.
Processing Superior BPDU Information
If a port receives superior root information (lower bridge ID, lower path cost, and so forth) than currently
stored for the port, the RSTP triggers a reconfiguration. If the port is proposed and is selected as the new
root port, RSTP forces all the other ports to synchronize.
If the BPDU received is an RSTP BPDU with the proposal flag set, the switch sends an agreement
message after all of the other ports are synchronized. If the BPDU is an 802.1D BPDU, the switch does
not set the proposal flag and starts the forward-delay timer for the port. The new root port requires twice
the forward-delay time to transition to the forwarding state.
If the superior information received on the port causes the port to become a backup or alternate port,
RSTP sets the port to the blocking state but does not send the agreement message. The designated port
continues sending BPDUs with the proposal flag set until the forward-delay timer expires, at which time
the port transitions to the forwarding state.
Processing Inferior BPDU Information
If a designated port receives an inferior BPDU (higher bridge ID, higher path cost, and so forth than
currently stored for the port) with a designated port role, it immediately replies with its own information.
Table 10-3 RSTP BPDU Flags
Bit Function
0 Topology change (TC)
1Proposal
2–3:
00
01
10
11
Port role:
Unknown
Alternate port
Root port
Designated port
4Learning
5Forwarding
6Agreement
7 Topology change acknowledgement (TCA)