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Example: Configuring Loop Protection to Prevent Interfaces from Transitioning from Blocking to Forwarding in a Spanning Tree

Requirements

• Junos OS Release 11.1 or later for the QFX Series
• Three switches in an RSTP topology

Note: By default, RSTP is enabled for the QFX Series.

Overview and Topology

A loop-free network in spanning-tree topologies is supported through the exchange of a special type of frame called a bridge protocol data unit (BPDU). Peer STP applications running on the switch interfaces use BPDUs to communicate. Ultimately, the exchange of BPDUs determines which interfaces block traffic (preventing loops) and which interfaces become root ports and forward traffic.

A blocking interface can transition to the forwarding state in error if the interface stops receiving BPDUs from its designated port on the segment. Such a transition error can occur when there is a hardware error on the switch or software configuration error between the switch and its neighbor. When this happens, a loop appears in the spanning tree. Loops in a Layer 2 topology cause broadcast, unicast, and multicast frames to continuously circle the looped network. As a switch processes a flood of frames in a looped network, its resources become depleted, and the ultimate result is a network outage.

Note: An interface can be configured for either loop protection or root protection, but not for both.

Three switches are displayed in Figure 1. In this example, they are configured for RSTP and create a loop-free topology. Interface xe-0/0/6 is blocking traffic between Switch 3 and Switch 1; thus, traffic is forwarded through interface xe-0/0/7 on Switch 2. BPDUs are being sent from the root bridge on Switch 1 to both of these interfaces.

This example shows how to configure loop protection on interface xe-0/0/6 to prevent it from transitioning from a blocking state to a forwarding state and creating a loop in the spanning-tree topology.

Figure 1: Network Topology for Loop Protection

Table 1 shows the components that will be configured for loop protection.

A spanning-tree topology contains ports that have specific roles:

• The root port is responsible for forwarding data to the root bridge.
• The alternate port is a standby port for the root port. When a root port goes down, the alternate port becomes the active root port.
• The designated port forwards data to the downstream network segment or device.

This configuration example uses an RSTP topology. However, you can also configure loop protection for STP or MSTP topologies at the [edit protocols (mstp | stp)] hierarchy level.

Configuration

CLI Quick Configuration

To quickly configure loop protection on interface xe-0/0/6:

Step-by-Step Procedure

1. Configure interface xe-0/0/6 on Switch 3:
   [edit protocols rstp]
user@switch# set interface xe-0/0/6bpdu-timeout-action block

Results

Check the results of the configuration:

Verification

To confirm that the configuration is working properly, perform these tasks:

• Displaying the Interface State Before Loop Protection Is Triggered
• Verifying That Loop Protection Is Working on an Interface

Displaying the Interface State Before Loop Protection Is Triggered

Purpose

Before loop protection is triggered on interface xe-0/0/6, confirm that the interface is blocked.

Action

Spanning tree interface parameters for instance 0

Interface    Port ID    Designated      Designated         Port    State  Role
                         port ID        bridge ID          Cost
xe-0/0/0.0     128:513      128:513  32768.0019e2503f00     20000  BLK    DIS  
xe-0/0/1.0     128:514      128:514  32768.0019e2503f00     20000  BLK    DIS  
xe-0/0/2.0     128:515      128:515  32768.0019e2503f00     20000  BLK    DIS  
xe-0/0/3.0     128:516      128:516  32768.0019e2503f00     20000  FWD    DESG 
xe-0/0/4.0     128:517      128:517  32768.0019e2503f00     20000  FWD    DESG 
xe-0/0/5.0     128:518      128:518  32768.0019e2503f00     20000  FWD    DESG 
xe-0/0/6.0     128:519        128:2  16384.00aabbcc0348     20000  BLK    ALT 
[output truncated]

Meaning

The output from the operational mode command show spanning-tree interface shows that xe-0/0/6.0 is the alternate port and is blocked.

Verifying That Loop Protection Is Working on an Interface

Purpose

Verify that the loop protection configuration on interface xe-0/0/6. RSTP has been disabled on interface xe-0/0/4 on Switch 1. This stops BPDUs from being sent to interface xe-0/0/6 and triggering loop protection on that interface.

Action

Spanning tree interface parameters for instance 0

Interface    Port ID    Designated      Designated         Port    State  Role
                         port ID        bridge ID          Cost
xe-0/0/0.0     128:513      128:513  32768.0019e2503f00     20000  BLK    DIS  
xe-0/0/1.0     128:514      128:514  32768.0019e2503f00     20000  BLK    DIS  
xe-0/0/2.0     128:515      128:515  32768.0019e2503f00     20000  BLK    DIS  
xe-0/0/3.0     128:516      128:516  32768.0019e2503f00     20000  FWD    DESG 
xe-0/0/4.0     128:517      128:517  32768.0019e2503f00     20000  FWD    DESG 
xe-0/0/5.0     128:518      128:518  32768.0019e2503f00     20000  FWD    DESG 
xe-0/0/6.0     128:519      128:519  32768.0019e2503f00     20000  BLK    DIS (Loop-Incon)
[output truncated]

Meaning

The operational mode command show spanning-tree interface shows that interface xe-0/0/6.0 has detected that BPDUs are no longer being forwarded to it and has moved into a loop-inconsistent state. The loop-inconsistent state prevents the interface from transitioning to a forwarding state. The interface recovers and transitions back to its original state as soon as it receives BPDUs.