LAN Broadcast Domain

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For the CCNA [Cisco Certified Network Associate], since switches do not route by IP [Internet Protocol] address, they will forward/flood all unknown frames or multicast frames to all segments connected calling a broadcast domain. That is, if you don’t know what to do, you broadcast to everyone you know, which causes traffic jams or packet collisions because of so many packets on the network.

Routers encapsulate (package) ethernet into IP packets and forward packets based on both MAC [Media Access Control] and IP addresses reducing traffic congestion across the IP WAN [Wide Area Network]. Transparent LAN switches build tables/lists based on MAC and then forward the packet from the port associated with the MAC address (unless the address is unknown or broadcast) and then flooded to all ports.

LAN switches and bridges that are Layer 2-Datalink devices do NOT filter broadcast traffic. That is, switches and bridges do NOT change the broadcast domain (area of service).

In other words, if the switch/bridge does not know the MAC addresses, they will flood/forward frames to ANYWHERE they don’t know the MAC address. A large number of flooded frames causes slow performance and is called a broadcast storm in a broadcast domain.

However, bridges/switches can reduce the number of collisions without changing the broadcast domain.