Design Reference

Chapter 10: Layer 2 loop prevention
This chapter provides information about how to use bandwidth and network resources efficiently,
and to prevent Layer 2 data loops.
Loop prevention and detection
In certain network designs, loops can form. For example, loops can form if you have incorrect
configuration or cabling.
There are two solutions to detect loops: Loop Detect, and Simple Loop Prevention Protocol (SLPP).
Both solutions perform the following functions:
• Detect the loop
• Automatically stop the loop
• Determine on which port the loop is occurring
• Shut down the port on which the loop is occurring
Avaya recommends the following loop-prevention and recovery features in order of preference:
1. SLPP
2. Loop Detect with Address Resolution Protocol (ARP)-Detect activated
For more information about SLPP and loop detection, see Avaya Virtual Services Platform 4000
Series Configuration — VLANs and Spanning Tree, NN46251-500.
SLPP
Use SLPP to protect the network against Layer 2 loops. If you configure and enable SLPP, the
switch sends a test packet to the VLAN. A loop is detected if the switch or a peer aggregation switch
on the same VLAN receives the original packet. If the switch detects a loop, the switch disables the
port. After the port is disabled, you must enable the port manually, or use port auto-enable to
reenable the port after a predefined interval.
Loops can be introduced into the network in many ways. One way is through the loss of a multilink
trunk configuration caused by user error or malfunction. This scenario does not introduce a
broadcast storm, but because all MAC addresses are learned through the looping ports, Layer 2
MAC learning is significantly affected. Spanning tree protocols cannot always detect such a
configuration issue, whereas SLPP reacts and disables the malfunctioning links, which minimizes
the impact on the network.
December 2014 Network Design Reference for Avaya VSP 4000 Series 45
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