A loopback interface is a special software-only interface that emulates a physical interface and allows the router to “connect” to itself. Packets routed to the loopback interface are rerouted back to the router and processed locally. Packets routed out the loopback interface but not destined for the loopback interface are dropped.

The vRouter supports multiple loopback interfaces. These interfaces, with unique IP addressing, can be used as preferred source addresses for routing protocols such as BGP. These interfaces can also be configured as null or blackhole interfaces.

The vRouter supports multiple IPv4 and IPv6 addresses on each loopback interface. These interfaces (lo and lo1 through loN ) have unique IP addressing, can be used as preferred source addresses for routing protocols such as BGP, and can be configured as null or blackhole interfaces.

The loopback interface provides a number of advantages.

• As long as the router is functioning, the loopback interface is always up, and so is very reliable. When even only one link to the router is functioning, the loopback interface can be accessed. The loopback interface thus eliminates the need to try each IP address of the router until it finds one that is still up.
• Because the loopback interface is always up, a routing session (such as a BGP session) can continue even if the outbound interface fails.
• You can simplify collection of management information by specifying the loopback interface as the interface for sending and receiving management information such as logs and SNMP traps.
• The loopback interface can be used to increase security by filtering incoming traffic with access control rules that specify the local interface as the only acceptable destination.
• In OSPF, you can advertise a loopback interface as an interface route into the network, regardless of whether physical links are up or down. This increases reliability by allowing traffic to take alternate paths if one or more physical links go down.
• In BGP, parallel paths can be configured to the loopback interface on a peer device. These parallel paths provide improved load sharing and redundancy.

The router automatically creates the loopback interface on startup with an interface name of lo. It also automatically configures the loopback address with standard IP addressing.

• According to RFC 5735, the 127.0.0.1/8 IPv4 address is assigned to the loopback address. This address is hidden from the show command output. Typically, the IPv4 address that is assigned to the loopback device is 127.0.0.1 for IPv4, although any address in the range from 127.0.0.0 through 127.255.255.255 is mapped to it.
• According to RFC 3513, the ::1/128 IPv6 address is assigned to the loopback interface.
• According to RFC 2606, the localhost domain name is mapped to the loopback addresses.

When configuring the router, it is good practice to take advantage of the reliability of the loopback interface with these practices:

• The host name of the router should be mapped to the loopback interface address, rather than to a physical interface.
• In OSPF and BGP, the router ID should be set to the loopback address. This prevents a possible dynamic recalculation and reassignment of the loopback address when physical interfaces are added or removed from the system. This action is disruptive to active BGP and OSPF sessions.

The vRouter has extensive support for IPv6, including IPv6 interface addressing. The commands for configuring IPv6 on the loopback interface are given in this chapter. A full description of IPv6 support is provided in the Ciena Vyatta Network OS IPv6 Support Configuration Guide.