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.

SUMMARY STEPS

1.    enable

2.    configure {terminal | memory | network}

3.    interface type number

4.    transmit-interface type number

5.    ip address ip-address mask

6.    ip mobile router-service roam

7.    exit

8.    interface type number

9.    ip address ip-address mask

10.    ip mobile router-service roam

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables higher privilege levels, such as privileged EXEC mode. Enter your password if prompted.
Step 2	configure {terminal | memory | network} Example: Router# configure terminal	Enters global configuration mode.
Step 3	interface type number Example: Router(config)# interface serial 1	Configures an interface type and enters interface configuration mode.
Step 4	transmit-interface type number Example: Router(config-if)# transmit-interface serial 2	Assigns a transmit interface to a receive-only interface. This is the uplink (receive-only) interface. In the example, this command specifies interface serial 2, connected to the downlink router, to be the transmit-only interface.
Step 5	ip address ip-address mask Example: Router(config-if)# ip-address 168.71.6.2 255.255.255.0	Sets a primary IP address for an interface. This is the IP address of a roaming interface.
Step 6	ip mobile router-service roam Example: Router(config-if)# ip mobile router-service roam	Enables the mobile router to specify on which configured interface it will discover foreign agents.
Step 7	exit Example: Router(config-if)# exit	Returns to global configuration mode.
Step 8	interface type number Example: Router(config)# interface serial 2	Configures an interface type and enters interface configuration mode. This is the downlink (transmit-only) interface that was specified in Step 4.
Step 9	ip address ip-address mask Example: Router(config-if)# ip-address 168.71.7.2 255.255.255.0	Sets a primary IP address for an interface. This is the IP address of a roaming interface.
Step 10	ip mobile router-service roam Example: Router(config-if)# ip mobile router-service roam	Enables the mobile router to specify on which configured interface it will discover foreign agents.

• Troubleshooting Tips

SUMMARY STEPS

1.    enable

2.    configure {terminal | memory | network}

3.    interface type number

4.    ip address ip-address mask

5.    ip irdp

6.    ip irdp maxadvertinterval seconds

7.    ip irdp minadvertinterval seconds

8.    ip irdp holdtime seconds

9.    ip mobile foreign-service

10.    exit

11.    router mobile

12.    exit

13.    ip mobile foreign-agent [care-of interface[interface-only transmit-only]]

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables higher privilege levels, such as privileged EXEC mode. Enter your password if prompted.
Step 2	configure {terminal | memory | network} Example: Router# configure terminal	Enters global configuration mode.
Step 3	interface type number Example: Router(config)# interface serial 1	Configures an interface and enters interface configuration mode.
Step 4	ip address ip-address mask Example: Router(config-if)# ip address 10.0.0.2 255.255.255.0	Sets a primary IP address of the interface.
Step 5	ip irdp Example: Router(config-if)# ip irdp	Enables IRDP processing on an interface.
Step 6	ip irdp maxadvertinterval seconds Example: Router(config-if)# ip irdp maxadvertinterval 4	(Optional) Specifies the maximum interval in seconds between advertisements.
Step 7	ip irdp minadvertinterval seconds Example: Router(config-if)# ip irdp minadvertinterval 3	(Optional) Specifies the minimum interval in seconds between advertisements.
Step 8	ip irdp holdtime seconds Example: Router(config-if)# ip irdp holdtime 10	(Optional) Length of time in seconds that advertisements are held valid. Default is three times the maxadvertintervalperiod.
Step 9	ip mobile foreign-service Example: Router(config-if)# ip mobile foreign-service	Enables foreign agent service on an interface. This command also appends Mobile IP information such as care-of address, lifetime, and service flags to the advertisement.
Step 10	exit Example: Router(config-if)# exit	Returns to global configuration mode.
Step 11	router mobile Example: Router(config)# router mobile	Enables Mobile IP on the router.
Step 12	exit Example: Router(config-router)# exit	Returns to global configuration mode.
Step 13	ip mobile foreign-agent [care-of interface[interface-only transmit-only]] Example: 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.

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.

SUMMARY STEPS

1.    show ip mobile visitor

2.    show ip mobile globals

3.    show ip mobile interface

DETAILED STEPS

	Command or Action	Purpose
Step 1	show ip mobile visitor Example: Router# show ip mobile visitor	Displays the table containing the visitor list of the foreign agent.
Step 2	show ip mobile globals Example: 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.
Step 3	show ip mobile interface 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
Foreign Agent
   Pending registrations expire after 15 secs
   Care-of addresses advertised
    Serial4/0 (11.0.0.2) - up, interface-only, transmit-only

The following example shows the mobile router configuration:

!
interface Loopback1
 ip address 20.0.4.1 255.255.255.0
!
interface Serial3/0
! Uplink interface 
 transmit-interface Serial3/1
  ip address 11.0.0.1 255.255.255.0
 ip mobile router-service roam
!
interface Serial3/1
! Downlink interface
 ip 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 11223344556677881122334455667788
ip mobile router
address 20.0.4.1 255.255.255.0
home-agent 43.0.0.3

The following example shows the foreign agent configuration:

!
interface Serial4/0
! Uplink interface
 ip address 11.0.0.2 255.255.255.0
 ip irdp
 ip irdp maxadvertinterval 10
 ip irdp minadvertinterval 5
 ip irdp holdtime 30
 ip mobile foreign-service
!
router mobile
!
ip mobile foreign-agent care-of Serial4/0 interface-only transmit-only