Configuring loopback and null interfaces 1

Configuring a loopback interface· 1

Introduction· 1

Configuration procedure· 1

Configuring a null interface· 2

Introduction· 2

Configuration procedure· 2

Displaying and maintaining loopback and null interfaces 2

This chapter describes how to configure a loopback interface and a null interface.

Configuring a 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.

·           Loopback interfaces can 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 switch generates. Because loopback interface addresses are stable unicast addresses, they are usually used as switch identifications. Therefore, when you configure a rule on an authentication or security server to permit or deny packets generated by a switch, you can simplify the rule by configuring it to permit or deny packets carrying the loopback interface address identifying the switch. 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 switch itself, so the switch does not forward these packets.

·           Because a loopback interface is always up, 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.

Configuration procedure

To configure a loopback interface:

Configuring a null interface

Introduction

Null interfaces are completely software-like logical interfaces. Null interfaces are always up. However, they can neither forward data packets nor have IP addresses and link layer protocols configured on them. 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. Null interface provides you a way to filter packets. That is, you can simply transmit unwanted traffic to a null interface rather than applying ACLs.

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 a null interface), you can have all the packets destined to the network segment 92.101.0.0/16 discarded.

Configuration procedure

To configure a null interface:

Displaying and maintaining loopback and null interfaces