Vport is a router BNG representation of a remote traffic aggregation point in the access network. It is a level in the hierarchical QoS model implemented within the BNG that requires QoS treatment.
When the BNG is connected to access network via LAG, a Vport construct within the BNG is instantiated per member link on that LAG. Each instance of the Vport in such a configuration receives the entire amount of configured bandwidth. When traffic is sprayed in a per-subscriber fashion over member links in an LAG without awareness of the Vport, it can lead to packet drops on one member link irrespective of the relative traffic priority on another LAG member link in the same Vport. The reason is that multiple Vport instances of the same Vport on different LAG member links are not aware of each other.
With a small number of subscribers per Vport and a great variation in bandwidth service offering per subscriber (from Mb/s to Gb/s), there is a great chance that the load distribution between the member links is heavily unbalanced. For example, if the lag consists of two member links on the same IOM, three 1Gb/s high priority subscribers can saturate the 2 Gb/s Vport bandwidth on one member link of the LAG. And all the while, twenty low priority 10 Mb/s subscribers that are using the other link are significantly under-utilizing available bandwidth on the corresponding Vport.
To remedy this situation, all traffic flowing through the same Vport must be hashed to a single LAG member link. This way, the traffic treatment is controlled by a single Vport instance, and achieve the wanted behavior where low priority 10 Mb/s subscribers traffic is affected before any traffic from the high priority subscribers.