
Configuring Ethernet, FDDI, and Token Ring Services
5-6
305259-A Rev 00
802.1Q Tagging
802.1Q tagging enables multiple VLANs to share a common connection to a
router. The router provides layer 3 routing services for the VLAN clients. The
router may provide standard routing services, that is, directing received frames
toward a remote destination; or it may function as a so-called “one-armed” router,
returning frames to the device from which it received them, but forwarding them
to a different logical entity.
Shared usage of a common physical port (often referred to as a tagged port) is
facilitated by the addition of two 2-byte fields within the standard Ethernet header
(Figure 5-4
).
Figure 5-4. IEEE 802.1Q Tagging
Destination address
(6 octets)
Source address
(6 octets)
Protocol type
(2 octets)
Data
(46 to 1496 octets)
FCS
(4 octets)
Destination address
(6 octets)
Source address
(6 octets)
Protocol type
(2 octets)
Tag protocol identifier
(2 octets)
Tag control information
(2 octets)
Data
(46 to 1496 octets)
FCS
(4 octets)
SN0016A
Ethernet encapsulation
Maximum size: 1514 octets
Maximum size: 1518 octets
IEEE 802.1Q encapsulation
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