Configuring loopback and null interfaces· 2

Configuring a loopback interface· 2

Introduction· 2

Configuration procedure· 2

Configuring a null interface· 3

Introduction· 3

Configuration procedure· 3

Displaying and maintaining loopback and null interfaces 3

Configuring a loopback interface

Introduction

A loopback interface is a virtual interface. The physical layer state and link layer protocols of a loopback interface are always up unless the loopback interface is manually shut down. A loopback interface is widely used in the following scenarios:

·          A loopback interface address can be configured 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. When you configure a rule on an authentication or security server to permit or deny packets that a device generates, you can simplify the rule by configuring it to permit or deny packets carrying the loopback interface address that identifies the device. When you use a loopback interface address as the source address of IP packets, make sure the peer is reachable through routes 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.

·          A loopback interface is often 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.

Configuration procedure

To configure a loopback interface:

You can configure settings such as IP addresses and IP routes on loopback interfaces. For more information, see Layer 3 Configuration Guide.

Configuring a 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 to the network segment 92.101.0.0/16 discarded.

Only one null interface, Null 0, is supported on your device. You cannot remove or create a null interface.

Configuration procedure

To configure a null interface:

Displaying and maintaining loopback and null interfaces