VLAN Trunking
A trunk is a link that carries traffic for more than one VLAN. Trunks multiplex
traffic from multiple VLANs. Trunks connect switches and allow ports
on multiple switches to be assigned to the same VLAN.
Two methods of identifying VLANs over trunk links are:
■ Inter-Switch Link (ISL)—A Cisco proprietary method that encapsulates
the original frame in a header, which contains VLAN information.
It is protocol-independent and can identify Cisco Discovery
Protocol (CDP) and bridge protocol data unit (BPDU) frames.
■ 802.1Q—Standards-based, tags the frames (inserts a field into the original
frame immediately after the source MAC address field), and
supports Ethernet and Token Ring networks.
When aframe comes into a switch port, the frame is tagged internally within
the switch with the VLAN number of the port. When it reaches the outgoing
port, the internal tag is removed. If the exit port is a trunk port, then its
VLAN is identified in either the ISL encapsulation or the 802.1Q tag. The
switch on the other end of the trunk removes the ISL or 802.1Q information,
checks the VLAN of the frame, and adds the internal tag. If the exit port is a
user port, then the original frame is sent out unchanged, making the use of
VLANs transparent to the user.
If a nontrunking port receives an ISL-encapsulated frame, the frame is
dropped. If the ISL header and footer cause the MTU size to be exceeded,
it might be counted as an error.
If a nontrunking port receives an 802.1Q frame, the source and destination
MAC addresses are read, the tag field is ignored, and the frame is switched
normally at Layer 2 .