Table Of Contents
Configuring Virtual Interfaces
Prerequisites for Configuring Virtual Interfaces
Information About Configuring Virtual Interfaces
Benefits of Virtual Interfaces
Loopback Interfaces Versus Loopback Mode
How to Configure Virtual Interfaces
Configuring a Loopback Interface
ICMP Unreachable Messages from Null Interfaces
Configuration Examples for Virtual Interfaces
Configuring a Loopback Interface: Example
Configuring a Null Interface: Example
Configuring Virtual Interfaces
First Published: May 12, 2009Last Updated: May 12, 2009Virtual interfaces are software-based interfaces that you create in the memory of the networking device using Cisco IOS XE 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 XE software:
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Loopback interfaces
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Contents
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Prerequisites for Configuring Virtual Interfaces
Before virtual interfaces can be used in your network, you must have some physical (hardware) interfaces configured and you must 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:
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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 0/0/0.1. 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 XE software supports four types of virtual interfaces:
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Benefits of Virtual Interfaces
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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
GigabitEthernet0 10.10.10.1 255.255.255.0GigabitEthernet1 10.10.11.1 255.255.255.0GigabitEthernet2 10.10.12.1 255.255.255.0GigabitEthernet3 10.10.13.1 255.255.255.0GigabitEthernet4 10.10.14.1 255.255.255.0GigabitEthernet5 10.10.15.1 255.255.255.0Router A DNS Entries Before Loopback
RouterA IN A 10.10.10.1IN A 10.10.11.1IN A 10.10.12.1IN A 10.10.13.1IN A 10.10.14.1IN A 10.10.15.1Interfaces 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 172.16.78.1 255.255.255.0GigabitEthernet0 10.10.10.1 255.255.255.0GigabitEthernet1 10.10.11.1 255.255.255.0GigabitEthernet2 10.10.12.1 255.255.255.0GigabitEthernet3 10.10.13.1 255.255.255.0GigabitEthernet4 10.10.14.1 255.255.255.0GigabitEthernet5 10.10.15.1 255.255.255.0Router A DNS Entries After Loopback
RouterA IN A 172.16.78.1The 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.
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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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 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 GigabitEthernet interface 0/0/0 might be named GigabitEthernet 0/0/0.1 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 XE software, see the "Implementing Tunnels" module of the Cisco IOS XE Interface and Hardware Component Configuration Guide, Release 2.
How to Configure Virtual Interfaces
This section contains the following tasks:
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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 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
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DETAILED STEPS
Step 1
enable
Example:Router> enable
Enables privileged EXEC mode.
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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.
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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.
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end
Example:Router(config-if)# end
Exits interface configuration mode and returns to privileged EXEC mode.
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show interfaces loopback number
Example:Router# show interfaces loopback 0
(Optional) Displays information about loopback interfaces.
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exit
Example:Router# exit
Exits privileged EXEC mode.
Examples
The following is sample output for the show interfaces loopback command.
Router# show interfaces loopbackLoopback0 is up, line protocol is upHardware is LoopbackInternet address is 10.20.1.2/24MTU 1514 bytes, BW 8000000 Kbit, DLY 5000 usec,reliability 255/255, txload 1/255, rxload 1/255Encapsulation LOOPBACK, loopback not setLast input never, output never, output hang neverLast clearing of "show interface" counters neverInput queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0Queueing strategy: fifoOutput queue: 0/0 (size/max)5 minute input rate 0 bits/sec, 0 packets/sec5 minute output rate 0 bits/sec, 0 packets/sec0 packets input, 0 bytes, 0 no bufferReceived 0 broadcasts, 0 runts, 0 giants, 0 throttles0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort0 packets output, 0 bytes, 0 underruns0 output errors, 0 collisions, 0 interface resets0 output buffer failures, 0 output buffers swapped outConfiguring 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
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DETAILED STEPS
Step 1
enable
Example:Router> enable
Enables privileged EXEC mode.
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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.
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no ip unreachables
Example:Router(config-if)# no ip unreachables
Prevents the generation of ICMP unreachable messages on an interface.
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Step 5
end
Example:Router(config-if)# end
Exits interface configuration mode and returns to privileged EXEC mode.
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show interfaces null [number] [accounting]
Example:Router# show interfaces null 0
(Optional) Displays information about null interfaces.
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Examples
The following is sample output for the show interfaces null command.
Router# show interfaces nullNull0 is up, line protocol is upHardware is UnknownMTU 1500 bytes, BW 10000000 Kbit, DLY 0 usec,reliability 0/255, txload 0/255, rxload 0/255Encapsulation ARPA, loopback not setLast input never, output never, output hang neverLast clearing of "show interface" counters neverInput queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 05 minute input rate 0 bits/sec, 0 packets/sec5 minute output rate 0 bits/sec, 0 packets/sec0 packets input, 0 bytes, 0 no bufferReceived 0 broadcasts, 0 runts, 0 giants, 0 throttles0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort0 packets output, 0 bytes, 0 underruns0 output errors, 0 collisions, 0 interface resets0 output buffer failures, 0 output buffers swapped outConfiguration Examples for Virtual Interfaces
This section contains the following examples:
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Configuring a Loopback Interface: Example
The following example shows how to configure a loopback interface, loopback 0.
interface loopback 0ip address 10.20.1.2 255.255.255.0endConfiguring 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 0no ip unreachablesendWhere to Go Next
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Additional References
The following sections provide references related to virtual interfaces.
Related Documents
Interface commands: complete command syntax, command mode, defaults, command history, usage guidelines, and examples
Cisco IOS Interface and Hardware Component Command Reference
All Cisco IOS XE commands
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Cisco IOS XE Interface and Hardware Component configuration modules
Cisco IOS XE Interface and Hardware Component Configuration Guide, Release 2
Configuration example showing how to use loopback interfaces with BGP
Sample Configuration for iBGP and eBGP With or Without a Loopback Address
Standards
No new or modified standards are supported, and support for existing standards has not been modified.
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MIBs
RFCs
No new or modified RFCs are supported, and support for existing RFCs has not been modified.
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Technical Assistance
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