So, multicast frames are forwarded to some or none, but perhaps not all ports. Multicasts go to devices that have subscribed to the particular multicast data stream. Whenever a switch sees all 1s in the destination MAC address, it forwards the frame out all active ports. This is then followed by three more bytes (24 bits) that generally map to the multicast IP address, and they work with both IPv4 and IPv6.īroadcasts, by definition, go to all devices. A multicast MAC address, by convention, begins with three special bytes: 01-00-5e. The broadcast address is just 48 bits of all 1s. There are a couple of special cases, though.

If it doesn’t already have that address in the MAC table, it creates an entry that associates the address with the port. Whenever it receives a frame from a port, it looks at the source MAC. The switch learns these MAC addresses by simply listening. The switch looks up the destination MAC in a dynamically generated internal table and forwards the frame out the port associated with the MAC. IPv6 addresses are 128 bits long, but the most common IPv6 address structure actually includes the MAC address to help simplify dynamic address generation.Įvery frame has a source MAC and a destination MAC.

Standard IPv4 addresses are 32 bits long, so we can uniquely identify many more devices using MAC address than using IP addresses. At the Ethernet layer, we tend to refer to packets as frames, but they’re approximately the same thing.Ī MAC address is just a 48-bit sequence that uniquely identifies the device.

An Ethernet switch uses Ethernet MAC addresses to decide how to forward frames. The first feature to talk about is called MAC Learning. So I thought it might be useful to have a brief discussion about how switches work and what features should or should not be used in normal office environments. In all cases, the errors turned out to be things that I don’t think should ever have been changed from their default settings. In separate incidents this past month, I’ve helped clients troubleshoot network problems that turned out to be due to misconfigured switches.

Why disable mac address learning