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5-6
C
5: T
HAPTER
RANSPARENT
Spanning Tree
and the Bridged
Network
Packet Looping in a
Bridged Network
B
RIDGING
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 traffic 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
standards require 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.
Loops can occur on a bridged network for various reasons. In a network in
which 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 configure the
extended network with loops, causing packets to be circulated indefinitely.
In the example of packet looping shown in Figure 5-3:
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 on to 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 indefinitely as they traverse the bridges.
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