Dell PowerConnect 8100 CLI Reference Guide - Page 781

36 VLAN Commands PowerConnect 802.1Q VLANs are an implementation of the Virtual Local Area Network, specification 802.1Q. Operating at Layer 2 of the OSI model, the VLAN is a means of parsing a single network into logical user groups or organizations as if they physically resided on a dedicated LAN segment of their own. In reality, this virtually defined community may have individual members scattered across a large, extended LAN. The VLAN identifier is part of the 802.1Q tag, which is added to an Ethernet frame by an 802.1Qcompliant switch or router. Devices recognizing 802.1Q-tagged frames maintain appropriate tables to track VLANs. The first 3 bits of the 802.1Q tag are used by 802.1p to establish priority for the packet. PowerConnect supports 802.1Q VLANs. As such, ports may simultaneously belong to multiple VLANs. VLANs allow a network to be logically segmented without regard to the physical locations of devices in the network. PowerConnect switching supports up to 1024 VLANs for forwarding. VLANs can be allocated by subnet and netmask pairs, thus allowing overlapping subnets. For example, subnet 10.10.128.0 with Mask 255.255.128.0 and subnet 10.10.0.0 with Mask 255.255.0.0 can have different VLAN associations. Double VLAN Mode An incoming frame is identified as tagged or untagged based on Tag Protocol Identifier (TPID) value it contains. The 802.1Q standard specifies a TPID value (0x8100) to recognize an incoming frame as tagged or untagged. Any valid Ethernet frame with a value 0x8100 in the 12th and 13th bytes is recognized as tagged frame. 802.1Q switches check the 12th and 13th bytes to decide the tag status of incoming frame. The PowerConnect switching component can be configured to enable the port in double-VLAN (DVLAN) mode. In this mode switch looks for 12th, 13th, 16th, and 17th bytes for the tag status in the incoming frame. The outer tag (S-TAG) TPID is identified with the 12th and 13th bytes values. The inner tag (C-TAG) TPID is identified with 16th and 17th bytes values. These VLAN Commands 781