Naming and Numbering Interfaces

Interfaces are configured with the interface command, followed by an interface name, followed by a port number. The space between the interface name and the port number is optional. For example:

interface serial 0 Serial port 0
interface serial 1 Serial port 1
interface ethernet 0 Ethernet port 0
interface ethernet 1 Ethernet port 1

On high-end routers, the interface cards are in slots; each slot has a series of ports. To specify these interfaces, use the slot/port naming scheme. For example, the Ethernet interface on port 5 of the card in slot 4 would be called:

interface ethernet 4/5

The interface command is followed by other commands that perform the actual configuration. If you're entering commands at the console, the interface command changes the prompt to Router(config-if)#.

VIP2 (Versatile Interface Processor) cards have two Ethernet ports per card. To accommodate these cards, use the syntax card/slot/port to specify a particular Ethernet interface. For example, Fast Ethernet card 2 on slot 1 on port 0 would be called:

interface fastethernet 2/1/0

 

5.1.1. Subinterfaces

Subinterfaces provide a way to have multiple logical configurations for the same interface; they are most commonly used in Frame Relay, ATM, and Fast Ethernet in switched environments. To specify a subinterface, add a period and the subinterface number to the regular interface name. For example:

interface serial 1.1
interface serial 1.2

On a high-end router that uses the slot/port notation, append the subinterface number to the port number:

interface serial 1/2.1
interface serial 1/2.2

Subinterface zero (0) refers to the actual interface; i.e., serial1 is equal to serial1.0.

Here's a simple example that shows how subinterfaces are typically used. Frame Relay permits subinterfaces in both point-to-point and multipoint modes. Each mode can have its own IP address and subnet mask. This multiple-IP address configuration can be accomplished only with the subinterface commands:

interface serial 1
 no shutdown
interface serial 1.1 point-to-point
 ip address 10.10.1.2 255.255.255.0
interface serial 1.2 point-to-multipoint
 ip address 10.10.2.2 255.255.255.0

In this example, we apply the no shutdown command to serial 1, which includes both subinterfaces. We then assign a different IP address to subinterface 1 and subinterface 2, each with a subnet mask of 255.255.255.0.

Most commands that apply to interfaces can also be applied to subinterfaces. However, a few commands can be applied only to an interface or to a subinterface.