A data link layer relay, which borders a collision domain. Bridges are usually restricted to interconnecting LANs of the same type. They localize station-to-station traffic by segregating a network into smaller populations. They listen promiscuously on all ports, and record the source identifier of the frame and the source port in a table. The table manages communication between the populations. The bridge uses the contents of the table to forward and filter frames. It will not forward the frame to a destination port if it is on the same port as the source frame.

A bridge forwards broadcast, multicast, and directed unicast traffic to all ports, which minimizes delay but can negatively affect performance. A bridge-based network is prone to broadcast storms in an extreme case. Broadcast frames must also be decoded, and the flooding of broadcasts can significantly reduce network performance. However, bridges are easy to administer, owing to the fact that they learn from the networks themselves.

Types of bridges include transparent bridges, source-route bridges (though that is debatable), source-route/transparent bridges, translating bridges, and encapsulating bridges.

at this point I bow deeply to Radia Perlman

constraints of bridges:

  • They must operate within the 802 addresses.
  • They cannot rerequest packets from the same source.
  • They do not offer firewall protection against broadcast storms.
  • They do not offer fragmentation and reassembly information.
  • Network size is limited to the tens of thousands.

advantages of bridges:

  • Transparent bridges require less configuration than routers
  • They allow an IP node to move within the bridged portion of the topology without changing its IP address
  • They forward unroutable protocols
  • Bridges are faster and cheaper than routers, primarily because the data link header is easier to parse than a network layer header.