Local-Area Networks and LAN Switching - CCIE Routing and Switching Exam Certification Guide - page 143

As shown in Figure 4-25, source-route translational bridges overcome the following issues:
•

Bit ordering of MAC addresses —As explained in an earlier section, Ethernet NICs
expect to receive the least-significant bit first for each octet; Token Ring considers the first
bit to be the most significant bit of each octet. Translational bridges reorder source and
destination addresses when translating between Token Ring and Ethernet.

•

MTU size —Ethernet's MTU is 1500 bytes; Token Ring has a MTU of 4472 bytes on 4
Mbps networks and 17,800 on 16 Mbps networks. Translational bridges usually set the
MTU on the Token Ring networks to 1500 to facilitate the transfer of frames to Ethernet.
They check the MTU of every frame.

•

Frame Status —Ethernet does not use frame status bits that are used in Token Ring. Some
bridges set the C (copied) bit to 1 when a frame is transferred from Token Ring to
Ethernet.

•

Explorer Frames —Ethernet does not use explorer frames. These frames are dropped
when converting from Token Ring to Ethernet segments.

•

RIF —Ethernet does not understand the concept of source-routed frames.

•

Spanning-tree algorithm —Token Ring bridges do not understand Ethernet's STP.

•

Frame Conversion —Ethernet V2 frames are converted to Token Ring SNAP frames, and
vice versa.

SR/TLB Figure 4-25

Virtual Ring Pseudo Ring

5 10 2 6 5

Bridge-group 1

Host A Token Ring

SR/TLB
Bridge

Host B

No RIF or explorer frames
change MAC representation

RIF, explorer frames
change MAC representation