Operation
3-4
Source Route and Source Route Translational Switching
In SR (Source Routing) and SRT (Source Route and Transparent) modes, the
received SR packets are stripped of their RIF during the translation to canonical
frame format. Packets that are queued for transmission out the Token Ring port
can have a RIF inserted from the RIF cache, or can be sent without the RIF as a TP
packet. This decision is automated when the port is set to SRT and is handled on a
packet by packet basis.
SRT Switching operates much the same as TP Switching, but strips the RIFs and
caches them locally. Frames transmitted out the Token Ring ports have RIFs
appended to them from the cache.
SR operates by transmitting frames over a designated route. Unlike Transparent
Bridging, all devices in a Source Route Bridged (SRB) network ÒknowÓ the
locations of other stations within the network. Using the RIF portion of the MAC
frame header, the source device determines the route for the frames it sends.
In order for source routing to work, the source station must determine the proper
route to reach the desired destination. To accomplish this:
1. The source station sends out frames called All Route Explorer (ARE) or
Spanning Tree Explorer (STE) frames onto the network. All Source Route
Bridges recognize these frames and forwards them to their outbound ports.
2. The receiving bridges insert their own route information to the RIF in the
MAC frame header and transmit the frame again.
3. Eventually, the original source device receives all of the ARE replies from the
other end stations on the network.
4. From this information, the source device can determine a desired route for
each frame it transmits.
Spanning Tree Algorithm - IEEE 802.1d
The 9T425-16, 9T427-16, and 9T428-16 promote maximum network use in
multiple bridge environments. A bridge learns the bridge topology of its network
from bridge protocol data that it receives from other bridges within the network.
The bridges then apply the Spanning Tree Algorithm (STA) to select a root bridge,
and then determine primary data paths within potential data loop conÞgurations.
BPDUs that are transmitted by the TR ports are sent to the bridge-group address
in MSB format (0X800143000000).