Cisco CRS-1 - Carrier Routing System Router Getting Started Manual page 171

Chapter 7 RP Redundancy and Hardware Administration on Cisco IOS XR Software
This section provides a brief description of the logical interfaces types. For more information, and for
instructions to configure logical interfaces, refer to the Cisco IOS XR Interface and Hardware
Component Configuration Guide.
Loopback Interfaces
You can specify a software-only interface called a loopback interface to emulate an interface. A loopback
interface is a virtual interface that is always up and allows Border Gateway Protocol (BGP), and remote
source-route bridging (RSRB) sessions to stay up even if the outbound interface is down.
You can use the loopback interface as the termination address for Open Shortest Path First (OSPF) or
BGP sessions, for RSRB connections, or to establish a Telnet session from the device console 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.
A loopback interface can also be used to set the device ID for OSPF. OSPF uses the device ID of a router
as a backup means of selecting a designated router (DR). Configuring a loopback interface for OSPF can
ensure that a given router is never selected (or will always be selected) as the DR.
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, meaning that the loopback interface also serves as the Null 0 interface.
Null Interfaces
A null interface functions in a manner similar 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 only
interface configuration command that you can specify for the null interface is no ip unreachables.
The null interface provides an alternative method of filtering traffic. You can avoid the overhead involved
with using access lists by directing undesired network traffic to the null interface.
Tunnel Interfaces
Tunneling provides a way to encapsulate arbitrary packets inside a transport protocol. This feature is
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 provides
the services necessary to implement any standard point-to-point encapsulation scheme. Because tunnels
are point-to-point links, you must configure a separate tunnel for each link.
Prerequisites for Interface Configuration
The following prerequisites must be met be when configuring an interface:
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For the interfaces to perform routing operations, at least one pair of MSC and PLIM cards must be
installed in the router. You can configure interfaces before the physical cards are installed. See the
"Preconfiguring Interfaces" section on page
The line card package must be activated on the router. This package is installed and activated by
default for all new systems. See
information on reinstalling, upgrading, or downgrading the line card driver software.
You must enter the interface network (IP) addresses and subnet mask. Consult your network
administrator or system planner for this information.
7-13.
Chapter 6, "Managing Cisco IOS XR Software Packages,"
Cisco CRS-1 Series Carrier Routing System Getting Started Guide
Configure Routing Interfaces
for
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