Cisco Nexus 5000 Series Nx-Os Layer 2 Switching Configuration Guide, Release 5.1(3)N1(1 - Cisco nexus 5000 series Configuration Manual

Configuring Rapid PVST+
duplex setting of the port. Full-duplex ports are assumed to be point-to-point ports, while half-duplex ports
are assumed to be shared ports.
Edge ports do not generate topology changes, but all other designated and root ports generate a topology
change (TC) BPDU when they either fail to receive three consecutive BPDUs from the directly connected
neighbor or the maximum age times out. At this point, the designated or root port sends out a BPDU with the
TC flag set. The BPDUs continue to set the TC flag as long as the TC While timer runs on that port. The value
of the TC While timer is the value set for the hello time plus 1 second. The initial detector of the topology
change immediately floods this information throughout the entire topology.
When Rapid PVST+ detects a topology change, the protocol does the following:
• Starts the TC While timer with a value equal to twice the hello time for all the non-edge root and
designated ports, if necessary.
• Flushes the MAC addresses associated with all these ports.
The topology change notification floods quickly across the entire topology. The system flushes dynamic
entries immediately on a per-port basis when it receives a topology change.
The TCA flag is used only when the switch is interacting with switches that are running legacy 802.1D
Note
STP.
The proposal and agreement sequence then quickly propagates toward the edge of the network and quickly
restores connectivity after a topology change.
Rapid PVST+ BPDUs
Rapid PVST+ and 802.1w use all six bits of the flag byte to add the role and state of the port that originates
the BPDU and the proposal and agreement handshake. The following figure shows the use of the BPDU flags
in Rapid PVST+.
Figure 16: Rapid PVST+ Flag Byte in BPDU
Another important change is that the Rapid PVST+ BPDU is type 2, version 2, which makes it possible for
the switch to detect connected legacy (802.1D) bridges. The BPDU for 802.1D is version 0.
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Cisco Nexus 5000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.1(3)N1(1)

Understanding Rapid PVST+
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