Reference Guide

1016 | VLAN
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Port-based VLANs
On FTOS, a VLAN is a user-defined group of ports (there is also the concept of protocol-based VLANs).
Ports in different VLANs do not communicate unless routing is configured between them. A port may
belong to more than one VLAN. Typically, ports connected to a host belong to only one VLAN, and ports
on an inter-switch link belong to more than one VLAN; these ports are sometimes called trunk ports.
Figure 58-1. VLAN Membership
VLAN Tagging
Since a port may belong to more than one VLAN, the switch must be able to identify the VLAN two which
a broadcast frame belongs. For this case, IEEE 802.1Q defines a method of marking frames to indicate the
VLAN on which the frame originated.
The marker, called a VLAN tag, is 4 bytes and is inserted after the source MAC in the Ethernet frame
header, as shown in Figure 58-2. The tag is preserved as the frame moves through the network so that
intermediate switches can forward the frame appropriately.
Figure 58-2. 802.1Q VLAN Tag
Ports that belong to more than one VLAN insert VLAN tags into frames and so they are called tagged
ports. Ports that belong to a single VLAN, do not insert VLAN tags into frames and are called untagged
ports. When you add a port to a VLAN, you must specify whether the port should be tagged or untagged.
Ports of an inter-switch
link typically belong to
multiple VLANs
VLAN 100
VLAN 200
Ports connected to end-stations
typically belong to a single VLAN
Preamble Destination
Address
Source
Address
Tag
Header
Ether
Type
Data Frame
Check
Sequence
TPID
(0x8100)
Priority
(0-7)
CFI
(0 or 1)
VLAN ID
(0-4095)
Signals that 802.1Q
information will follow
Cannonical
Form Indicator