A virtual loopback interface is a virtual interface with a single endpoint that is always up or active. Any packet that the system transmits over a virtual loopback interface is immediately received by the same interface. Loopback interfaces emulate a physical interface.

In Cisco IOS XR Software, virtual loopback interfaces perform these functions:

• Loopback interfaces can act as a termination address for routing protocol sessions. This allows routing protocol sessions to stay up even if the outbound interface is down.

• You can ping the loopback interface to verify that the router IP stack is working properly.

In applications where other routers or access servers attempt to reach a virtual loopback interface, you must 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 to 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.

A null 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 null interface provides an alternative method of filtering traffic. You can avoid the overhead that is involved with using access lists by directing undesired network traffic to the null interface.

The only interface configuration command that you can specify for the null interface is the ipv4 unreachables command. With the ipv4 unreachables command, if the software receives a nonbroadcast packet destined for itself that uses a protocol it does not recognize, it sends an Internet Control Message Protocol (ICMP) protocol unreachable message to the source. If the software receives a datagram that it cannot deliver to its ultimate destination because it knows of no route to the destination address, it replies to the originator of that datagram with an ICMP host unreachable message. By default, the system enables the ipv4 unreachables command. If we do not want ICMP to send protocol unreachable, then you need to configure using the ipv4 icmp unreachable disable command.

By default, the system creates the Null 0 interface during boot process and you cannot remove it. You can configure the ipv4 unreachables command for this interface, but most configuration is unnecessary because this interface just discards all the packets thats the system sends.

Use the show interfaces null0 command to display the Null 0 interface.

Configuring an IPv4 virtual address enables you to access the router from a single virtual address with a management network without prior knowledge of which RP is active. An IPv4 virtual address persists across route switch processor (RP) failover situations. For this to happen, the virtual IPv4 address must share a common IPv4 subnet with a management Ethernet interface on both the RPs.

On a router where each RP has multiple management Ethernet interfaces, the virtual IPv4 address maps to the management Ethernet interface on the active RP that shares the same IP subnet.

The standby RP is available and in a state in which it can take over the work from the active RPs should that prove necessary. Conditions that necessitate the standby RP to become the active RP and assume the active RP’s duties include:

• Failure detection by a watchdog

• Administrative command to take over

• Removal of the active RP from the chassis

If a second RP is not present in the chassis while the first is in operation, a second RP may be inserted and automatically becomes the standby RP. The standby RP may also be removed from the chassis with no effect on the system other than loss of RP redundancy.

After failover, the virtual interfaces all are present on the standby (now active) RP. Their state and configuration are unchanged and there has been no loss of forwarding (in the case of tunnels) over the interfaces during the failover. The routers use nonstop forwarding (NSF) over bundles and tunnels through the failover of the host RP.

Note

The user need not configure anything to guarantee that the standby interface configurations are maintained. Protocol configuration such as tacacs source-interface, snmp-server trap-source, ntp source, logging source-interface do not use the virtual management IP address as their source by default. Use the ipv4 virtual address use-as-src-addr command to ensure that the protocol uses the virtual IPv4 address as its source address. Alternatively, you can also configure a loopback address with the designated or desired IPv4 address and set that as the source for protocols such as TACACS+ using the tacacs source-interface command.

This task explains how to configure a basic null interface.

RP/0/RP0/CPU0:router# configure

RP/0/RP0/CPU0:router#(config)# interface null 0

RP/0/RP0/CPU0:router(config-null0)# end

RP/0/RP0/CPU0:router# show interfaces null 0

This task explains how to configure an IPv4 virtual interface.

RP/0/RP0/CPU0:router# configure

RP/0/RSP0/CPU0:router(config)# ipv4 virtual address 10.3.32.154/8
RP/0/RSP0/CPU0:router(config-null0)# end
or
RP/0/RSP0/CPU0:router(config-null0)# commit

RP/0/RSP0/CPU0:router# configure
RP/0/RSP0/CPU0:router(config)# ipv4 virtual address 10.3.32.154/8
RP/0/RSP0/CPU0:router(config-null0)# commit