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Cisco Mobile Networks Asymmetric LinkAn 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 onlyone way traffic, the inherent mapping in the foreign agent of the return path to the mobile router for incomingmessages does not apply. The Cisco Mobile Networks--Asymmetric Link feature solves this problem byextending the use of mobile networks to networks where the mobile router has unidirectional links to theforeign agent. The foreign agent is able to transmit packets back to the mobile router over a different linkthan the one on which it receives packets from the mobile router.Feature Specifications for the Cisco Mobile Networks: Asymmetric LinkFeature HistoryReleaseModification12.2(13)TThis feature was introduced.Supported PlatformsRefer to Feature Navigator as referenced below. Finding Feature Information, page 2 Restrictions for Cisco Mobile Networks Asymmetric Link, page 2 Information About Cisco Mobile Networks Asymmetric Link, page 2 How to Configure Mobile Networks in an Asymmetric Link Environment, page 4 Configuration Examples for Cisco Mobile Networks Asymmetric Link, page 9 Additional References, page 10 Command Reference, page 12 Glossary, page 12IP Mobility: Mobile Networks Configuration Guide, Cisco IOS Release 15M&T1

Cisco Mobile Networks Asymmetric LinkFinding Feature InformationFinding Feature InformationYour software release may not support all the features documented in this module. For the latest caveats andfeature information, see Bug Search Tool and the release notes for your platform and software release. Tofind information about the features documented in this module, and to see a list of the releases in which eachfeature is supported, see the feature information table.Use Cisco Feature Navigator to find information about platform support and Cisco software image support.To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.Restrictions for Cisco Mobile Networks Asymmetric LinkThis feature can be used only on serial interfaces.Information About Cisco Mobile Networks Asymmetric LinkUnidirectional Routing in Cisco Mobile NetworksWith unidirectional routing, registration requests from the mobile router travel a slightly different route thanin bidirectional routing. The mobile router uses different interfaces to transmit and receive. Advertisementsare received on the mobile router interface that is connected to the uplink equipment. This interface is configuredto be receive-only (transmit-interface command) and another interface connected to the downlink traffic isconfigured to be transmit-only. When the mobile router receives an advertisement from the foreign agent onthe 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 hearsadvertisements on the interface connected to the uplink equipment. The registration request is sent out themobile 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 foreignagent receives the registration request, it looks up the care-of address. If the care-of address is associated withan asymmetric interface, the foreign agent treats the mobile router as a visitor on that interface and forwardsthe registration request to the home agent. The home agent sends a registration reply to the foreign agentcare-of address, which will then be forwarded to the mobile router through the uplink interface.IP Mobility: Mobile Networks Configuration Guide, Cisco IOS Release 15M&T2

Cisco Mobile Networks Asymmetric LinkUnidirectional Routing in Cisco Mobile NetworksThe figure below shows how packets are routed within the mobile network using unidirectional routing.Figure 1: Unidirectional Routing in an Asymmetric Communications EnvironmentIP Mobility: Mobile Networks Configuration Guide, Cisco IOS Release 15M&T3

Cisco Mobile Networks Asymmetric LinkHow to Configure Mobile Networks in an Asymmetric Link EnvironmentHow to Configure Mobile Networks in an Asymmetric LinkEnvironmentEnabling Mobile Router Services for Unidirectional InterfacesSUMMARY STEPS1. enable2. configure {terminal memory network}3. interface type number4. transmit-interface type number5. ip address ip-address mask6. ip mobile router-service roam7. exit8. interface type number9. ip address ip-address mask10. ip mobile router-service roamDETAILED STEPSStep 1Command or ActionPurposeenableEnables higher privilege levels, such as privileged EXEC mode.Enter your password if prompted.Example:Router enableStep 2configure {terminal memory network}Enters global configuration mode.Example:Router# configure terminalStep 3interface type numberConfigures an interface type and enters interface configurationmode.Example:Router(config)# interface serial 1Step 4transmit-interface type numberAssigns a transmit interface to a receive-only interface.IP Mobility: Mobile Networks Configuration Guide, Cisco IOS Release 15M&T4

Cisco Mobile Networks Asymmetric LinkEnabling Mobile Router Services for Unidirectional InterfacesCommand or ActionPurpose This is the uplink (receive-only) interface.Example:Router(config-if)# transmit-interface serial2Step 5ip address ip-address mask In the example, this command specifies interface serial2, connected to the downlink router, to be thetransmit-only interface.Sets a primary IP address for an interface. This is the IP address of a roaming interface.Example:Router(config-if)# ip-address 168.71.6.2255.255.255.0Step 6Enables the mobile router to specify on which configuredinterface it will discover foreign agents.ip mobile router-service roamExample:Router(config-if)# ip mobile router-serviceroamStep 7Returns to global configuration mode.exitExample:Router(config-if)# exitStep 8interface type numberConfigures an interface type and enters interface configurationmode.Example:Router(config)# interface serial 2Step 9ip address ip-address mask This is the downlink (transmit-only) interface that wasspecified in Step 4.Sets a primary IP address for an interface. This is the IP address of a roaming interface.Example:Router(config-if)# ip-address 168.71.7.2255.255.255.0Step 10Enables the mobile router to specify on which configuredinterface it will discover foreign agents.ip mobile router-service roamExample:Router(config-if)# ip mobile router-serviceroamIP Mobility: Mobile Networks Configuration Guide, Cisco IOS Release 15M&T5

Cisco Mobile Networks Asymmetric LinkEnabling Foreign Agent Services for Unidirectional InterfacesTroubleshooting Tips With back-to-back serial interfaces (DTE to DTE), you need to disable keepalives with the no keepaliveinterface configuration command. The forwarding table will appear "normal." Use the debug ip packet and trace commands to displaythe packets that are being routed unidirectionally.Enabling Foreign Agent Services for Unidirectional InterfacesSUMMARY STEPS1. enable2. configure {terminal memory network}3. interface type number4. ip address ip-address mask5. ip irdp6. ip irdp maxadvertinterval seconds7. ip irdp minadvertinterval seconds8. ip irdp holdtime seconds9. ip mobile foreign-service10. exit11. router mobile12. exit13. ip mobile foreign-agent [care-of interface[interface-only transmit-only]]DETAILED STEPSStep 1Command or ActionPurposeenableEnables higher privilege levels, such as privileged EXEC mode.Enter your password if prompted.Example:Router enableStep 2configure {terminal memory network}Enters global configuration mode.Example:Router# configure terminalIP Mobility: Mobile Networks Configuration Guide, Cisco IOS Release 15M&T6

Cisco Mobile Networks Asymmetric LinkEnabling Foreign Agent Services for Unidirectional InterfacesStep 3Command or ActionPurposeinterface type numberConfigures an interface and enters interface configuration mode.Example:Router(config)# interfaceStep 4serial1ip address ip-address maskSets a primary IP address of the interface.Example:Router(config-if)# ip address 10.0.0.2255.255.255.0Step 5Enables IRDP processing on an interface.ip irdpExample:Router(config-if)# ip irdpStep 6ip irdp maxadvertinterval seconds(Optional) Specifies the maximum interval in seconds betweenadvertisements.Example:Router(config-if)# ip irdp maxadvertinterval4Step 7ip irdp minadvertinterval seconds(Optional) Specifies the minimum interval in seconds betweenadvertisements.Example:Router(config-if)# ip irdp minadvertinterval3Step 8ip irdp holdtime seconds(Optional) Length of time in seconds that advertisements areheld valid.Example: Default is three times the maxadvertintervalperiod.Router(config-if)# ip irdp holdtime 10Step 9Enables foreign agent service on an interface.ip mobile foreign-serviceExample:Router(config-if)# ip mobile foreign-serviceStep 10exit This command also appends Mobile IP information suchas care-of address, lifetime, and service flags to theadvertisement.Returns to global configuration mode.Example:Router(config-if)# exitIP Mobility: Mobile Networks Configuration Guide, Cisco IOS Release 15M&T7

Cisco Mobile Networks Asymmetric LinkEnabling Home Agent ServicesStep 11Command or ActionPurposerouter mobileEnables Mobile IP on the router.Example:Router(config)# router mobileStep 12Returns to global configuration mode.exitExample:Router(config-router)# exitStep 13ip mobile foreign-agent [care-ofinterface[interface-only transmit-only]]Example:Router(config)# ip mobile foreign-agentcare-of serial 1 interface-onlytransmit-onlyEnables foreign agent service. The interface-only keyword causes the interface typespecified in the interface argument to advertise only itsown address as the care-of address. The transmit-only keyword informs Mobile IP that theinterface acts as an uplink so for registration and replypurposes, treat registration requests received for thiscare-of address as having arrived on the transmit-onlyinterface. Any care-of address can be configured as interface onlybut only serial interfaces can be configured as transmitonly.Enabling Home Agent ServicesThere are no changes to the home agent configuration with the introduction of the Cisco MobileNeworks--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 ConfigurationSUMMARY STEPS1. show ip mobile visitor2. show ip mobile globals3. show ip mobile interfaceIP Mobility: Mobile Networks Configuration Guide, Cisco IOS Release 15M&T8