"Dot1q" (what we network admins use as a shortform for 802.1q) is a standard for using VLAN tags to put many logically-separated networks on to the same physical equipment. Conceptually, this isn't terribly difficult to work with. The complications come when you start working with VLAN trunks, native / untagged VLANs, and VLAN tags over a larger network!
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There are other VLAN trunking standards, including Cisco's proprietary ISL (Inter-Switch Link) protocol. However, 802.1q is the IEEE standard, and even Cisco recommends using the standard dot1q VLAN protocol on their equipment, as it creates less overhead for the network to deal with.
VLAN 0 and VLAN 4095 are reserved by the 802.1q spec, so they can't be used to create a VLAN tag. However, some networking entities (such as VMware ESXi) use these values to represent no VLAN--untagged access traffic (VLAN 0) or all VLANs--a VLAN trunk (VLAN 4095).
Some VLAN IDs may also be used internally. For example, Cisco Catalyst switches use VLANs 1002-1005 for Token Ring or FDDI networks (alternatives to Ethernet), and therefore you cannot redefine or delete these VLAN IDs on Catalysts!
25 окт 2024
