When you use Virtual Inter-Switch Trunk (vIST), all critical network traffic runs on this link. If vIST fails, network protocols such as RIP, VRRP, OSPF, and VLACP go down and eventually cause a network outage.
vIST uses an SPBM tunnel to virtually connect two nodes that can be anywhere in the SPBM cloud. Even if the two vIST nodes are directly connected by an MLT link, the vIST VLAN does not have MLT ports as members. Instead, it is configured to be an SPBM C-VLAN.
Note
For more information on vIST and a configuration example, see MultiLink Trunking and Split MultiLink Trunking.
The vIST tunnel is up as long as there is SPBM connectivity between the IST peers. If there is a vIST failure, check the following procedure for some possible reasons:
enable
configure terminal
show virtual-ist
show isis spbm i-sid discover
Important
The I-SID associated with the vIST VLAN should be the same on the vIST peer, and this I-SID should not be used anywhere else in the network.
show vlan mac-address-entry <1-4059>
show ip arp vlan <vid>