Cisco Nexus 3000 Series Nx-Os Layer 2 Switching Configuration Guide, Release 5.0(3)U1(1 - Cisco Nexus 3000 series Configuration Manual

Configuring Rapid PVST+
• Root ports—If Rapid PVST+ selects a new root port, it blocks the old root port and immediately transitions
the new root port to the forwarding state.
• Point-to-point links—If you connect a port to another port through a point-to-point link and the local
port becomes a designated port, it negotiates a rapid transition with the other port by using the
proposal-agreement handshake to ensure a loop-free topology.
Rapid PVST+ achieves rapid transition to the forwarding state only on edge ports and point-to-point links.
Although the link type is configurable, the system automatically derives the link type information from the
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.

Cisco Nexus 3000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U1(1)

Understanding Rapid PVST+
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