CISCO IOS in a Nutshell

Ethernet and Fast Ethernet Interfaces

Give an Ethernet or Fast Ethernet interface an IP address, and it’s ready to go. It’s a good idea to give the interface a description, but that’s not required. Also, as with all interfaces, don’t forget the no shutdown command. For Fast Ethernet interfaces, you can also specify the interface speed and whether it’s operating at full or half duplex:

interface Ethernet0
  description Internal Office Network
  ip address 10.10.1.1 255.255.255.248
  no shutdown
interface FastEthernet1
  description Fast Network for fileservers
  ip address 10.10.1.17 255.255.255.248
  ! Set the speed to 100 (which is the default)
  speed 100
  ! Set the duplex to full
  duplex full
  no shutdown

Ethernet Encapsulation

By default, an Ethernet uses ARPA encapsulation (standard Ethernet Version 2.0). However, you can specify another encapsulation type using the encapsulation command. For example:

interface ethernet 0
   encapsulation arpa
   ip address 10.10.1.10 255.255.255.248

The possible encapsulation types are shown in Table 5-3.

Table 5-3. Ethernet encapsulation types

Keyword	Encapsulation type
arpa	Standard Ethernet Version 2.0
isol	IEEE 802.3 Encapsulation
snap	IEEE 803.3 Encapsulation per RFC 1042

All the devices connected to an Ethernet must share the same encapsulation type; if they do not, they will not be able to communicate. If you’re not sure of the encapsulation type, you’re probably using ARPA.