Configuring Virtual Interfaces

Table Of Contents

Configuring Virtual Interfaces

Contents

Prerequisites for Configuring Virtual Interfaces

Information About Configuring Virtual Interfaces

Virtual Interfaces

Benefits of Virtual Interfaces

Loopback Interfaces

Loopback Interfaces Versus Loopback Mode

Null Interfaces

Subinterfaces

Tunnel Interfaces

How to Configure Virtual Interfaces

Configuring a Loopback Interface

Prerequisites

Examples

Configuring a Null Interface

ICMP Unreachable Messages from Null Interfaces

Restrictions

Examples

Configuration Examples for Virtual Interfaces

Configuring a Loopback Interface: Example

Configuring a Null Interface: Example

Where to Go Next

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Feature Information for Configuring Virtual Interfaces

Configuring Virtual Interfaces

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Virtual interfaces are software-based interfaces that you create in the memory of the networking device using Cisco IOS commands. Virtual interfaces do not have a hardware component such as the RJ-45 female port on a 100BASE-T Fast Ethernet network interface card. This module describes the four common types of virtual, or logical, interfaces that can be configured using Cisco IOS software:

•Loopback interfaces

•Null interfaces

•Subinterfaces

•Tunnel interfaces

Module History

This module was first published on May 2, 2005, and last updated on May 2, 2005.

Finding Feature Information in This Module

Your Cisco IOS software release may not support all features. To find information about feature support and configuration, use the "Feature Information for Configuring Virtual Interfaces" section.

Contents

•Prerequisites for Configuring Virtual Interfaces

•Information About Configuring Virtual Interfaces

•How to Configure Virtual Interfaces

•Configuration Examples for Virtual Interfaces

•Where to Go Next

•Additional References

•Feature Information for Configuring Virtual Interfaces

Prerequisites for Configuring Virtual Interfaces

Before virtual interfaces can be used in your network, you must have some physical (hardware) interfaces configured and be able to communicate between the networking devices on which you wish to use virtual interfaces.

Information About Configuring Virtual Interfaces

To configure virtual interfaces, you should understand the following concepts:

•Virtual Interfaces

•Benefits of Virtual Interfaces

•Loopback Interfaces

•Loopback Interfaces Versus Loopback Mode

•Null Interfaces

•Subinterfaces

•Tunnel Interfaces

Virtual Interfaces

Virtual interfaces are network interfaces that are not associated with a physical interface. Physical interfaces have some form of physical element—for example, an RJ-45 male connector on an Ethernet cable. Virtual interfaces exist only in software; there are no physical elements. You identify an individual virtual interface using a numerical ID after the virtual interface name. For example: loopback 0, tunnel 1, and fastethernet 0/0.1. The ID is unique per virtual interface type to make the entire name string unique; for example both a loopback 0 interface and a null 0 interface can exist, but two loopback 0 interfaces cannot exist in a single networking device.

Cisco IOS software supports four types of virtual interfaces;

•loopback

•null

•subinterface

•tunnel.

Benefits of Virtual Interfaces

•A loopback interface can provide a stable interface on which you can assign a Layer 3 address such as an IP or IPX address. This address can be configured as the source address when the networking device needs to send data for protocols such as NetFlow or Cisco Discovery Protocol (CDP) to another device in your network and you want the receiving device to always see the same source IP address from the networking device. This is an issue in networks with multiple equal-cost paths because under normal circumstances the packets that are generated by a networking device use the IP address from the outbound interface as the source address for the packets and because in a network with two or more equal-cost paths from the networking device to the receiving host each packet might use a different outbound interface.

•A null interface provides an alternative method of filtering without the overhead involved with using access lists. For example, instead of creating an outbound access list that prevents traffic to a destination network from being transmitted out an interface, you can configure a static route for the destination network that points to the null interface.

•Subinterfaces were invented as a method of virtually subdividing a physical interface into two or more interfaces so that the IP routing protocols would see the network connection to each remote networking device as a separate physical interface even though the subinterfaces share a common physical interface. One of the first uses of subinterfaces was to resolve the problem with split horizon on Frame Relay WANs.

•The following are several situations in which tunneling (encapsulating traffic in another protocol) is useful:

–To enable multiprotocol local networks over a single-protocol backbone.

–To provide workarounds for networks that use protocols that have limited hop counts; for example, RIP version 1, AppleTalk.

–To connect discontiguous subnetworks.

–To allow virtual private networks across WANs.

Loopback Interfaces

You can specify a software-only interface called a loopback interface to emulate a physical interface. Loopback interfaces are supported on all platforms. A loopback interface is a virtual interface on a Cisco router that remains up (active) after you issue the no shutdown command until you disable it with the shutdown command. Unlike subinterfaces, loopback interfaces are independent of the state of any physical interface.

The loopback interface can be considered stable because once you enable it, it will remain up until you shut it down. This makes loopback interfaces ideal for assigning Layer 3 addresses such as IP addresses when you want a single address as a reference that is independent of the status of any physical interfaces in the networking device. A good example of this is using the IP address of a loopback interface as the IP address for the DNS host address for the networking device. Before loopback interfaces were available, network administrators had to configure a DNS host entry for every interface on a router that had an IP address assigned to it because they could never be certain which interface IP address might be available at any given time for managing the router. In the sample interface configuration and DNS entries for Router A shown below, you can see that there is a DNS entry for each interface.

Router A Interface Configuration Before Loopback

Ethernet0 10.10.10.1 255.255.255.0

Ethernet1 10.10.11.1 255.255.255.0

Ethernet2 10.10.12.1 255.255.255.0

Ethernet3 10.10.13.1 255.255.255.0

Ethernet4 10.10.14.1 255.255.255.0

Ethernet5 10.10.15.1 255.255.255.0

Router A DNS Entries Before Loopback

RouterA   IN  A  10.10.10.1

          IN  A  10.10.11.1

          IN  A  10.10.12.1

          IN  A  10.10.13.1

          IN  A  10.10.14.1

          IN  A  10.10.15.1

Interfaces on networking devices can fail, and they can also be taken out of service for maintenance. If any of the interfaces in Router A fails or is taken out of service, another networking device will not be able to access that interface. When you configure a networking device with a loopback interface and assign it an IP address that is advertised throughout the network, the networking device will be reachable by using this IP address as long as the networking device has at least one network interface capable of sending and receiving IP traffic. In the sample interface configuration and DNS entries for Router A after a loopback interface is configured, you can see that there is now only one DNS entry that can be used to reach the router over any of its physical interfaces.

Router A Interface Configuration After Loopback

Loopback 172.16.78.1 255.255.255.0

Ethernet0 10.10.10.1 255.255.255.0

Ethernet1 10.10.11.1 255.255.255.0

Ethernet2 10.10.12.1 255.255.255.0

Ethernet3 10.10.13.1 255.255.255.0

Ethernet4 10.10.14.1 255.255.255.0

Ethernet5 10.10.15.1 255.255.255.0

Router A DNS Entries After Loopback

RouterA   IN  A  172.16.78.1

The configured IP address of the loopback interface—172.16.78.1—can be used as the source address for packets generated by the router and forwarded to networking management applications and routing protocols. Unless this loopback interface is explicitly shut down, it is always reachable.

You can use the loopback interface as the termination address for OSPF or BGP sessions. A loopback interface can also be used to establish a Telnet session from the console port of the device to its auxiliary port when all other interfaces are down. In applications where 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.

IP 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. Under these two conditions, the loopback interface can behave like a null interface.

Loopback Interfaces Versus Loopback Mode

Loopback interfaces provide a stable source interface to ensure that the IP address assigned to the interface is always reachable as long as the IP routing protocols continue to advertise the subnet assigned to the loopback interface. Loopback mode, however, is used to test and diagnose issues with WAN (serial) links such as bit loss or data corruption. The idea is to configure a loop to return the data packets that were received by the interface back out the same interface to the device that originated the traffic. Loopback mode is used to troubleshoot problems by checking that the data packets are returned in the same condition in which they were sent. Errors in the data packets indicate a problem with the WAN infrastructure. Many types of serial interfaces have their own form of loopback command syntax that is entered under interface or controller configuration mode.

For more details about loopback mode, see the "Configuring Serial Interfaces" chapter.

Null Interfaces

The null interface is a virtual network interface that is similar to the loopback interface. Whereas traffic to the loopback interface is directed to the router itself, traffic sent to the null interface is discarded. This interface is always up and can never forward or receive traffic; encapsulation always fails. The null interface functions similarly to the null devices available on most operating systems.

Null interfaces are used as a low-overhead method of discarding unnecessary network traffic. For example, if you do not want your network users to be able to reach certain IP subnets, you can create static IP routes for the subnets that point to the null interface of a networking device. Using the static IP routes takes less CPU time for the networking device than using IP access lists. The static-route configuration is also easier to configure than IP access lists because it is done in global configuration mode instead of in interface configuration mode.

The null interface may not be configured with an address. Traffic can be sent to this interface only by configuring a static route where the next hop is the null interface—represented by Null 0. One example of configuring the next hop to be the null interface is to create a route to an aggregate network that can then be announced through the BGP, or to ensure that traffic to a particular range of addresses is not propagated through the router, perhaps for security purposes.

The router always has a single null interface. By default, a packet sent to the null interface causes the router to respond by sending an ICMP unreachable message to the source IP address of the packet. You can configure the router either to send these responses or to drop the packets silently.

Subinterfaces

Subinterfaces are associated with physical interfaces. Subinterfaces are enabled when the physical interface with which they are associated is enabled, and subinterfaces are disabled when the physical interface is shut down.

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Note Subinterfaces can be enabled and shut down independently of the physical port with which they are associated. However, you cannot enable a subinterface of a physical interface that has been shut down.

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Subinterfaces are created by subdividing the physical interface into two or more virtual interfaces on which you can assign unique Layer 3 network addresses such as IP subnets. One of the first uses of subinterfaces was to resolve the problem with split horizon on Frame Relay WANs. Split horizon is a behavior associated with IP routing protocols such as RIP and OSPF in which IP subnets are not advertised back out the same physical interface that they were learned over. Split horizon was implemented to prevent routing loops in IP networks. A routing loop can be created when the networking devices at both ends of a network connection advertise the same IP routes to each other. Split horizon was an issue for Frame Relay multipoint network interfaces—interfaces that connect to two or more remote networking devices over a single physical interface—because the default behavior of many networking devices was to implement split horizon, which means that the networking device did not advertise the IP routes that were learned over an interface back out the interface to other devices that were also reachable via the same physical interface. Subinterfaces were invented as a method of virtually subdividing a physical interface into two or more interfaces so that the IP routing protocols would see the network connection to each remote networking device as a separate physical interface even though the subinterfaces share a common physical interface. Although TCP/IP now disables split horizon limitations by default, protocols such as AppleTalk and IPX are still constrained by split horizon.

Subinterfaces are identified by a prefix that consists of the hardware interface descriptor (IDB) followed by a period and then by a number that is unique for that prefix. The full subinterface number must be unique to the networking device. For example, the first subinterface for Ethernet interface 0/0 might be named Ethernet 0/0.1 where .1 indicates the subinterface.

Tunnel Interfaces

Tunneling provides a way to encapsulate arbitrary packets inside a transport protocol. Tunnels are 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.

There are several ways to implement tunnel interfaces depending on the connectivity that you need to provide. One common use for tunnels is to carry data traffic for a network protocol such as IPX over devices in your network that do not support IPX. For instance, if your network uses IPX in sites at the edge of your network but not in the core of your network, you can connect the IPX sites at the network edges by tunneling IPX in IP over the core of the network.

For more details about the various types of tunneling techniques available using Cisco IOS software, see the "Implementing Tunnels" module.

How to Configure Virtual Interfaces

This section contains the following tasks:

•Configuring a Loopback Interface

•Configuring a Null Interface

Configuring a Loopback Interface

This task explains how to configure a loopback interface. A loopback interface can be considered stable because once you enable it, it will remain up until you shut it down. This makes loopback interfaces ideal for assigning Layer 3 addresses such as IP addresses to when you want to have a single address to use as a reference that is independent of the status of any of the physical interfaces in the networking device.

Prerequisites

The IP address for the loopback interface must be unique and not in use by another interface.

SUMMARY STEPS

1. enable

2. configure terminal

3. interface loopback number

4. ip address ip-address mask [secondary]

5. end

6. show interfaces loopback number

7. exit

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. •Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	interface loopback number Example: Router(config)# interface loopback 0	Specifies a loopback interface and enters interface configuration mode. •Use the number argument to specify the number of the loopback interface that you want to create or configure. Note There is no limit on the number of loopback interfaces that you can create.
Step 4	ip address ip-address mask [secondary] Example: Router(config-if)# ip address 10.20.1.2 255.255.255.0	Specifies an IP address for the loopback interface and enables IP processing on the interface. •Use the ip-address and mask arguments to specify the subnet for the loopback address.
Step 5	end Example: Router(config-if)# end	Exits interface configuration mode and returns to privileged EXEC mode.
Step 6	show interfaces loopback number Example: Router# show interfaces loopback 0	(Optional) Displays information about loopback interfaces. •Use the number argument to display information about one particular loopback interface. Note Only the syntax applicable to this task is used in this example. For more details, see the Cisco IOS Interface and Hardware Component Command Reference, Release 12.4.
Step 7	exit Example: Router# exit	Exits privileged EXEC mode.

Examples

The following is sample output for the show interfaces loopback command.

Router# show interfaces loopback

Loopback0 is up, line protocol is up 

  Hardware is Loopback

  Internet address is 10.20.1.2/24

  MTU 1514 bytes, BW 8000000 Kbit, DLY 5000 usec, 

     reliability 255/255, txload 1/255, rxload 1/255

  Encapsulation LOOPBACK, loopback not set

  Last input never, output never, output hang never

  Last clearing of "show interface" counters never

  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0

  Queueing strategy: fifo

  Output queue: 0/0 (size/max)

  5 minute input rate 0 bits/sec, 0 packets/sec

  5 minute output rate 0 bits/sec, 0 packets/sec

     0 packets input, 0 bytes, 0 no buffer

     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles

     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort

     0 packets output, 0 bytes, 0 underruns

     0 output errors, 0 collisions, 0 interface resets

     0 output buffer failures, 0 output buffers swapped out

Configuring a Null Interface

This task explains how to configure a null interface. Null interfaces provide an alternative method to access control lists for filtering traffic. All unwanted traffic can be directed to the null interface; the null interface cannot receive or forward traffic, or allow its traffic to be encapsulated.

The only interface configuration command that you can specify for the null interface is the no ip unreachables command.

ICMP Unreachable Messages from Null Interfaces

By default, a packet sent to the null interface causes the router to respond by sending an Internet Control Message Protocol (ICMP) unreachable message to the source IP address of the packet. You can configure the router either to send these responses or to drop the packets silently.

To disable the sending of ICMP unreachable messages in response to packets sent to the null interface, use the no ip unreachables command in interface configuration mode. To reenable the sending of ICMP unreachable messages in response to packets sent to the null interface, use the ip unreachables command in interface configuration mode.

Restrictions

Only one null interface can be configured on each networking device.

SUMMARY STEPS

1. enable

2. configure terminal

3. interface null number

4. no ip unreachables

5. end

6. show interfaces null [number] [accounting]

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. •Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	interface null number Example: Router(config)# interface null 0	Specifies a null interface and number, and enters interface configuration mode. •The number argument is always 0.
Step 4	no ip unreachables Example: Router(config-if)# no ip unreachables	Prevents the generation of ICMP unreachable messages on an interface. •This command affects all types of ICMP unreachable messages.
Step 5	end Example: Router(config-if)# end	Exits interface configuration mode and returns to privileged EXEC mode.
Step 6	show interfaces null [number] [accounting] Example: Router# show interfaces null 0	(Optional) Displays information about null interfaces. •For null interfaces, the number argument is always 0. Note Only the syntax applicable to this task is used in this example. For more details, see the Cisco IOS Interface and Hardware Component Command Reference, Release 12.4.

Examples

The following is sample output for the show interfaces null command.

Router# show interfaces null

Null0 is up, line protocol is up 

  Hardware is Unknown

  MTU 1500 bytes, BW 10000000 Kbit, DLY 0 usec, 

     reliability 0/255, txload 0/255, rxload 0/255

  Encapsulation ARPA, loopback not set

  Last input never, output never, output hang never

  Last clearing of "show interface" counters never

  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0

  5 minute input rate 0 bits/sec, 0 packets/sec

  5 minute output rate 0 bits/sec, 0 packets/sec

     0 packets input, 0 bytes, 0 no buffer

     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles

     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort

     0 packets output, 0 bytes, 0 underruns

     0 output errors, 0 collisions, 0 interface resets

     0 output buffer failures, 0 output buffers swapped out

Configuration Examples for Virtual Interfaces

This section contains the following examples:

•Configuring a Loopback Interface: Example

•Configuring a Null Interface: Example

Configuring a Loopback Interface: Example

The following example shows how to configure a loopback interface, loopback 0.

interface loopback 0

 ip address 10.20.1.2 255.255.255.0

 end

Configuring a Null Interface: Example

The following example shows how to configure a null interface and to drop the ICMP unreachable messages. All packets sent to the null interface are dropped and in this example, the ICMP messages usually sent in response to packets being sent to the null interface are dropped.

interface null 0

 no ip unreachables

 end

Where to Go Next

•If you want to implement tunnels in your network, see the "Implementing Tunnels" module.

•If you want to implement other types of interfaces such as LAN or serial in your network, see the "Configuring LAN Interfaces" or the "Configuring Serial Interfaces" chapters.

Additional References

The following sections provide references related to virtual interfaces.

Related Documents

Related Topic	Document Title
Interface commands: complete command syntax, command mode, defaults, command history, usage guidelines, and examples	Cisco IOS Interface and Hardware Component Command Reference, Release 12.4
Configuration example showing how to use loopback interfaces with BGP	Sample Configuration for iBGP and eBGP With or Without a Loopback Address

Standards

Standard	Title
No new or modified standards are supported, and support for existing standards has not been modified.	—

MIBs

MIB	MIBs Link
No new or modified MIBs are supported, and support for existing MIBs has not been modified.	To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL: http://www.cisco.com/go/mibs

RFCs

RFC	Title
No new or modified RFCs are supported, and support for existing RFCs has not been modified.	—

Technical Assistance

Description	Link
The Cisco Technical Support website contains thousands of pages of searchable technical content, including links to products, technologies, solutions, technical tips, and tools. Registered Cisco.com users can log in from this page to access even more content.	http://www.cisco.com/techsupport

Feature Information for Configuring Virtual Interfaces

Table 1 lists the features in this module and provides links to specific configuration information. Only features that were introduced or modified in Cisco IOS Release 12.2(1) or later appear in the table.

Not all commands may be available in your Cisco IOS software release. For details on when support for specific commands was introduced, see the command reference documents.

If you are looking for information on a feature in this technology that is not documented here, see the "<<x-ref to the title of the information product roadmap—Section format>>."

Cisco IOS software images are specific to a Cisco IOS software release, a feature set, and a platform. Use Cisco Feature Navigator to find information about platform support and Cisco IOS software image support. Access Cisco Feature Navigator at http://www.cisco.com/go/fn. You must have an account on Cisco.com. If you do not have an account or have forgotten your username or password, click Cancel at the login dialog box and follow the instructions that appear.

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Note Table 1 lists only the Cisco IOS software release that introduced support for a given feature in a given Cisco IOS software release train. Unless noted otherwise, subsequent releases of that Cisco IOS software release train also support that feature.

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Table 1 Feature Information for Configuring Virtual Interfaces 

Feature Name	Releases	Feature Configuration Information
This table is intentionally left blank because no features were introduced or modified in Cisco IOS Release 12.2(1) or later. This table will be updated when feature information is added to this module.	—	—

Copyright © 2005 Cisco Systems, Inc. All rights reserved.
This module first published May 2, 2005. Last updated May 2, 2005.