This topic describes the virtual loopback interface, its always-up single-endpoint behavior, and its typical uses as a routing-protocol session endpoint and a target for ping verification on the Cisco 8000 Series Router.
The virtual loopback interface is a virtual interface that
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remains active at all times
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emulates a physical interface, and
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processes transmitted packets locally.
Virtual loopback interfaces perform these functions in Cisco IOS XR Software:
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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.
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You can ping the loopback interface to verify that the router IP stack is working properly.
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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.
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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.