The first four traffic flow examples are based on SMLT aggregation switches and operations.
Assuming a and b1/b2 are communicating through Layer 2, traffic flows from A to switch E and is forwarded over its direct link to B. Traffic coming from b1 or b2 to a is sent by B on one of its multilink trunk ports.
B can send traffic from b1 to a on the link to switch E, and traffic from b2 to a on the link to F. In the case of traffic from b1, switch E forwards the traffic directly to switch A, while traffic from b2, which arrived at F, is forwarded across the vIST to E and then to A.
Traffic from b1/b2 to c1/c2 is always sent by switch B through its multilink trunk to the core. No matter at which switch E or F arrives at, it is sent directly to C through the local link.
Traffic from a to d (and d to a) is forwarded across vIST because it is the shortest path. The link is treated as a standard link; SMLT and vIST parameters are not considered.
Traffic from f to c1/c2 is sent out directly from F. Return traffic from c1/c2 passes through one active VRRP Master for each IP subnet. The traffic is passed across vIST if switch C sends it to E.
In an SMLT environment, the two aggregation switches share the same forwarding database by exchanging forwarding entries using the vIST. The entry for 00:E0:7B:B3:04:00 is shown on switch C as an entry learned on MLT-1, but because SMLT Remote is true, this entry was actually learned from switch B. On B, that same entry is shown as directly learned through MLT-1 because SMLT Remote is false.
The following illustration shows the network topology.
After a packet arrives at switch C destined for 00:E0:7B:B3:04:00, if the SMLT Remote status is true, the switch tries to send the packet to MLT-1 rather than through vIST. Traffic rarely traverses vIST unless there is a failure. If this same packet arrives at B, it is then forwarded to MLT-1 on the local ports.