Slpp, Loop Detect, And Extended Cp-Limit; Simple Loop Prevention Protocol (Slpp) - Avaya 8800 Planning And Engineering, Network Design

Layer 2 loop prevention

SLPP, Loop Detect, and Extended CP-Limit

Split MultiLink Trunking (SMLT) based network designs form physical loops for redundancy that
logically do not function as a loop. Under certain adverse conditions, incorrect configurations or
cabling, loops can form.
The two solutions to detect loops are Loop Detect and Simple Loop Prevention Protocol (SLPP).
Loop Detect and SLPP detect a loop and automatically stop the loop. Both solutions determine on
which port the loop is occurring and shuts down that port.
Control packet rate limit (CP-Limit) controls the amount of multicast and broadcast traffic sent to the
SF/CPU from a physical port. CP-Limit protects the SF/CPU from being flooded with traffic from a
single, unstable port. The CP-Limit functionality only protects the switch from broadcast and control
traffic with a QoS value of 7.
Do not use only the CP-Limit for loop prevention. Avaya recommends the following loop prevention
and recovery features in order of preference:
• SLPP
• Extended CP-Limit (Ext-CP-Limit) HardDown
• Loop Detect with ARP-Detect activated, when available
For information about configuring CP-Limit and SLPP, see Avaya Ethernet Routing Switch
8800/8600 Administration, NN46205-605. For more information about loop detection, see Avaya
Ethernet Routing Switch 8800/8600 Configuration — VLANs and Spanning Tree, NN46205-517.

Simple Loop Prevention Protocol (SLPP)

Beginning with Software Release 4.1, Avaya recommends that you use Simple Loop Prevention
Protocol (SLPP) to protect the network against Layer 2 loops. When you configure and enable
SLPP, the switch sends a test packet to the VLAN. A loop is detected if the switch or if a peer
aggregation switch on the same VLAN receives the original packet. If a loop is detected, the switch
disables the port. To enable the port requires manual intervention. As an alternative, you can use
port Auto Recovery to reenable the port after a predefined interval. For more information on Auto
Recovery, see Administration (NN46205–605).
SLPP prevents loops in an SMLT network, but it also works with other configurations, including
Spanning Tree networks.
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 impacted. Spanning Tree cannot always detect such a configuration
issue, whereas SLPP reacts and disables the malfunctioning links, minimizing the impact on the
network.
In addition to using SLPP for loop prevention, you can use the extended CP-Limit softdown feature
to protect the SF/CPU against Denial of Service (DOS) attacks where required. The extended CP-
Limit harddown option should only be used as a loop prevention mechanism in Software Release
3.7.x.
June 2016 Planning and Engineering — Network Design
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