Configuring Loopback and null interfaces 1

Loopback interface· 1

Introduction· 1

Configuring a loopback interface· 1

Null interface· 2

Introduction· 2

Configuring interface Null 0· 2

Displaying and maintaining loopback and null interfaces 3

Loopback interface

Introduction

A loopback interface is a software-only virtual interface. It delivers the following benefits.

·           The physical layer state and link layer protocols of a loopback interface are always up unless the loopback interface is manually shut down.

·           You must assign a loopback interface an IP address with an all-F mask to save the IP address resources. When you assign an IPv4 address whose mask is not 32-bit, the system automatically changes the mask into a 32-bit mask. When you assign an IPv6 address whose mask is not 128-bit, the system automatically changes the mask into a 128-bit mask.

·           You can enable routing protocols on a loopback interface, and a loopback interface can send and receive routing protocol packets.

Because of the benefits mentioned above, loopback interfaces are widely used in the following scenarios：

·           You can configure a loopback interface address as the source address of the IP packets that the device generates. Because loopback interface addresses are stable unicast addresses, they are usually used as device identifications. Therefore, when you configure a rule on an authentication or security server to permit or deny packets generated by a device, you can simplify the rule by configuring it to permit or deny packets carrying the loopback interface address identifying the device. Note that, when you use a loopback interface address as the source address of IP packets, make sure that the route from the loopback interface to the peer is reachable by performing routing configuration. All data packets sent to the loopback interface are considered as packets sent to the device itself, so the device does not forward these packets.

·           Because a loopback interface is always up unless you manually shut it down, it can be used in dynamic routing protocols. For example, if no router ID is configured for a dynamic routing protocol, the highest loopback interface IP address is selected as the router ID. In BGP, to avoid BGP sessions being interrupted by physical port failure, you can use a loopback interface as the source interface of BGP packets.

Configuring a loopback interface

To configure a loopback interface:

Null interface

Introduction

A null interface is a completely software-based logical interface, and is always up. However, you cannot use it to forward data packets or configure an IP address or link layer protocol on it. With a null interface specified as the next hop of a static route to a specific network segment, any packets routed to the network segment are dropped. The null interface provides a simpler way to filter packets than ACL. You can filter uninteresting traffic by transmitting it to a null interface instead of applying an ACL.

For example, by executing the ip route-static 92.101.0.0 255.255.0.0 null 0 command (which configures a static route leading to null interface 0), you can have all the packets destined for the network segment 92.101.0.0/16 discarded.

Currently, only one null interface, interface Null 0, is supported on your router. You cannot remove or create a null interface.

Configuring interface Null 0

To configure interface Null 0:

Displaying and maintaining loopback and null interfaces