What three things must be configured on a router interface to make it operational?

An IP (Internet Protocol) address, a no shutdown to activate it (router interfaces are down by default), and optionally a description.

1. IP Address: At least one IPv4 or IPv6 address

   R1(config-if)# ip address 192.168.10.1 255.255.255.0
   R1(config-if)# ipv6 address 2001:db8:acad:1::1/64
   

2. Activated (no shutdown): Router interfaces are shutdown by default (unlike switches)

   R1(config-if)# no shutdown
   

3. Description (optional but recommended): Up to 240 characters for documentation

   R1(config-if)# description Link to LAN 1
   

Key difference from switches: Router LAN (Local Area Network) and WAN (Wide Area Network) interfaces are administratively down by default. You must explicitly enable them with no shutdown. Switch ports are enabled by default.

Interface types on a router:

• GigabitEthernet (e.g., G0/0/0, G0/0/1) — LAN connections
• Serial (e.g., S0/1/0) — WAN connections
• HWIC (High-Speed WAN Interface Card) slots — for DSL (Digital Subscriber Line), cable, and other WAN interface cards