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
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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
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.
The figure below 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
Router(config)# ip mobile foreign-agent care-of serial 1 interface-only transmit-only
Enables foreign agent service.
The interface-only keyword causes the interface type specified in the interface argument to advertise only its own address as the care-of address.
The transmit-only keyword informs Mobile IP that the interface acts as an uplink so for registration and reply purposes, treat registration requests received for this care-of address as having arrived on the transmit-only interface.
Any care-of address can be configured as interface only but only serial interfaces can be configured as transmit only.
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
Command or Action
Router# show ip mobile visitor
Displays the table containing the visitor list of the foreign agent.
Router# show ip mobile globals
Displays global information for mobile agents.
Relevant fields in the display output will indicate interface-only and transmit-only status if configured.
See the display output following this table for an example.
Router# show ip mobile interface
Displays advertisement information for interfaces that are providing foreign agent service or are home links for mobile nodes.
What to Do Next
The following example shows interface-only and transmit-only configured on the foreign agent:
Router# show ip mobile globals
IP Mobility global information:
Home Agent is not enabled
Pending registrations expire after 15 secs
Care-of addresses advertised
Serial4/0 (188.8.131.52) - up, interface-only, transmit-only
Configuration Examples for Cisco Mobile Networks Asymmetric Link
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.
The following example shows the mobile router configuration:
ip address 184.108.40.206 255.255.255.0
! Uplink interface
ip address 220.127.116.11 255.255.255.0
ip mobile router-service roam
! Downlink interface
ip address 18.104.22.168 255.255.255.
ip mobile router-service roam
ip mobile secure home-agent 22.214.171.124 spi 100 key hex 11223344556677881122334455667788
ip mobile router
address 126.96.36.199 255.255.255.0
Foreign Agent Example
The following example shows the foreign agent configuration:
! Uplink interface
ip address 188.8.131.52 255.255.255.0
ip irdp maxadvertinterval 10
ip irdp minadvertinterval 5
ip irdp holdtime 30
ip mobile foreign-service
ip mobile foreign-agent care-of Serial4/0 interface-only transmit-only
For additional information related to the Cisco Mobile Networks--Asymmetric Link feature, refer to the following sections:
Mobile IP configuration tasks
"Configuring Mobile IP" chapter in theCisco IOS IP Configuration Guide,Release 12.2.
Mobile IP commands
"Mobile IP Commands" chapter in theCisco IOS IP Command Reference, Volume 1 of 3: Addressing and Services , Release 12.2.
Cisco Mobile Networks commands
"Cisco Mobile Networks" feature document, Release 12.2(4)T.
No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.
No new or modified MIBs are supported by this feature, and support for existing MIBs has not been modified by this feature.
To obtain lists of supported MIBs by platform and Cisco IOS release, and to download MIB modules, go to the Cisco MIB website on Cisco.com at the following URL:
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 email@example.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:
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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The following commands are introduced or modified in the feature or features documented in this module. For information about these commands, see the Cisco IOS IP Mobility Command Reference at http://www.cisco.com/en/US/docs/ios/ipmobility/command/reference/imo_book.html. For information about all Cisco IOS commands, go to the Command Lookup Tool at http://tools.cisco.com/Support/CLILookup or to the Cisco IOS Master Commands List .
care-ofaddress--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.
foreignagent--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.
homeagent--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 .
mobilerouter--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.
satellitecommunications--The use of geostationary orbiting satellites to relay information.