On 7280R/R2, "Configurable TPID" enables a TPID to be configured per switchport (default TPID on switchport is 0x8100). A packet is considered tagged (both from ingress and egress point of view) on an interface if and only if TPID of the outermost VLAN tag on the packet matches the TPID configured on the interface.? Up to 3 distinct (non-default) TPID values may be recognized per chip.

Source port filtering is enabled by default to prevent traffic from egressing out the same interface it ingressed on.

Dot1q (802.1Q) is a tunneling protocol that encapsulates traffic from multiple customer (c-tag) VLANs in an additional single outer service provider (s-tag) VLAN for transit across a larger network structure that includes traffic from all customers. Tunneling eliminates the service provider requirement that every VLAN be configured from multiple customers, avoiding overlapping address space issues.