Design Reference

Chapter 8: 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.
Avaya Virtual Services Platform 4000 Series uses Simple Loop Prevention Protocol (SLPP) as the
solution to detect loops. SLPP performs 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
For more information about SLPP and loop detection, see Configuring VLANs and Spanning Tree
on Avaya Virtual Services Platform 4000 Series, 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.
SLPP configuration considerations and recommendations
SLPP uses an individual VLAN hello packet mechanism to detect network loops. Sending hello
packets on an individual VLAN basis allows SLPP to detect VLAN-based network loops for
untagged and tagged IEEE 802.1Q VLAN link configurations. You determine to which VLANs a
switch sends SLPP test packets. All port members of the SLPP-enabled VLAN replicate the
packets.
June 2015 Network Design Reference for Avaya VSP 4000 Series 43
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