This MIB module provides information on Dual Brain Virtual Link
Trunk(VLT) feature which is a control plane mechanism to provide
Layer2 multipathing between access network devices (switches or 
servers) and the core network.

VLT represents a single logical layer 2 domain from the view of 
downstream devices that have LAG bundles terminating on separate
chassis in the virtual link trunk domain.

However, the two VLT chassis are independent L2/L3 switches for 
devices in the upstream network.

A sample of VLT scenario:

---------------------------------------------------------------
|                       _______________                       |
|                      |  Edge router  |                      |
|                      |_______________|                      |
|                     /                 \                     |
|                    /                   \                    |
| VLT               /                     \                   |
|      ____________/_______________________\____________    . |
|     |    _______/      Back Up Link       \_______    |  /|\|
|     |   |       |-------------------------|       |   |   | |
|     |   |       |         -------         |       |   | L3| |
|     |   | S4810 |________| Inter |________| S4810 |   |_____|
|     |   |       |________|Chassis|________|       |   | L2| |
|     |   |       |        | Link  |        |       |   |   | |
| VLT port \_____\          -------          /____/VLT port\|/|  
|     |_____\_____\_________________________/____/_____|    . | 
|            \     \      -----------------/    /             |
|             \     \----/------------         /              |
|              \        /             \       /               |
|               \      /               \     /                |
|Lag(active)<--- \    /                 \   /--->Lag(active)  |
|              ___\__/___________________\_/______            |
|             |  _______     Stacking   ______    |           |
|             | |  S60  |--------------|  S60 |   |           |
|             | |__\____|              |____/_|   |           |
|             |_____\______________________/______|           |
|                    \                    /-->Nic teaming     |
|                     \__________________/                    |
|                    | Nic1          Nic2 |                   |
|                    |       Server       |                   |
|                    |____________________|                   |
---------------------------------------------------------------

Benefits of VLT are as follows: 
 > Allows a single device to use LAG across two upstream devices
 > Eliminates Spanning Tree Protocol (STP) blocked ports
 > Provides a loop-free topology
 > Uses all available uplink bandwidth
 > Provides fast convergence if either the link or a device fails
 > Provides link-level resiliency
 > Assures high availability

              GLOSSARY AND ABBREVIATIONS

VLT - Virtual Link Trunk 
       The combined port channel between the VLT peer devices and
       the downstream device.

VLT Peer device 
       One of a pair of devices that are connected with the 
       special port channel known as the chassis interconnect 
       trunk.

VLT Chassis Interconnect Trunk
       The link used to synchronize states between the VLT peer 
       devices. 

VLT domain 
       This domain includes both VLT peer devices, the VLT chassis
       interconnect trunk, and all of the port channels in the VLT
       connected to the downstream devices. 

VLT Backup link 
       The backup link monitors the vitality of a VLT peer device.
       The backup trunk sends configurable, periodic heart beat 
       messages between VLT peer devices.

ICL 
       Abbreviation for Chassis InterConnect Link.