Specifications

2-21
TOKEN RING OVERVIEW
Source Routing
Upon receiving an SRF, a bridge looks for the following information in the
RIF:
• The ring number of the receiving port on the bridge
• The bridge number
• The ring number of the port on the bridge through which the bridge
forwards the SRF
If the bridge finds the above information, it transmits the SRF to the next
ring. However, a bridge discards the SRF if any of the following situations
exist:
• The bridge does not find the above information.
• The bridge finds the information, but the RIF length field is zero (0) or
four (4) or an odd number.
The length of the RIF is determined in part by the cumulative value of
the Route Designators (RDs) added to the RIF by each bridge traversed
during the route-discovery process.
• There are multiple combinations of the above information in the RIF
field, indicating a possible loop in the internetwork configuration.
Bridge and Ring Numbering
Each ring is allocated a unique number, using a 12-bit pattern that is
determined by the bridges.
Likewise, each bridge uses a 4-bit pattern to create a unique bridge
identity. With a Routing field of 18 bytes, 2 bytes reserved for a control
field, the RIF can hold information about 8 rings connected by 7 bridges in
a series.
Control Field
The Control Field consists of 5 distinct sections, as follows:
• Broadcast - 3 bits - indicates whether there is broadcast Routing
Information or not
• Length - 5 bits - indicates length of Routing Information field
• Direction - 1 bit - indicates order of scan of RI field
• Largest Frame - 4 bits - indicates largest frame that a bridge will
forward
• Reserved - 3 bits