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Table Of Contents
Cisco Mobile Networks—Asymmetric Link
Restrictions for Cisco Mobile Networks—Asymmetric Link
Information About Cisco Mobile Networks—Asymmetric Link
Unidirectional Routing in Cisco Mobile Networks
How to Configure Mobile Networks in an Asymmetric Link Environment
Enabling Mobile Router Services for Unidirectional Interfaces
Enabling Foreign Agent Services for Unidirectional Interfaces
Verifying Cisco Mobile Networks—Asymmetric Link Configuration
Configuration Examples for Cisco Mobile Networks—Asymmetric Link
Cisco Mobile Networks—Asymmetric Link
An asymmetric link environment such as satellite communications, with a separate uplink and downlink, provides challenges for the mobile router and foreign agent. Because each unidirectional link provides only one way traffic, the inherent mapping in the foreign agent of the return path to the mobile router for incoming messages does not apply. The Cisco Mobile Networks—Asymmetric Link feature solves this problem by extending the use of mobile networks to networks where the mobile router has unidirectional links to the foreign agent. The foreign agent is able to transmit packets back to the mobile router over a different link than the one on which it receives packets from the mobile router.
Feature Specifications for the Cisco Mobile Networks—Asymmetric Link
12.2(13)T
This feature was introduced.
Refer to Feature Navigator as referenced below.
Determining Platform Support Through Cisco Feature Navigator
Cisco IOS software is packaged in feature sets that support specific platforms. To obtain updated information about platform support for this feature, access Cisco Feature Navigator. Cisco Feature Navigator dynamically updates the list of supported platforms as new platform support is added for the feature.
Cisco Feature Navigator is a web-based tool that enables you to determine which Cisco IOS software images support a specific set of features and which features are supported in a specific Cisco IOS image. You can search by feature or release. In the release section, you can compare releases side by side to display both the features unique to each software release and the features that releases have in common.
To access Cisco Feature Navigator, you must have an account on Cisco.com. If you have forgotten or lost your account information, send a blank e-mail to cco-locksmith@cisco.com. An automatic check will verify that your e-mail address is registered with Cisco.com. If the check is successful, account details with a new random password will be e-mailed to you. Qualified users can establish an account on Cisco.com by following the directions at the following URL:
Cisco Feature Navigator is updated regularly when major Cisco IOS software releases and technology releases occur. For the most current information, go to the Cisco Feature Navigator home page at the following URL:
Availability of Cisco IOS Software Images
Platform support for particular Cisco IOS software releases is dependent on the availability of the software images for those platforms. Software images for some platforms may be deferred, delayed, or changed without prior notice. For updated information about platform support and availability of software images for each Cisco IOS software release, refer to the online release notes or Cisco Feature Navigator.
Contents
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Information About Cisco Mobile Networks—Asymmetric Link
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Restrictions for Cisco Mobile Networks—Asymmetric Link
This feature can be used only on serial interfaces.
Information About Cisco Mobile Networks—Asymmetric Link
To configure the Cisco Mobile Networks—Asymmetric Link feature, you need to understand the following concept:
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Unidirectional Routing in Cisco Mobile Networks
With unidirectional routing, registration requests from the mobile router travel a slightly different route than in bidirectional routing. The mobile router uses different interfaces to transmit and receive. Advertisements are received on the mobile router interface that is connected to the uplink equipment. This interface is configured to be receive-only (transmit-interface command) and another interface connected to the downlink traffic is configured to be transmit-only. When the mobile router receives an advertisement from the foreign agent on the uplink, it takes the care-of address advertised by that foreign agent to use in the registration request. However, the mobile router has been configured to send traffic to a downlink router even though it hears advertisements on the interface connected to the uplink equipment. The registration request is sent out the mobile router's downlink interface to the care-of address given in the the foreign agent's uplink interface.
The downlink router routes the registration request using normal routing to the foreign agent. When the foreign agent receives the registration request, it looks up the care-of address. If the care-of address is associated with an asymmetric interface, the foreign agent treats the mobile router as a visitor on that interface and forwards the registration request to the home agent. The home agent sends a registration reply to the foreign agent care-of address, which will then be forwarded to the mobile router through the uplink interface.
Figure 1 shows how packets are routed within the mobile network using unidirectional routing.
Figure 1 Unidirectional Routing in an Asymmetric Communications Environment
How to Configure Mobile Networks in an Asymmetric Link Environment
This section contains the following procedures:
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Enabling Mobile Router Services for Unidirectional Interfaces
To configure this task of enabling mobile router services for a unidirectional interface, use the following commands:
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DETAILED STEPS
Troubleshooting Tips
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Enabling Foreign Agent Services for Unidirectional Interfaces
To enable foreign agent services for unidirectional interfaces, use the following commands:
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DETAILED STEPS
Enabling Home Agent Services
There are no changes to the home agent configuration with the introduction of the Cisco Mobile Neworks—Asymmetric Link feature. Configure the home agent as described in the "Cisco Mobile Networks" feature document introduced in Cisco IOS Release 12.2(4)T.
Verifying Cisco Mobile Networks—Asymmetric Link Configuration
To verify that the asymmetric link configuration on the foreign agent is working, perform the following optional steps:
SUMMARY STEPS
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DETAILED STEPS
The following example shows interface-only and transmit-only configured on the foreign agent:
Router# show ip mobile globalsIP Mobility global information:Home Agent is not enabledForeign AgentPending registrations expire after 15 secsCare-of addresses advertisedSerial4/0 (11.0.0.2) - up, interface-only, transmit-onlyConfiguration Examples for Cisco Mobile Networks—Asymmetric Link
This section provides the following configuration examples:
In the following examples, a home agent provides service for one mobile router. The mobile router detects the foreign agent advertisements on the uplink interface and sends the registration request on the downlink interface to the advertised care-of address of the foreign agent.
Mobile Router Example
The following example shows the mobile router configuration:
!interface Loopback1ip address 20.0.4.1 255.255.255.0!interface Serial3/0! Uplink interfacetransmit-interface Serial3/1ip address 11.0.0.1 255.255.255.0ip mobile router-service roam!interface Serial3/1! Downlink interfaceip address 12.0.0.1 255.255.255.ip mobile router-service roam!router mobile!ip mobile secure home-agent 43.0.0.3 spi 100 key hex 11223344556677881122334455667788ip mobile routeraddress 20.0.4.1 255.255.255.0home-agent 43.0.0.3Foreign Agent Example
The following example shows the foreign agent configuration:
!interface Serial4/0! Uplink interfaceip address 11.0.0.2 255.255.255.0ip irdpip irdp maxadvertinterval 10ip irdp minadvertinterval 5ip irdp holdtime 30ip mobile foreign-service!router mobile!ip mobile foreign-agent care-of Serial4/0 interface-only transmit-onlyAdditional References
For additional information related to the Cisco Mobile Networks—Asymmetric Link feature, refer to the following sections:
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Related Documents
Standards
No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.
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MIBs
To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:
http://tools.cisco.com/ITDIT/MIBS/servlet/index
If Cisco MIB Locator does not support the MIB information that you need, you can also obtain a list of supported MIBs and download MIBs from the Cisco MIBs page at the following URL:
http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml
To access Cisco MIB Locator, you must have an account on Cisco.com. If you have forgotten or lost your account information, send a blank e-mail to cco-locksmith@cisco.com. An automatic check will verify that your e-mail address is registered with Cisco.com. If the check is successful, account details with a new random password will be e-mailed to you. Qualified users can establish an account on Cisco.com by following the directions found at this URL:
RFCs
No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature.
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Technical Assistance
Command Reference
This section documents modified commands. All other commands used with this feature are documented in the Cisco IOS Release 12.2 command reference publications or the "Cisco Mobile Networks" feature documentation, Release 12.2(4)T.
ip mobile foreign-agent
To enable foreign agent service, use the ip mobile foreign-agent command in global configuration mode. To disable this service, use the no form of this command.
ip mobile foreign-agent [care-of interface [interface-only transmit-only] | reg-wait seconds]
no ip mobile foreign-agent [care-of interface [interface-only transmit-only] | reg-wait seconds]
Syntax Description
Defaults
Disabled
Command Modes
Global configuration
Command History
12.0(1)T
This command was introduced.
12.2(13)T
The interface-only and transmit-only keywords were introduced.
Usage Guidelines
This command enables foreign agent service when at least one care-of address is configured. When no care-of address exists, foreign agent service is disabled.
The foreign agent is responsible for relaying the registration request to the home agent, setting up a tunnel to the home agent, and forwarding packets to the mobile node. The show commands used to display relevant information are shown in parentheses in the following paragraph.
When a registration request comes in, the foreign agent will ignore requests when foreign agent service is not enabled on an interface or no care-of address is advertised. If a security association exists for a visiting mobile node, the visitor is authenticated. The registration bitflag is handled as described in Table 1 . The foreign agent checks the validity of the request. If successful, the foreign agent relays the request to the home agent, appending an FH authentication extension if a security association for the home agent exists. The pending registration timer of 15 seconds is started (show ip mobile visitor pending command). At most, five outstanding pending requests per mobile node are allowed. If a validity check fails, the foreign agent sends a reply with error code to the mobile node (reply codes are listed in Table 2). A security violation is logged when visiting mobile node authentication fails (show ip mobile violation command).
When a registration reply comes in, the home agent is authenticated (show ip mobile secure home-agent command) if a security association exists for the home agent (IP source address or home agent address in reply). The reply is relayed to the mobile node.
When registration is accepted, the foreign agent creates or updates the visitor table, which contains the expiration timer. If no binding existed before this registration, a virtual tunnel is created, a host route to the mobile node via the interface (of the incoming request) is added to the routing table (show ip route mobile command), and an ARP entry is added to avoid sending ARP requests for the visiting mobile node. Visitor binding is removed (along with its associated host route, tunnel, and ARP entry) when the registration lifetime expires or deregistration is accepted.
When registration is denied, the foreign agent will remove the request from the pending registration table. The table and timers of the visitor will be unaffected.
When a packet destined for the mobile node arrives on the foreign agent, the foreign agent will deencapsulates the packet and forwards it out its interface to the visiting mobile node, without sending ARP requests.
The care-of address must be advertised by the foreign agent. This adddress is used by the mobile node to register with the home agent. The foreign agent and home agent use this address as the source and destination point of tunnel, respectively. The foreign agent is not enabled until at least one care-of address is available. The foreign agent will advertise on interfaces configured with the ip mobile foreign-service command.
Only care-of addresses with interfaces that are up are considered available.
The interface-only and transmit-only keywords are used in an aysmmetric link environment, such as satellite communications, where separate uplinks and downlinks exist. The ip mobile foreign-agent care-of interface interface-only command enables the interface to advertise only its own address as the care-of address. All other care-of addresses are not advertised. Other foreign agent interfaces configured for foreign-service will not advertise interface-only care-of addresses. The ip mobile foreign-agent care-of interface transmit-only command informs Mobile IP that the interface acts as an uplink. Registration requests and replies received for this care-of address are treated as transmit-only. This interface will not hear any solicitations.
Table 1 lists mobile node registration request service bitflags.
Table 2 lists foreign agent reply codes.
Examples
The following example enables foreign agent service on ethernet interface 1, advertising 10.0.0.1 as the care-of address:
ip mobile foreign-agent care-of Ethernet0interface Ethernet0ip address 10.0.0.1 255.0.0.0interface Ethernet1ip mobile foreign-serviceThe following example enables foreign agent service on serial interface 4, advertising 10.0.0.2 as the only care-of address. The uplink interface is configured as a transmit-only interface.
ip mobile foreign-agent care-of Serial4 interface-only transmit-onlyinterface Serial4! Uplink interfaceip address 10.0.0.2 255.255.255.0ip irdp!ip mobile foreign-service!Related Commands
show ip mobile globals
To display global information for mobile agents, use the show ip mobile globals EXEC command.
show ip mobile globals
Syntax Description
This command has no arguments or keywords.
Command Modes
EXEC
Command History
Usage Guidelines
This command shows the Mobile IP services that are running on the router. Note the deviation from RFC 2006: the foreign agent will not display busy or registration required information. Both are handled on a per-interface basis (refer to the show ip mobile interface command), not at the global foreign agent level.
Examples
The following is sample output from the show ip mobile globals command:
Router# show ip mobile globalsIP Mobility global information:Home AgentRegistration lifetime: 10:00:00 (36000 secs)Broadcast enabledReplay protection time: 7 secsReverse tunnel enabledICMP Unreachable enabledVirtual networks20.0.0.0/8Foreign Agent is not enabled, no care-of address0 interfaces providing serviceEncapsulations supported: IPIP and GRETunnel fast switching enabledDiscovered tunnel MTU aged out after 1:00:00Table 3 describes the significant fields shown in the display.
Glossary
care-of address—The termination point of the tunnel to a mobile node or mobile router. This can be a colocated care-of address, by which the mobile node or mobile router acquires a local address and detunnels its own packets, or a foreign agent care-of address, by which a foreign agent detunnels packets and forwards them to the mobile node or mobile router.
foreign agent—A router on the visited network of a foreign network that provides routing services to the mobile node while registered. The foreign agent detunnels and delivers packets to the mobile node or mobile router that were tunneled by the home agent of the mobile node. For packets sent by a mobile node, the foreign agent may serve as a default router for registered mobile nodes.
home agent—A router that forwards packets to mobile nodes or the mobile router while they are away from home. It keeps current location information for registered mobile nodes called a mobility binding.
mobile router—A mobile node that is a router. It provides for the mobility of one or more entire networks moving together, perhaps on an airplane, a ship, a train, an automobile, a bicycle, or a kayak. The nodes connected to a network served by the mobile router may themselves be fixed nodes or mobile nodes or routers.
satellite communications—The use of geostationary orbiting satellites to relay information.
Note
