Virtual IST (vIST) provides the ability to dual-home hosts, servers and other network devices to a pair of Multi-Chassis Link Aggregation (MC-LAG) enabled devices. The MC-LAG nodes appear to the connected devices as one link-aggregated group. So, although the physical connection is spread between two individual network nodes, logically they appear as a single connection.
Transparent Port UNI (T-UNI) over vIST peers extends the capability of dual-home hosts on the SPB cloud to achieve higher network resiliency. The MACs learnt on the T-UNI interface of any one vIST peer is synchronized with the other peer through MAC synchronization.
In the following figure, the T-UNI access switch ACCESS-1 is dual-homed into vIST peer hosts VIST-PEER 1 and VIST-PEER 2. At ACCESS-1, a link aggregation is created to connect to the SPBM cluster. On the VIST peers, an SMLT is created towards ACCESS-1. Depending on the link aggregation hashing logic, traffic is hashed on to VIST-PEER 1 and VIST-PEER 2. The MACs learnt on the T-UNI interfaces of either host is synchronized with the other host.
If one of the links between ACCESS-1 and the vIST cluster goes down, all traffic is serviced through the other link. The same applies when any of the vIST peers go down. Since MAC learning on both peers are synchronized, both peers can switch traffic with the same efficiency.
If you configure a T-UNI service as a single-homed service on a vIST-enabled node, you must configure the same ISID service without port/MLT being mapped to ISID, on the other vIST peer node. Failure to perform this configuration on the vIST peer node can result in the loss of traffic to the single-homed T-UNI service in various scenarios.