Cabletron Systems STH-24 manual Source Route Bridging

Operation

Source Route Bridging

Source Route Bridging operates by transmitting frames over a designated route. Unlike Transparent Bridging, all devices in a Source Route Bridged (SRB) network “know” the locations of other stations within the network. Using a portion of the MAC frame header known as the RIF (Routing Information Field), 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 Single Route Explorer (SRE) frames onto the network. All Source Route Bridges recognize these frames and forwards them to their outbound ports.

2.The receiving bridges append their own route information to the Route Information Field (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

The 9T122-24 promotes 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 configurations.

Spanning Tree Algorithm is a hierarchy (or tree) of priorities that bridges establish between themselves. This hierarchy guarantees that primary and redundant data paths are clearly defined at all times, so that the network is continuously available to users.

In a multiple-bridge environment, one bridge in the network establishes itself as the root bridge. As the root, this bridge has priority over all other bridges. In a Spanning Tree, all of the bridges must determine which bridge is the root, and then determine their own relative priority within the network.

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