Cisco IOS Interface Configuration Guide

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Cisco IOS Interface Configuration Guide

Configuring Logical Interfaces

Use the information in this chapter to understand and configure the types of logical, or virtual, interfaces

supported on Cisco routers and access servers. This chapter includes the following sections:

• Configuring a Loopback Interface

• Configuring a Null Interface

• Configuring a Tunnel Interface

For examples of configuration tasks, see the “Logical Interface Configuration Examples” section.

For hardware technical descriptions and information about installing interfaces, refer to the hardware

installation and configuration publication for your product. For complete descriptions of the logical

interface commands, refer to the “Interface Commands” chapter of the Cisco IOS Interface Command

Reference. To locate documentation of other commands that appear in this chapter, use the command

reference master index or search online.

To identify the hardware platform or software image information associated with a feature, use the

Feature Navigator on Cisco.com to search for information about the feature or refer to the software

release notes for a specific release. For more information, see the Identifying Supported Platforms in

“Using Cisco IOS Software.”

Configuring a Loopback Interface

You can specify a software-only interface called a loopback interface to emulate an interface. Loopback

interfaces are supported on all platforms. A loopback interface is a virtual interface that is always up and

allows Border Gateway Protocol (BGP) and remote source-route bridging (RSRB) sessions to stay up

even if the outbound interface is down.

You can use the loopback interface as the termination address for BGP sessions, for RSRB connections,

or to establish a Telnet session from the device’s console to its auxiliary port when all other interfaces

are down. You can also use a loopback interface to configure IPX-PPP on asynchronous interfaces. To

do so, you must associate an asynchronous interface with a loopback interface configured to run IPX. In

applications in which other routers or access servers attempt to reach this loopback interface, you should

configure a routing protocol to distribute the subnet assigned to the loopback address.

Packets routed to the loopback interface are rerouted back to the router or access server and processed

locally. IP packets routed out the loopback interface but not destined to the loopback interface are

dropped. This means that the loopback interface serves as the Null 0 interface also.

Configuring Logical Interfaces

Configuring a Null Interface

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Cisco IOS Interface Configuration Guide

Note Loopback does not work on an X.21 DTE because the X.21 interface definition does not include a

loopback definition.

To specify a loopback interface and enter interface configuration mode, use one of the following

commands in global configuration mode:

For more general information about loopback interfaces, see the “Running Interface Loopback

Diagnostics” section in the “Features for Any Interface” chapter.

Configuring a Null Interface

The Cisco IOS software supports a “null” interface. This pseudo-interface functions similarly to the null

devices available on most operating systems. This interface is always up and can never forward or

receive traffic; encapsulation always fails. The only interface configuration command that you can

specify for the null interface is no ip unreachables.

The null interface provides an alternative method of filtering traffic. You can avoid the overhead involved

with using access lists by directing undesired network traffic to the null interface.

To specify the null interface, use the following command in global configuration mode:

Specify null 0 (or null0) as the interface type and number. The null interface can be used in any command

that has an interface type as an argument. The following example configures a null interface for IP route

127.0.0.0:

ip route 127.0.0.0 255.0.0.0 null 0

Configuring a Tunnel Interface

Tunneling provides a way to encapsulate arbitrary packets inside a transport protocol. This feature is

implemented as a virtual interface to provide a simple interface for configuration. The tunnel interface

is not tied to specific “passenger” or “transport” protocols, but rather, it is an architecture that is designed

to provide the services necessary to implement any standard point-to-point encapsulation scheme.

Because tunnels are point-to-point links, you must configure a separate tunnel for each link.

Command Purpose

Router(config)# interface loopback number Enters interface configuration.

Router(config)# interface loopback

slot/port

Enters interface configuration for Cisco 7200 series or Cisco 7500

series routers.

Router(config)# interface loopback

slot/port-adapter/port

Enters interface configuration for Cisco 7500 series routers.

Command Purpose

Router(config)# interface null 0 Enters interface configuration.

Configuring Logical Interfaces

Configuring a Tunnel Interface

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Tunneling has the following three primary components:

• Passenger protocol, which is the protocol that you are encapsulating (AppleTalk, Banyan VINES,

CLNS, DECnet, IP, or IPX)

• Carrier protocol, which is one of the following encapsulation protocols:

– Generic route encapsulation (GRE), Cisco’s multiprotocol carrier protocol

– Cayman, a proprietary protocol for AppleTalk over IP

– EON, a standard for carrying CLNP over IP networks

– NOS, IP over IP compatible with the popular KA9Q program

– Distance Vector Multicast Routing Protocol (DVMRP) (IP in IP tunnels)

• Transport protocol, which is the protocol used to carry the encapsulated protocol (IP only)

Figure 22 illustrates IP tunneling terminology and concepts.

Figure 22 IP Tunneling Terminology and Concepts

To understand the process of tunneling, consider connecting two AppleTalk networks with a

non-AppleTalk backbone, such as IP. The relatively high bandwidth consumed by the broadcasting of

Routing Table Maintenance Protocol (RTMP) data packets can severely hamper the backbone’s network

performance. This problem can be solved by tunneling AppleTalk through a foreign protocol, such as IP.

Tunneling encapsulates an AppleTalk packet inside the foreign protocol packet, which is then sent across

the backbone to a destination router. The destination router then removes the encapsulation from the

AppleTalk packet and, if necessary, routes the packet to a normal AppleTalk network. Because the

encapsulated AppleTalk packet is sent in a directed manner to a remote IP address, bandwidth usage is

greatly reduced. Furthermore, the encapsulated packet benefits from any features normally enjoyed by

IP packets, including default routes and load balancing.

802.3 802.2 CLNP TP4 VT

Ethernet IP GRE CLNP TP4 VT

Normal packet

Tunnel packet

Passenger protocol

Encapsulation protocol

Transport protocol

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Configuring Logical Interfaces

Configuring a Tunnel Interface

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Advantages of Tunneling

The following are several situations in which encapsulating traffic in another protocol is useful:

• To provide multiprotocol local networks over a single-protocol backbone.

• To provide workarounds for networks containing protocols that have limited hop counts;

for example, AppleTalk (see Figure 23).

• To connect discontinuous subnetworks.

• To allow virtual private networks across WANs.

Figure 23 Providing Workarounds for Networks with Limited Hop Counts

Special Considerations for Configuring Tunnel Interfaces

The following are considerations and precautions to observe when you configure tunneling:

• Encapsulation and the removal of encapsulation at the tunnel end points are slow operations; in

general, only processor switching is supported. However, fast switching of GRE tunnels was

introduced in Cisco IOS Release 11.1 for the Cisco 2500 series and the Cisco 4000 series of routers.

• Consider security and topology issues. Be careful not to violate access control lists. You can

configure a tunnel with a source and destination that are not restricted by firewall routers.

• Tunneling might create problems with transport protocols that have limited timers (for example,

DECnet) because of increased latency.

• Be aware of the environments across which you create tunnels. You might be tunneling across fast

FDDI rings or through slow 9600-bps phone lines; some passenger protocols function poorly in

mixed media networks.

• Multiple point-to-point tunnels can saturate the physical link with routing information.

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If the path between two computers has more than 15 hops, they cannot communicate with each other,

but it is possible to hide some of the hops inside

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