Configuration Guide

Table Of Contents
16.2SMLT BEB – Virtual Inter-Switch Trunk (vIST)
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October 2021
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A traditional IST uses direct physical links configured as an MLT between a pair of cluster switches. Unlike
a traditional IST, a vIST instead uses a virtual IST channel between a pair of cluster switches. This IST
virtual channel is supported across the SPBM cloud and is not dependent on local physical ports. Hence,
this eliminates the single point of failure with a dedicated MLT. The vIST is always up as long as there is
SPBM connectivity between the vIST peers. Also, the vIST devices do not have to be the same type.
For the vIST, an IST VLAN must be provisioned as would be required with a normal IST with an IPv4
address. In addition, an SPBM ISID must be assigned to the vIST VLAN on both cluster switches. This ISID
value must be unique and cannot be assigned to other VLANs in the network.
With vIST, any VLAN to be assigned to an SMLT interface must have an I-SID configured and thus becomes
a C-VLAN. For each C-VLAN provisioned (C-VLAN = VLAN with ISID) on a vIST cluster/SMLT- pair, an
SPBM ISID must be assigned on both cluster switches regardless of the additional services used on this
VLAN (L2VSN, IP Shortcuts, or L3VSN). For an L2VSN service, this ISID can be used on other BEB
switches in the network to form an L2VSN service. For an IP L3VSN service, a separate ISID value is used
to identify the L3VSN service.