Bridging - LAN Switches

• Bridges also called LAN switches is a class of switch that is used to forward packets between LANs
• extended LAN - a collection of LANs connected by one or more bridges
• simplest bridge accepts LAN frames on inputs and forward them out on all other outputs

Learning Bridges

• maintain a forwarding table
• should the bridge receive a frame that is addressed to a host not currently in the table, it forwards the frame out on all the other ports

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Spanning Tree Algorithm

• problem with learning bridges - the preceding strategy works just fine until the extended LAN has a loop in it, in which case it fails in a horrible way

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• solution - have the bridges run a distributed spanning tree algorithm

  • TODO explain implementation of algorithm

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Broadcast and Multicast

• preceding discussion has focused on how bridges forward unicast frames from one LAN to another
• bridges must support broadcast and multicast
• broadcast is simple—each bridge forwards a frame with a destination broadcast address out on each active (selected) port other than the one on which the frame was received
• multicast can be implemented exactly like broadcast

Limitations of Bridges

• problem with scale:

  • spanning tree scales linearly
  • forwards all broadcast frames

• solution

  • use Virtual LAN (VLAN) - allows a single extended LAN to be partitioned into several seemingly separate LANs
  • each LAN is assigned an identifier and packets can only travel from one segment/LAN to another if both segments/LAN have the same identifier

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