Table of Contents
Advertisement
field), it does not retransmit that frame. When the destination station receives the frame, it re-
sponds using the route noted in the frame. Multiple frames may reach the destination station
because of different routes that exist, and each of these multiple frames received will be sent
back to the originating station by the route that it took to get to the target station. The sending
station will choose which route to save in its routing table according to different rules. Examples
of these rules include: the frame that was received first, or the frame that took the shortest path
(went through the least number of bridges).
There also exists a limited broadcast mode, chosen by the SRLB bit in the SET MODE 2 command,
in which only those stations that are in limited broadcast mode may rebroadcast a broadcast
frame. This reduces the number of frames the receiving station gets by strategically placing the
limited broadcast stations, thus reducing the amount of traffic on the network.
C.3.1 Source Routing Implementation
The source routing mechanism is implemented when the source address (SA) JIG bit is equal to
one. In the IEEE 802.4 standard, the SA IIG bit equal to one is reserved for future use. When using
source routing, the routing field is a subset of the data field of the 802.4 standard. The frame
format is shown as follows:
NORMAL 802.4 FRAME
SD
FC
DA
SA
DATA
FCS
ED
SOURCE ROUTING IMPLEMENTED
SD
FC
DA
SA
RI
DATA
FCS
ED
IG=1
The routing information (RI) field is present only when the SA field least significant bit, referred
to as the routing information indicator, or RII bit, is set to one. The RI field can be from 2 to 30
octets. If the RII bit is equal to zero, a routing information field is not present. If this bit is one in
a control frame, the frame is considered invalid and is treated as a noise burst. If this bit is one
in a data frame and recognize source routing (RSR) is not enabled in the TBe, the TBe treats the
frame as a normal frame. If this bit is one and RSR is enabled, the frame is accepted if the
destination address matches the specified criteria set by the host or if the RI field specifies that
the frame is to be routed to another segment via the TBe. In this case, the RI field is copied to
the first data buffer like normal data. Note that lower bridge mode and recognize source routing
mode are mutually exclusive modes. If both mode bits are set, RSR dominates. The routing
information field has the format shown below.
RC Field
-------
-------
RC
- - - - RD Fields - - - - - - - -
Where:
RD1
RC
The Routing Control (2 Octets)
RD
Route Designators (2 Octets Each)
MOTOROLA
C-4
RD2
RDn
MC68824 USER'S MANUAL
Advertisement
Chapters
Table of Contents
