Cisco Systems N3KC3048TP1GE Configuring Rapid PVST+, Rapid PVST+ Interoperation with 802.1s MST

Configuring Rapid PVST+

Rapid PVST+ Interoperation with 802.1s MST

BPDU version 0, 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.

The switch interoperates with legacy 802.1D switches as follows:

•Notification—Unlike 802.1D BPDUs, 802.1w does not use TCN BPDUs. However, for interoperability with 802.1D switches, Cisco NX-OS processes and generates TCN BPDUs.

•Acknowledgement—When an 802.1w switch receives a TCN message on a designated port from an 802.1D switch, it replies with an 802.1D configuration BPDU with the TCA bit set. However, if the TC-while timer (the same as the TC timer in 802.1D) is active on a root port connected to an 802.1D switch and a configuration BPDU with the TCA set is received, the TC-while timer is reset.

This method of operation is required only for 802.1D switches. The 802.1w BPDUs do not have the TCA bit set.

•Protocol migration—For backward compatibility with 802.1D switches, 802.1w selectively sends 802.1D configuration BPDUs and TCN BPDUs on a per-port basis.

When a port is initialized, the migrate-delay timer is started (specifies the minimum time during which 802.1w BPDUs are sent), and 802.1w BPDUs are sent. While this timer is active, the switch processes all BPDUs received on that port and ignores the protocol type.

If the switch receives an 802.1D BPDU after the port migration-delay timer has expired, it assumes that it is connected to an 802.1D switch and starts using only 802.1D BPDUs. However, if the 802.1w switch is using 802.1D BPDUs on a port and receives an 802.1w BPDU after the timer has expired, it restarts the timer and starts using 802.1w BPDUs on that port.

Note If you want all switches to renegotiate the protocol, you must restart Rapid PVST+.

Rapid PVST+ Interoperation with 802.1s MST

Rapid PVST+ interoperates seamlessly with the IEEE 802.1s Multiple Spanning Tree (MST) standard. No user configuration is needed.

Configuring Rapid PVST+

Rapid PVST+, which has the 802.1w standard applied to the Rapid PVST+ protocol, is the default STP setting in the software.

You enable Rapid PVST+ on a per-VLAN basis. The software maintains a separate instance of STP for each VLAN (except on those VLANS on which you disable STP). By default, Rapid PVST+ is enabled on the default VLAN and on each VLAN that you create.

Enabling Rapid PVST+

Once you enable Rapid PVST+ on the switch, you must enable Rapid PVST+ on the specified VLANs.

Rapid PVST+ is the default STP mode. You cannot simultaneously run MST and Rapid PVST+.

	Cisco Nexus 3000 NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U3(1)
78	OL-26590-01