Local-Area Networks and LAN Switching - CCIE Routing and Switching Exam Certification Guide - page 139

RIF Figure 4-22

RIF Header Route descriptor

X X X X X

4-bit bridge
number

Type Length MTU unused 12-bit ring number

D-bit

AC FC DA SA RIF LLC TYPE Data

Token Ring SNAP Header

The first two most significant bits indicate the type of frame:
•

00 —Non Broadcast (specific route). Indicates that this is a regular frame that should be
routed to the destination following the information in the route descriptors.

•

10 —All-routes broadcast. Indicates this is an All-rings explorer packet that should take
all possible routes to the destination.

•

11 —Single-route broadcast. Indicates that this is a limited broadcast that should take one
path to the destination.

The next bit is unused.

The 5 least significant bits in the first byte indicate the total length of the RIF field including the
2-byte header. A RIF with only one bridge hop is 6-bytes long.

The most significant bit in the second byte is the D-bit (direction bit). A value of 0 indicates
forward direction, which means the source to the destination. The route descriptors are read
from left to right when the D-bit equals 0. A value of 1 indicates reverse direction, meaning the
destination station has seen it and the frame travels from the destination to the source. The route
descriptors are read right to left when the D-bit equals 1.

The next 3 bits indicate the maximum transmission frame size for the 802.5 frame. This is the
maximum frame size that this station is willing to accept. A value of 011 indicates 4136 bytes,
and a value of 100 indicates 8232 bytes. (Some references might say the values are 4472 and
8144, respectively.) Table 4-9 shows possible maximum frame size values.