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Configuring 802.1Q Tagging

308623-14.00 Rev 00

5-7

The IEEE has not yet standardized values for the tag protocol identifier (TPID)

field, leaving vendors to provide their own proprietary values. The Accelar family

of routing switches, for example, writes a value of 8100 (hexadecimal) to this

field.

The tag control information (TCI) field contains a unique value that identifies the

VLAN on which the frame originated. This value is assigned during the

configuration of the layer 2 device.

The addition of the four bytes required for the TPID and TCI fields raises the

possibility of generating frames up to 1518 bytes in length, four bytes larger than

the maximum packet size specified by Ethernet. Consequently, for frames on

which 802.1Q tagging is enabled, BayRS accepts such outsized frames.

Router Processing of Tagged Frames

802.1Q tagging is supported only on 100BASE-T interfaces that connect the

Nortel Networks router to an 802.1Q-compliant switch or routing switch. With

802.1Q tagging enabled, the physical connection between the router and the

adjacent device supports multiple virtual connections.

The number of connections is equal to the number of virtual connections plus a

default physical connection that provides transit services for other non-VLAN

traffic that may be received from or forwarded to the adjacent device.

Upon receipt of a frame across a virtual connection, a circuit manager strips the

four bytes of 802.1Q header information and directs a now standard Ethernet

frame to a connection-specific routing process. The routing process consults its

forwarding table and, in turn, directs the frame to a circuit manager handling the

next-hop connection. If that connection is a non-tagged, non-virtual connection,

processing is completed as for any other standard Ethernet frame.

However, if the next-hop connection is a tagged, virtual connection, the circuit

manager inserts the four bytes of 802.1Q header information that identify that

VLAN into the standard Ethernet header. After performing the 802.1Q

encapsulation, the circuit manager forwards the frame across the virtual

connection toward the destination VLAN.

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Avaya Configuring Ethernet, FDDI, and Token Ring Services Manual do Utilizador Página 83