3Com SUPERSTACK II 2000 TR, Manual

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2-6 C
HAPTER
2: TR-
IN
-FE C
ONCEPTS
The Tag Header carries a Tag Protocol Identifier (TPID)
that identifies the frame as a Tagged Frame. The
Header also carries Tag Control Information (TCI),
which consists of the following fields:
■
User Priority
■
TR-encap
■
VLAN Identifier (VID)
The User Priority field allows the tagged frame to
carry user priority information across bridged LANs
where individual LAN segments may not be able to
signal for priority information. An 802.3/Ethernet rep-
resents such a segment.
The primary purpose of the TR-encap flag is to permit
tunneling of Tagged Frames between two 802.5 LAN
segments across a VLAN tagged region. It also allows
an end station on an 802.5 subnetwork to communi-
cate with an end station on an 802.3 subnetwork.
The VLAN Identifier (VID) field uniquely identifies the
VLAN to which a frame belongs. See the next section,
“Virtual LANs (VLANS),” for a complete explanation
of VLANs and the VLAN Identifier (also known as
VLAN ID or VID). The Route Control field is added to
the frame to carry TR specific information such as
Transparent or SR information. The payload is TR spe-
cific in content.
Once the tunneling process is complete, the com-
bined Token Ring-in Fast Ethernet frame resembles
the frame shown in Figure 2-4 (simplified). The pro-
cess is reversed to de-encapsulate the frame.
Figure 2-4 Token Ring-in-Fast Ethernet Frame.
cSA User Data
Header Information for Ethernet/Token Ring (E/TR) Frames
Payload
Key
cDA–Canonical Destination Address Field
cSA–Canonical Source Address Field
cDA TPID
RCNTL RDs PAD
TPID–Tag Protocol Identifier Field
TCI–Tag Control Information Field, which includes User Priority, TR-encap, VLAN ID
LEN–Length in octets of RCNTL + RDs + User Data Fields
RCNTL–Route Control Field
RDs–Route Descriptor (0...n) from the original frame
PAD–Ethernet frames must be at least 64 bytes in length
FCS
FCS–Frame Check Sequence
LEN TCI