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8.1 | |
VLAN Concepts | |
| 8.1.5 | |
VLAN types |
There are three basic VLAN memberships for
determining and controlling how a packet gets assigned:
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The frame headers are encapsulated or modified to reflect a VLAN ID before the frame is sent over the link between switches. Before forwarding to the destination device, the frame header is changed back to the original format. The number of VLANs in a switch vary depending on several factors:
In addition, an important consideration in defining the size of the switch and the number of VLANs is the IP addressing scheme. For example, a network using a 24-bit mask to define a subnet has a total of 254 host addresses allowed on one subnet. Given this criterion, a total of 254 host addresses are allowed in one subnet. Because a one-to-one correspondence between VLANs and IP subnets is strongly recommended, there can be no more than 254 devices in any one VLAN. It is further recommended that VLANs should not extend outside of the Layer 2 domain of the distribution switch. There are two major methods of frame
tagging, Inter-Switch Link (ISL) and 802.1Q. ISL used to be the most
common, but is now being replaced by 802.1Q frame tagging.
LAN emulation (LANE) is a way to make an Asynchronous Transfer Mode (ATM) network simulate an Ethernet network. There is no tagging in LANE, but the virtual connection used implies a VLAN ID. As packets are received by the switch from any attached end-station device, a unique packet identifier is added within each header. This header information designates the VLAN membership of each packet. The packet is then forwarded to the appropriate switches or routers based on the VLAN identifier and MAC address. Upon reaching the destination node the VLAN ID is removed from the packet by the adjacent switch and forwarded to the attached device. Packet tagging provides a mechanism for controlling the flow of broadcasts and applications while not interfering with the network and applications.
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