Table of Contents

1 Loopback and Null Interface Configuration· 1-1

Loopback Interface· 1-1

Introduction to Loopback Interface· 1-1

Configuring a Loopback Interface· 1-1

Null Interface· 1-2

Introduction to Null Interface· 1-2

Configuring Interface Null 0· 1-2

Displaying and Maintaining Loopback and Null Interfaces· 1-3

This chapter includes these sections:

l          Loopback Interface

l          Null Interface

l          Displaying and Maintaining Loopback and Null Interfaces

Loopback Interface

Introduction to Loopback Interface

A loopback interface is a software-only 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 can be configured with an IP address. For the purpose of saving IP address resources, the IP address is automatically configured with a 32-bit mask. Routing protocols can be enabled on a loopback interface, and a loopback interface is capable of sending and receiving routing protocol packets.

Loopback interfaces are widely used. For example, you can configure a loopback interface address as the source address of all the IP packets that the AP 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 an AP, you can streamline the rule by configuring it to permit or deny packets carrying the loopback interface address identifying the AP. Note that, when a loopback interface is used for source address binding (that is, assigning an IP address to this loopback interface), make sure that the route from the loopback interface to the peer is reachable; all data packets sent to the loopback interface are considered as packets sent to the AP itself, so the AP does not forward these packets.

Because a loopback interface is always up, it can be used for some other special purposes. 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. For another example, to avoid BGP sessions being interrupted by physical port failure, you can use a loopback interface as the source interface of BGP packets. However, you must ensure that the IP address of the loopback interface on the BGP peer is reachable. If EBGP connection is involved, you need to allow the establishment of EBGP connections to non-directly-connected peers.

Configuring a Loopback Interface

Follow these steps to configure a loopback interface:

Null Interface

Introduction to Null Interface

A null interface is a completely software-based logical interface. A null interface is always up. However, you can neither use it to forward data packets nor 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 you a simpler way to filter packets than ACL. In other words, 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.

Currently, the AP supports only one null interface, Null 0. You can neither remove this null interface nor create a new one.

Configuring Interface Null 0

Follow these steps to enter null interface view:

Displaying and Maintaining Loopback and Null Interfaces