Prior to MLT and other aggregation techniques, parallel links were underutilized due to Spanning Tree Protocol’s loop protection.

SMLT, DSMLT and R-SMLT technologies removes this limitation by allowing the physical ports to be split between two switches.

For each packet that needs to be transmitted, one of the physical links is selected based on a load-balancing algorithm (usually involving a hash function operating on the source and destination MAC address information).

The IST is a (standard) MLT connection between the aggregation switches which allows the exchange of information regarding traffic forwarding and the status of individual SMLT links.

For example, when one switch receives a response to an ARP request from an end station on a port that is part of an SMLT, it will inform its peer switch across the IST and request the peer to update its own ARP table with a record pointing to its own connection with the corresponding SMLT ID.

In general, normal network traffic does not traverse the IST unless this is the only path to reach a host which is connected only to the peer switch.

In general the failure of any one component results in a traffic disruption lasting less than half a second (normal less than 100 millisecond[3][4]) making SMLT appropriate in environments running time- and loss-sensitive applications such as voice and video.

In a network using SMLT, it is often no longer necessary to run a spanning tree protocol of any kind since there are no logical bridging loops introduced by the presence of the IST.

This eliminates the need for spanning tree reconvergence or root-bridge failovers in failure scenarios which causes interruptions in network traffic longer than time-sensitive applications are able to cater for.

Routed-SMLT (R-SMLT) is a computer networking protocol developed at Nortel as an enhancement to split multi-link trunking (SMLT) enabling the exchange of Layer 3 information between peer nodes in a switch cluster for resiliency and simplicity for both L3 and L2.

The R-SMLT protocol works with SMLT and distributed Split Multi-Link Trunking (DSMLT) technologies to provide sub-second failover (normally less than 100 milliseconds)[7] so no outage is noticed by end users.

R-SMLT takes care of packet forwarding in core router failures and works with any of the following protocol types: IP Unicast Static Routes, RIP1, RIP2, OSPF, BGP and IPX RIP.