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In Token Ring, What is a RIF and how do I read it?
by Fred Ingham

The RIF or Routing Information Field in Token Ring is a map through the network for bridging beyond the current ring. The RIF shows the path from the originating ring to the destination ring with a path consisting of ring, bridge combinations.The RIF directly follows the source address in the frame when present.

The first bit of the Source Address is the Routing Information Indicator (RII) and is 0 for frames not leaving the ring and 1 for frames leaving the ring and having a RIF.   ( If the MAC address is 10 00 5a 12 34 56 then the source address will appear as 90 00 5a 12 34 56 for a frame with a RIF.) In the IBM version which is implemented by Cisco and most other vendors the maximum number of ring-bridge combinations in the RIF is 8 or a maximum of 7 hops. The last bridge will always be shown as 0.

The information contained in the RIF consists of the Routing Control Field (2 Bytes) and multiple Route Designation Fields (each 2 bytes). 

*************************************************************************
Routing Information Field (RIF)
RCF          RD            RD           RD ...    
2 Bytes     2 Bytes    2 Bytes    2 Bytes ... (min of 2 Bytes, max of 18 Bytes total)

RCF = Routing Control Field
RD = Route Designator Field
*************************************************************************

The Routing Control Field (RCF) consists of five subfields:

*************************************************************************
Routing Control Field (RCF)

            Byte 0:                 Byte 1:
bit   0 1 2 3 4 5 6 7     0 1 2 3 4 5 6 7
      B B B L L L L L      D F F F R R R R

B = Broadcast Indicators
L = Length Bits (length of RIF)
D = Direction Bit
F = Largest Frame Bits
R = Reserved Bits (always 0)
************************************************************************

Broadcast Indicators (3 bits)
     0xx Non Broadcast
    10x all-routes Broadcast
    11x Single route Broadcast (spanning explorer)

Length of the RIF (5 bits)
The length must be an even number between 2 and 18. (IEEE 802.5 allows for a longer RIF but 18 is the limit in all field implementations)

Direction Bit (1 bit)
    0 read left to right
    1 read right to left

Largest Frame Bits (3 bits)
  000       up to 516 bytes
  001       1500
  010       2052
  011       4472
  100       8144
  101       11407
  110       17800
  111       used in all-route broadcast frames

Reserved Bits (4 bits)
Transmitted as 0's

Route Designator Field
The Route Designator Field consists of two subfields:

Ring Number (12 bits) values between 0x001 and 0xfff (1 to 4095 decimal)
Bridge Number (4 bits) values between 0x1 and 0xf (1 to 15 decimal)

**********************************************************
Route Designator Field (RD)

              Byte 0                 Byte 1
bit    0 1 2 3 4 5 6 7     0 1 2 3 4 5 6 7
       R R R R R R R R     R R R R B B B B

R = Ring Number (12 bits)
B = Bridge Number (4 bits)
**********************************************************

Another very good resource that deals with RIFs in extreme detail is: IBM Token-Ring Network Architecture Reference, SC30-3374-02, Sep 1989

Examples (all RIFs in hexidecimal):

RIF 1:  C8 10 00 11 1f 41 00 20  
This indicates a single route broadcast frame with a RIF of 8 bytes with a maximum frame size of 1500 bytes. The path is read left to right as ring 1 bridge 1 to ring 500(0x1f4) bridge 1 to ring 2.  This is a two hop path.

The RIF is often written as C810 001 1 if4 1 002 0 to make the ring, bridge combination easier to read.  Note that rings here are expressed in hexidecimal  but must be configured in cisco routers by the decimal equivalents (that is, 500 not 1F4). 

RIF 2:  1290 001 1 1f4 1 002 2 300 1 034 2 2f2 3 003 1 f00 0
This RIF indicates a non broadcast frame with a RIF of 18 bytes with a maximum frame size of 1500 bytes with the path read from right to left. The path is ring 3840(0xf00) bridge 1 to ring 3 bridge 3 to ring  754 (0x2f2) bridge 2 to   ring 52 (0x34) bridge 1 to ring 768 (0x300) bridge 2 to ring 2 bridge 1 to ring 500 (0x1f4) bridge 1 to ring 1.   This RIF is the maximum length allowable and goes thru 7 bridges (hops).

Reference:  IBM Token Ring Architecture Reference, SC30-3374-02, 3rd ed, Sep 1989.