Bridging
Þlters a packet if it determines that the packetÕs destination address exists on the same side of the bridge as the source address.
If two or more bridges are connected to the same Ethernet LAN segmentÑplaced in parallelÑonly a single bridge must be allowed to forward data frames onto that segment. If two or more bridges were forwarding data frames onto the same Ethernet segment, the network would soon be ßooded.
With a data loop in the topology, bridges would erroneously associate a single source address with multiple bridge ports, and keep proliferating data by forwarding packets in response to the
To avoid such data storms, Transparent bridges communicate with one another on the network by exchanging Bridge Protocol Data Units (BPDUs) to determine the network topology and collectively implement a Spanning Tree Algorithm (STA) that selects a controlling bridge for each LAN segment. This ensures that only a single data route exists between any two end stations and that topology information remains current.
About Source Route Bridging
Source Routing is typically used to connect two or more Token Ring network segments. Source Route bridges differ from Transparent bridges in that they do not build and then use a physical address database to make forwarding decisions. Instead, the source end station transmits packets with a header that contains routing information (added by bridges in the network topology during a route discovery process between end stations); once a route has been determined, a Source Route bridge simply reads the header of a source routed packet to determine whether it is a participant in routing the packet.
In Source Routing, sending and receiving devices employ broadcast packetsÑknown as explorer packetsÑto determine the most efÞcient route for a message to travel. Generally, before a station sends a message, it will Þrst send a test packet to all stations on the same ring; if the sending station receives a response to this packet, it assumes that the destination station is on the same ring and therefore it will not include routing information in frames sent to that station in the future. Any further packets issued between stations will appear to be
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. They achieve this 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
Bridging Basics |
