Processing Superior Bpdu Information; Processing Inferior Bpdu Information - Cisco 7604 Configuration Manual
Ios software configuration guide
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Understanding RSTP
Table 19-3 RSTP BPDU Flags (continued)
Bit
6
7
The sending router sets the proposal flag in the RSTP BPDU to propose itself as the designated router
on that LAN. The port role in the proposal message is always set to the designated port.
The sending router 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 TCN BPDU. It uses the topology change (TC) flag to show the
topology changes. However, for interoperability with 802.1D routers, the RSTP router processes and
generates TCN BPDUs.
The learning and forwarding flags are set according to the state of the sending port.
Processing Superior BPDU Information
A superior BPDU is a BPDU with root information (such as lower switch ID or lower path cost) that is
superior to what is currently stored for the port.
If a port receives a superior BPDU, 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 router sends an agreement
message after all of the other ports are synchronized. If the BPDU is an 802.1D BPDU, the router 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 port or an alternate
port, RSTP sets the port to the blocking state and sends an 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
An inferior BPDU is a BPDU with root information (such as higher switch ID or higher path cost) that
is inferior to what is currently stored for the port.
If a designated port receives an inferior BPDU, it immediately replies with its own information.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
19-14
Function
Agreement
Topology change acknowledgement (TCA)
Chapter 19
Configuring Standard-Compliant IEEE MST
OL-4266-08
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