Cabletron Systems CSX200, CSX400 manual Bridging Basics

Bridging

If the sending station does not receive a response to the test packet, it will send explorer packets to the destination; the explorer packets will be propagated by the networkÕs bridges as either All Paths Explorer (APE) packets or as Spanning Tree Explorer (STE) packets. The task of both packet types is to get the destination station to return speciÞc route information to the sending station (by including an identiÞer for each ring the explorer packet traversed and for each bridge between any rings).

Since the data ßow on a Source Routed network is determined by end stations (unlike a Transparently bridged network), a looped bridge topology is not an issue for data ßow. APE packets are sent from the source station over every possible bridge path to the end station. The original APE frame contains no routing information (e.g., bridge numbers and ring numbers). As the frame is propagated along all available paths to the destination station, each bridge along the way adds its own bridge and ring numbers to the packetÕs RIF before forwarding it, thereby providing route information.

In response to each received APE packet, the destination station directs a reply to the sending station. On receiving the replies, the sending station ideally assumes that the Þrst returned reply contains the most efÞcient route. The sending station then stores the route information and uses it to send subsequent transmissions to the same station.

Because APE frames do increase network trafÞc, some sites may use STE explorer frames as an alternate method of route discovery. With STE exploration, a Spanning Tree Algorithm (either conÞgured automatically via BPDUs or manually via management) is maintained for the sole purpose of determining how to direct an explorer frame during route discovery.

During the discovery process, a source station will send out STE explorer frames into a bridged topology. If a bridge is in a forwarding state according to Spanning Tree, it will forward an explorer frame onto its attached LAN segment (appending the Bridge and LAN Segment IdentiÞers in the appropriate area of the RIF); if the bridge is Þltering, it will discard the explorer frames. In this fashion, only a single explorer frame will reach each individual LAN segment.

Ultimately, the destination station will receive only a single STE packet, and will respond with APE packets (that return to the sending station on all possible bridge paths) or an STE packet (that returns to the sending station via in the reverse route of the STE explorer packet).

Although the Spanning Tree Algorithm determines the bridge path an STE takes to the destination station, during future communication between the stations, bridges along the route will use Source Routing to forward the packet (i.e., the bridges will read the Routing Information Field in the header of speciÞcally routed frames to decide whether to forward them).

Bridging Basics 4-3