3Com 2000 TR manual 4Token Ring-in-Fast Ethernet Frame

2-6CHAPTER 2: TR-IN-FE CONCEPTS

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.

Header Information for Ethernet/Token Ring (E/TR) Frames

Payload

cDA cSA TPID TCI LEN RCNTL RDs User Data PAD FCS

Key

cDA–Canonical Destination Address Field

cSA–Canonical Source Address Field

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–Frame Check Sequence

Figure 2-4Token Ring-in-Fast Ethernet Frame.