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CHAPTER 5: TRANSPARENT BRIDGING

Spanning Tree and the Bridged Network

When transparent bridges are used to attach networks with redundant links, packets can loop and rapidly multiply on the attached LANs. These additional packets create traf�c that might unnecessarily clog the LAN. A loop exists if more than one path can be used to forward a packet from one station to another. To solve this problem, IEEE 802.1d bridging includes Spanning Tree Protocol, an algorithm that dynamically maps out a loopless network topology (a subset of the entire topology), ensuring that only one active path exists between every pair of LANs.

Packet Looping in a Bridged Network

Loops can occur on a bridged network for various reasons. In a network where reliability is key, network administrators often implement redundant links so that, although individual bridges might fail, the �networks� (data pathways) between stations remain active. Loops can also occur by accident. For instance, when more than one bridge is used to connect various LANs, the network manager might inadvertently con�gure the extended network with loops, causing packets to be circulated inde�nitely. A potential example of packet looping is shown in Figure 5-3. In this example: 1 Packet 1 is transmitted on LAN 1. 2 Bridges A, B, and C (connected to both LAN 1 and LAN 2) receive Packet 1 and forward it onto LAN 2, creating packets 1a, 1b, and 1c, respectively. 3 Bridge A receives Packets 1b and 1c on LAN 2 and forwards them onto LAN 1; at the same time, Bridge B receives Packets 1a and 1c on LAN 2 and forwards them onto LAN 1. Bridge C follows this same pattern. When multiple bridges receive the same packet, they each transmit a new copy of the packet onto the attached LANs. Consequently, the packets will loop and multiply inde�nitely as they traverse the bridges.