Table Of Contents
Configuring Logical Interfaces
Configure a Loopback Interface
IP Tunneling Configuration Task List
Configure the Tunnel Destination
Configure End-to-End Checksumming
Configure a Tunnel Identification Key
Configure a Tunnel Interface to Drop Out-of-Order Datagrams
Configure Asynchronous Host Mobility
Logical Interface Configuration Examples
Routing Two AppleTalk Networks across an IP-Only Backbone Example
Routing a Private IP Network and a Novell Net across a Public Service Provider Example
Configuring Logical Interfaces
Use the information in this chapter to understand and configure the types of logical, or virtual, interfaces supported on Cisco routers and access servers. This chapter includes the following configuration instructions and examples:
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Configure a Loopback Interface
For examples of configuration tasks, see "Logical Interface Configuration Examples" at the end of this chapter.
For hardware technical descriptions and information about installing interfaces, refer to the hardware installation and configuration publication for your product. For complete descriptions of the logical interface commands, refer to the "Interface Commands" chapter of the Cisco IOS Interface Command Reference. To locate documentation of other commands that appear in this chapter, use the command reference master index or search online.
Configure a Loopback Interface
You can specify a software-only interface called a loopback interface to emulate an interface. It is supported on all platforms. A loopback interface is a virtual interface that is always up and allows BGP and RSRB sessions to stay up even if the outbound interface is down.
You can use the loopback interface as the termination address for BGP sessions, for RSRB connections, or to establish a Telnet session from the device's console to its auxiliary port when all other interfaces are down. You can also use a loopback interface to configure IPX-PPP on asynchronous interfaces. To do so, you must associate an asynchronous interface with a loopback interface configured to run IPX. 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.
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. This means that the loopback interface serves as the Null 0 interface also.
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To specify a loopback interface and enter interface configuration mode, use one of the following commands in global configuration mode:
See the section "Run Interface Loopback Diagnostics" in the "Overview of Interface Configuration" chapter in this publication.
Configure a Null Interface
The Cisco IOS software supports a "null" interface. This pseudo-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 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.
To specify the null interface, use the following command in global configuration mode:
Specify null 0 (or null0) as the interface type and number. The null interface can be used in any command that has an interface type as an argument. The following example configures a null interface for IP route 127.0.0.0:
ip route 127.0.0.0 255.0.0.0 null 0Configure a Tunnel Interface
Tunneling provides a way to encapsulate arbitrary packets inside of 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 is designed to provide 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.
Tunneling has the following three primary components:
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illustrates IP tunneling terminology and concepts.
Figure 22 IP Tunneling Terminology and Concepts
To understand the process of tunneling, consider connecting two AppleTalk networks with a non-AppleTalk backbone, such as IP. The relatively high bandwidth consumed by the broadcasting of Routing Table Maintenance Protocol (RTMP) data packets can severely hamper the backbone's network performance. This problem can be solved by tunneling AppleTalk through a foreign protocol, such as IP. Tunneling encapsulates an AppleTalk packet inside the foreign protocol packet, which is then sent across the backbone to a destination router. The destination router then de-encapsulates the AppleTalk packet and, if necessary, routes the packet to a normal AppleTalk network. Because the encapsulated AppleTalk packet is sent in a directed manner to a remote IP address, bandwidth usage is greatly reduced. Furthermore, the encapsulated packet benefits from any features normally enjoyed by IP packets, including default routes and load balancing.
Advantages of Tunneling
The following are several situations where encapsulating traffic in another protocol is useful:
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Figure 23 Providing Workarounds for Networks with Limited Hop Counts
Special Considerations
The following are considerations and precautions to observe when you configure tunneling:
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, packets from Host 1 will travel across networks w, q, and z to get to Host 2 instead of taking the path w, x, y, z because it "appears" shorter.Figure 24 Tunnel Precautions: Hop Counts
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%TUN-RECURDOWN Interface Tunnel 0temporarily disabled due to recursive routingIP Tunneling Configuration Task List
If you want to configure IP tunneling, you must perform the tasks in the following sections:
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The tasks in the following tunnel configuration sections are optional:
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For commands that monitor IP tunnels, see the section "Monitor and Maintain the Interface" in the "Interface Configuration Overview" chapter. For examples of configuring tunnels, see the section "IP Tunneling Examples" at the end of this chapter.
Specify the Tunnel Interface
To specify a tunnel interface and enter interface configuration mode, use one of the following commands in global configuration mode:
Configure the Tunnel Source
You must specify the tunnel interface's source address by using the following command in interface configuration mode:
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Configure the Tunnel Destination
You must specify the tunnel interface's destination by using the following command in interface configuration mode:
Configure the Tunnel Mode
The encapsulation mode for the tunnel interface defaults to generic route encapsulation (GRE), so this command is considered optional. However, if you want a mode other than GRE, you must configure it by using the following command in interface configuration mode:
If you are tunneling AppleTalk, you must use either the AppleTalk Update Routing Protocol (AURP), Cayman or GRE tunneling mode. Cayman tunneling is designed by Cayman Systems and enables routers and access servers to interoperate with Cayman GatorBoxes. You can have Cisco devices at either end of the tunnel, or you can have a GatorBox at one end and Cisco router or access server at the other end. Use Distance Vector Multicast Routing Protocol (DVMRP) mode when a router or access server connects to a mrouted router to run DVMRP over a tunnel. It is required to configure Protocol-Independent Multicast (PIM) and an IP address on a DVMRP tunnel.
CautionDo not configure a Cayman tunnel with an AppleTalk network address.
If you use GRE, you must have only Cisco routers or access servers at both ends of the tunnel connection. When you use GRE to tunnel AppleTalk, you must configure an AppleTalk network address and a zone. Use the following commands to tunnel AppleTalk using GRE:
Configure End-to-End Checksumming
Some passenger protocols rely on media checksums to provide data integrity. By default, the tunnel does not guarantee packet integrity. By enabling end-to-end checksums, the Cisco IOS software drops corrupted packets. To enable such checksums on a tunnel interface, use the following command in interface configuration mode:
Configure a Tunnel Identification Key
You can enable an ID key for a tunnel interface. This key must be set to the same value on the tunnel endpoints. Tunnel ID keys can be used as a form of weak security to prevent misconfiguration or injection of packets from a foreign source.
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The tunnel ID key is available with GRE only.
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To configure a tunnel ID key, use the following command in interface configuration mode:
Configure a Tunnel Interface to Drop Out-of-Order Datagrams
You can optionally configure a tunnel interface to drop datagrams that arrive out of order. This is useful when carrying passenger protocols that function poorly when they receive packets out of order (for example, LLC2-based protocols). This option is available with GRE only.
To use this option, use the following command in interface configuration mode:
tunnel sequence-datagrams
Configure a tunnel interface to drop out-of-order datagrams.
Configure Asynchronous Host Mobility
Increasingly, remote users are accessing networks through dial-up telephone connections. In contrast to local users who can connect directly into the network, remote users must first dial into an access server.
The access server supports a packet tunneling strategy that extends the internetwork—in effect creating a virtual private link for the mobile user. When a user activates asynchronous host mobility, the access server on which the remote user dials into becomes a remote point-of-presence (POP) for the user's home network. Once logged in, users experience a server environment identical to the one that they experience when they connect directly to the "home" access server.
Once the network layer connection is made, data packets are tunneled at the physical and/or data link layer instead of at the protocol layer. In this way, raw data bytes from dial-in users are transported directly to the "home" access server, which processes the protocols.
illustrates the implementation of asynchronous host mobility on an extended internetwork. A mobile user connects to an access server on the internetwork and, by activating asynchronous host mobility, is connected to a "home" access server configured with the appropriate user name. The user sees an authentication dialog or prompt from the "home" system and can proceed as if he or she were connected directly to that device.
Figure 25 Asynchronous Host Mobility
The remote user implements asynchronous host mobility by executing the tunnel command in user EXEC mode. The tunnel command sets up a network layer connection to the specified destination. The access server accepts the connection, attaches it to a virtual terminal (VTY), and runs a command parser capable of running the normal dial-in services. After the connection is established, data is transferred between the modem and network connection with a minimum of interpretations. When communications are complete, the network connection can be closed and terminated from either end.
Refer to the Cisco Access Connection Guide for information about setting up the network layer connection with the tunnel command.
Logical Interface Configuration Examples
This section provides examples that illustrate configuration of IP tunnels.
IP Tunneling Examples
The following example shows an IP tunneling configuration with commented (!) explanations:
!Creates the interfaceinterface tunnel 0!enables IPX on the interfacenovell network 1e!enables appletalkappletalk cable-range 4001-4001 128!enables IPip address 10.1.2.3. 255.255.255.0!enables DECnetDECnet cost 4!sets the source address, or interface, for packetstunnel source ethernet 0!determines where the encapsulated packets are to gotunnel destination 131.108.14.12!sets the protocoltunnel mode gre!computes a checksum on passenger packets if protocol doesn't already have reliable !checksumtunnel checksum needed!sets the id keytunnel key 42!set to drop out of order packetstunnel sequence-datagramsRouting Two AppleTalk Networks across an IP-Only Backbone Example
is an example of connecting multiprotocol subnetworks across a single-protocol backbone. The configurations of router A and router B follow .
Figure 26 Connecting AppleTalk Networks across an IP-Only Backbone
Router A
interface ethernet 0description physics department AppleTalk lanappletalk cable-range 4001-4001 32!interface fddi 0description connection to campus backboneip address 36.0.8.108 255.255.255.0interface tunnel 0tunnel source fddi 0tunnel destination 36.0.21.20appletalk cable-range 5313-5313 1Router B
interface ethernet 0description chemistry department appletalk lanappletalk cable-range 9458-9458 3!interface fddi 0description connection to campus backboneip address 36.0.21.20 255.255.255.0interface tunnel 0tunnel source fddi 0tunnel destination 36.0.8.108appletalk cable-range 5313-5313 2Routing a Private IP Network and a Novell Net across a Public Service Provider Example
is an example of routing a private IP network and a Novell network across a public service provider. The configuration of router A and router B follow .
Figure 27 Creating Virtual Private Networks Across WANs
Router A
interface ethernet 0description boston officeip address 10.1.1.1 255.255.255.0novell network 1e!interface serial 0description connection to NEARnetip address 192.13.2.1 255.255.255.0!interface tunnel 0tunnel source serial 0tunnel destination 131.108.5.2ip address 10.1.2.1 255.255.255.0novell network 1fRouter B
interface ethernet 0description menlo park officeip address 10.1.3.1 255.255.255.0novell network 31!interface serial 4description connection to BARRnetip address 131.108.5.2 255.255.255.0!interface tunnel 0tunnel source serial 4tunnel destination 192.13.2.1ip address 10.1.2.2 255.255.255.0novell network 1f
