How do Packets Move Around the Network?
When a packet is received by a hub, it is repeated (simultaneously) to one or more ports according to the following rules: • If the packet didn’t come from the uplink port and the hub is not the root hub, then the packet is repeated to the uplink port. • The packet is repeated to all ports that have other hubs plugged into them. These ports are called downlink ports. • The packet is repeated to all ports that are in promiscuous mode (sometimes called monitor mode). • If the destination address in the packet is a broadcast address or multicast address, then it is sent to all active ports. If not, then see rule five. • For non-broadcast/multicast packets, the hub uses its CAM to see if the device matching the packet’s destination address is plugged into the hub. If it is, then the packet is also repeated to that port. These rules have two important side effects: • Every hub sees all packets on the network, and each port that is in promiscuous mode also receives all packets. • A port in n
