Design Reference

Chapter 9: Layer 2 switch clustering and
SMLT
Split MultiLink Trunking (SMLT) enables node redundancy by allowing aggregated link groups to be
dual-homed across a pair of aggregating devices. This introduces an extra level of redundancy and
failure protection. SMLT is introduced into existing subnetworks to provide this redundancy without
the need to upgrade installed equipment. Bandwidth availability and network resiliency are improved
by allowing all aggregation paths in a dual-homed configuration to be active and to forward traffic. In
the event of a link failure, traffic failover is fast. An SMLT aggregation device pair uses a virtual
interswitch trunk (vIST) to exchange information and appear as a single, logical path aggregation
end point to dual-homed devices. vIST signalling protects against single points of failure such as link
outages by detecting and modifying information about forwarding data paths.
The following sections describe SMLT and its implementation.
Related Links
Split MultiLink Trunk configuration on page 50
Split MultiLink Trunk configuration
SMLT improves Layer 2 resiliency by providing switch failure redundancy with subsecond failover in
addition to standard MLT link failure protection and flexible bandwidth scaling functionality. Use
SMLT to connect a device that supports link aggregation to two distinct SMLT endpoints to form a
triangle. These SMLT switches form a switch cluster and are referred to as an vIST core switch pair.
Switch clusters are always formed as a pair but you can combine pairs of clusters in either a square
or full-mesh fashion to increase the size and port density of the switch cluster.
SMLT and VLACP
Avaya recommends the use of Virtual Link Aggregation Control Protocol (VLACP) for all SMLT
access links configured as MultiLink Trunks to ensure both end devices can communicate. Virtual
Services Platform 4000 does not support LACP and VLACP on the same links simultaneously.
VLACP for SMLT also protects against CPU failures by causing traffic to switch or reroute to the
SMLT peer if the CPU fails or stops responding.
The following table provides the recommended values for VLACP in an SMLT environment:
50 Network Design Reference for Avaya VSP 4000 Series June 2015
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