user manual
Table Of Contents
- Cisco Nexus 3000 NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U3(1)
- Contents
- Preface
- New and Changed Information for this Release
- Overview
- Configuring Ethernet Interfaces
- Information About Ethernet Interfaces
- Configuring Ethernet Interfaces
- Configuring the UDLD Mode
- Changing an Interface Port Mode
- Configuring Interface Speed
- Disabling Link Negotiation
- Configuring the CDP Characteristics
- Enabling or Disabling CDP
- Enabling the Error-Disabled Detection
- Enabling the Error-Disabled Recovery
- Configuring the Error-Disabled Recovery Interval
- Configuring the Debounce Timer
- Configuring the Description Parameter
- Disabling and Restarting Ethernet Interfaces
- Displaying Interface Information
- Displaying Input Packet Discard Information
- Default Physical Ethernet Settings
- Configuring VLANs
- Configuring Private VLANs
- Information About Private VLANs
- Guidelines and Limitations for Private VLANs
- Configuring a Private VLAN
- Enabling Private VLANs
- Configuring a VLAN as a Private VLAN
- Associating Secondary VLANs with a Primary Private VLAN
- Configuring an Interface as a Private VLAN Host Port
- Configuring an Interface as a Private VLAN Promiscuous Port
- Configuring a Promiscuous Trunk Port
- Configuring an Isolated Trunk Port
- Configuring the Allowed VLANs for PVLAN Trunking Ports
- Configuring Native 802.1Q VLANs on Private VLANs
- Verifying the Private VLAN Configuration
- Configuring Access and Trunk Interfaces
- Configuring Switching Modes
- Configuring Rapid PVST+
- Information About Rapid PVST+
- Understanding STP
- Understanding Rapid PVST+
- Rapid PVST+ and IEEE 802.1Q Trunks
- Rapid PVST+ Interoperation with Legacy 802.1D STP
- Rapid PVST+ Interoperation with 802.1s MST
- Configuring Rapid PVST+
- Enabling Rapid PVST+
- Enabling Rapid PVST+ per VLAN
- Configuring the Root Bridge ID
- Configuring a Secondary Root Bridge
- Configuring the Rapid PVST+ Port Priority
- Configuring the Rapid PVST+ Pathcost Method and Port Cost
- Configuring the Rapid PVST+ Bridge Priority of a VLAN
- Configuring the Rapid PVST+ Hello Time for a VLAN
- Configuring the Rapid PVST+ Forward Delay Time for a VLAN
- Configuring the Rapid PVST+ Maximum Age Time for a VLAN
- Specifying the Link Type
- Restarting the Protocol
- Verifying Rapid PVST+ Configurations
- Information About Rapid PVST+
- Configuring Multiple Spanning Tree
- Information About MST
- Configuring MST
- MST Configuration Guidelines
- Enabling MST
- Entering MST Configuration Mode
- Specifying the MST Name
- Specifying the MST Configuration Revision Number
- Specifying the Configuration on an MST Region
- Mapping and Unmapping VLANs to MST Instances
- Mapping Secondary VLANs to Same MSTI as Primary VLANs for Private VLANs
- Configuring the Root Bridge
- Configuring a Secondary Root Bridge
- Configuring the Port Priority
- Configuring the Port Cost
- Configuring the Switch Priority
- Configuring the Hello Time
- Configuring the Forwarding-Delay Time
- Configuring the Maximum-Aging Time
- Configuring the Maximum-Hop Count
- Configuring PVST Simulation Globally
- Configuring PVST Simulation Per Port
- Specifying the Link Type
- Restarting the Protocol
- Verifying MST Configurations
- Configuring STP Extensions
- About STP Extensions
- Information About STP Extensions
- Configuring STP Extensions
- STP Extensions Configuration Guidelines
- Configuring Spanning Tree Port Types Globally
- Configuring Spanning Tree Edge Ports on Specified Interfaces
- Configuring Spanning Tree Network Ports on Specified Interfaces
- Enabling BPDU Guard Globally
- Enabling BPDU Guard on Specified Interfaces
- Enabling BPDU Filtering Globally
- Enabling BPDU Filtering on Specified Interfaces
- Enabling Loop Guard Globally
- Enabling Loop Guard or Root Guard on Specified Interfaces
- Verifying STP Extension Configuration
- About STP Extensions
- Configuring LLDP
- Configuring the MAC Address Table
- Configuring IGMP Snooping
- Configuring Traffic Storm Control
- INDEX
Processing Inferior BPDU Information
An inferior BPDU is a BPDU with root information (such as a 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.
Spanning-Tree Dispute Mechanism
The software checks the consistency of the port role and state in the received BPDUs to detect unidirectional
link failures that could cause bridging loops.
When a designated port detects a conflict, it keeps its role, but reverts to a discarding state because disrupting
connectivity in case of inconsistency is preferable to opening a bridging loop.
The following figure illustrates a unidirectional link failure that typically creates a bridging loop. Switch A
is the root bridge, and its BPDUs are lost on the link leading to switch B. The 802.1w-standard BPDUs include
the role and state of the sending port. With this information, switch A can detect that switch B does not react
to the superior BPDUs it sends and that switch B is the designated, not root port. As a result, switch A blocks
(or keeps blocking) its port, thus preventing the bridging loop. The block is shown as an STP dispute.
Figure 13: Detecting Unidirectional Link Failure
Port Cost
Rapid PVST+ uses the short (16-bit) pathcost method to calculate the cost by default. With the short
pathcost method, you can assign any value in the range of 1 to 65535. However, you can configure the
switch to use the long (32-bit) pathcost method, which allows you to assign any value in the range of 1
to 200,000,000. You configure the pathcost calculation method globally.
Note
The STP port path-cost default value is determined from the media speed and path-cost calculation method
of a LAN interface. If a loop occurs, STP considers the port cost when selecting a LAN interface to put into
the forwarding state.
Table 9: Default Port Cost
Long Path-cost Method of Port CostShort Path-cost Method of Port
Cost
Bandwidth
2,000,00010010 Mbps
Cisco Nexus 3000 NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U3(1)
76 OL-26590-01
Configuring Rapid PVST+
Understanding Rapid PVST+