ARP and Switching

Hosts ask "who has this IP?" to find a MAC address; the switch quietly learns which MAC lives behind which port. Flood when unsure, forward once learned.

ARP: IP → MAC

Switch: MAC → port

MAC aging: 300 s

Scenario

Animation Speed: 1200ms

Switch MAC table · t = 0 s

empty

Host ARP caches

empty

Four hosts on one Ethernet switch, all in 10.0.0.0/24. IP addresses are how the hosts name each other, but a frame on this LAN is delivered by MAC address — so before sending, a host has to find out the MAC behind an IP (ARP). The switch, meanwhile, starts knowing nothing about which MAC sits on which port.

The Switch's Three Rules

Learnsource MAC → incoming port

Floodbroadcast or unknown destination

Forwardknown destination → its one port

A fourth, filter, applies when the destination is on the port the frame came in on: the switch drops it. A hub has none of these rules — it repeats every frame out every port.

Knowledge Check

Question 1 of 3

What destination MAC address does an ARP request use, and what about the reply?

DSAverse

ARP and Switching

The Switch's Three Rules

Knowledge Check

See also

ARP and Switching

The Switch's Three Rules

Knowledge Check

See also