Port Priority; Rapid Pvst+ And Ieee 802.1Q Trunks; Rapid Pvst+ Interoperation With Legacy 802.1D Stp - Cisco Nexus 7000 Series Configuration Manual

Configuring Rapid PVST+ Using Cisco NX-OS
If a loop occurs, STP considers the port cost when selecting a LAN interface to put into the forwarding state.
You can assign the lower cost values to LAN interfaces that you want STP to select first and higher cost values
to LAN interfaces that you want STP to select last. If all LAN interfaces have the same cost value, STP puts
the LAN interface with the lowest LAN interface number in the forwarding state and blocks other LAN
interfaces.
On access ports, you assign the port cost by the port. On trunk ports, you assign the port cost by the VLAN;
you can configure the same port cost to all the VLANs on a trunk port.

Port Priority

If a redundant path occurs and multiple ports have the same path cost, Rapid PVST+ considers the port priority
when selecting which LAN port to put into the forwarding state. You can assign lower priority values to LAN
ports that you want Rapid PVST+ to select first and higher priority values to LAN ports that you want Rapid
PVST+ to select last.
If all LAN ports have the same priority value, Rapid PVST+ puts the LAN port with the lowest LAN port
number in the forwarding state and blocks other LAN ports. The possible priority range is from 0 through
224 (the default is 128), configurable in increments of 32. The device uses the port priority value when the
LAN port is configured as an access port and uses the VLAN port priority values when the LAN port is
configured as a trunk port.

Rapid PVST+ and IEEE 802.1Q Trunks

The 802.1Q trunks impose some limitations on the STP strategy for a network. In a network of Cisco network
devices connected through 802.1Q trunks, the network devices maintain one instance of STP for each VLAN
allowed on the trunks. However, non-Cisco 802.1Q network devices maintain only one instance of STP for
all VLANs allowed on the trunks, which is the Common Spanning Tree (CST).
When you connect a Cisco network device to a non-Cisco device through an 802.1Q trunk, the Cisco network
device combines the STP instance of the 802.1Q VLAN of the trunk with the STP instance of the non-Cisco
802.1Q network device. However, all per-VLAN STP information that is maintained by Cisco network devices
is separated by a cloud of non-Cisco 802.1Q network devices. The non-Cisco 802.1Q cloud that separates the
Cisco network devices is treated as a single trunk link between the network devices.

Rapid PVST+ Interoperation with Legacy 802.1D STP

Rapid PVST+ can interoperate with devices that are running the legacy 802.1D protocol. The device knows
that it is interoperating with equipment running 802.1D when it receives a BPDU version 0. The BPDUs for
Rapid PVST+ are version 2. If the BPDU received is an 802.1w BPDU version 2 with the proposal flag set,
the device sends an agreement message after all of the other ports are synchronized. If the BPDU is an 802.1D
BPDU version 0, the device 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 device interoperates with legacy 802.1D devices as follows:
• Notification—Unlike 802.1D BPDUs, 802.1w does not use TCN BPDUs. However, for interoperability
with 802.1D devices, the device processes and generates TCN BPDUs.
Cisco Nexus 7000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.x
Rapid PVST+ and IEEE 802.1Q Trunks
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