IEEE 802.1 Transparent Bridges

1. Before you all went away for break, we were discussing a technique for interconnecting multiple LANS intended to:

   • Enable one to connect moderately incompatible networks (IEEE Ethernets, Fast Ethernets, Token rings).

   • Overcome the length limitations imposed to keep contention protocol behavior reasonable.

   • Provide a way to limit congestion on a LAN by replacing the "everything is broadcast to everyone" approach with an attempt to break the LAN into sub-LANs and limit traffic to the sub-LAN on which it originated whenever possible.

2. If one ignores the concern about congestion, it is simple to construct bridges that solve all the other problems.

   • Each bridge reads in every packet sent (successfully) on any network to which it is connected. called ports.

   • Once the packet is received, the bridge retransmits it onto all of the networks to which it is connected except the one from which it was received (following the standard MAC protocols of these networks). This is called flooding.

3. It also isn't hard to design a bridge that solves the congestion problem without much help from the stations on connected networks.

   • Each bridge keeps a little table a packets sources and the ports the associated packets arrived on.

     • When a packet is received, look the destination of the packet up in the source/port table.

     • If no entry is found, just flood the packet.

     • If an entry is found:

       • If the port associated with the address is the port on which the packet arrived, ignore the packet.

       • Otherwise, just forward the packet to the network attached through the associated port.

4. Both of these schemes only work if the network is loop free.

   • If there were a cycle, the flooding process would be disastrous.

   • If there were a cycle, packets from a given source might be observed on several different ports of a single bridge.