Layer 2 Switching Commands 785Double VLAN ModeAn incoming frame is identified as tagged or untagged based on Tag ProtocolIdentifier (TPID) value it contains. The IEEE 802.1Q standard specifies aTPID value (0x8100) to recognize an incoming frame as tagged or untagged.Any valid Ethernet frame with a value of 0x8100 in the 12th and 13th bytes isrecognized as a tagged frame.Dell EMC Networking N-Series switches can be configured to enable the portin double-VLAN (QinQ) mode. In this mode, the switch looks for 12th, 13th,16th, and 17th bytes for the tag status in the incoming frame. The outer tagTPID is identified by the 12th and 13th bytes values. The inner tag TPID isidentified by 16th and 17th bytes values. These two TPID values can bedifferent or the same. VLAN normalization, source MAC learning, andforwarding are based on the outer value in a received frame.NOTE: DVLAN is not available on the N3000 Series switches when utilizing theAGGREGATION ROUTER image.Independent VLAN LearningIndependent VLAN Learning (IVL) allows unicast address-to-port mappingsto be created based on a MAC Address in conjunction with a VLAN ID.This arrangement associates the MAC Address only with the VLAN on whichthe frame was received. Therefore, frames are forwarded based on theirunicast destination address as well as their VLAN membership. Thisconfiguration affords multiple occurrences of an address in the forwardingdatabase. Each address associates with a unique VLAN. Care must be takenin the administration of networks, as multiple instances of a MAC address,each on a different VLAN, can quickly eat up address entries.Each VLAN is associated with its own forwarding database. Hence thenumber of forwarding databases equals the number of VLANs supported.The MAC address stored is supplemented by a 2-byte VLAN ID. The first 2bytes of a forwarding database entry contain the VLAN ID associated, and thenext 6 bytes contain the MAC address. There is a one-to-one relationshipbetween VLAN ID and FID (forwarding database ID).
