Identifying VLANs

Identifying VLANs
VLANs can span multiple connected switches, which (as we stated earlier) Cisco calls a switch
fabric. Switches within the switch fabric must keep track of frames as they are received on the switch
ports, and they must keep track of the VLAN they belong to as the frames traverse the switch fabric.
Switches use frame tagging to perform this function. Switches can then direct frames to the appropriate
port.
458 Chapter 14  VLANs, Trunks, and VTP
There are two types of links in a switched environment:
Access link An access link is a link that is part of only one VLAN, which is referred to as the
native VLAN of the port. Any device attached to an access link is unaware of a VLAN membership.
This device just assumes it is part of a broadcast domain, with no understanding of the
physical network. Switches remove any VLAN information from the frame before it is sent to
an access-link device. Access-link devices cannot communicate with devices outside of their
VLAN unless the packet is routed through a router.
Trunk link Trunks can carry multiple VLANs. Originally named after the trunks of the telephone
system, which carry multiple telephone conversations, a trunk link is used to connect switches to
other switches, to routers, or even to servers. Trunk links are supported on FastEthernet or Gigabit
Ethernet only. To identify the VLAN that a frame belongs to, Cisco switches support two identification
techniques: Inter-Switch Link (ISL) and 802.1Q. Trunk links are used to transport VLANs
between devices and can be configured to transport all VLANs or just a few VLANs. Trunk links still
have a native VLAN, and that VLAN is used if the trunk link fails.