Asymmetric Links

Mobile routers can be deployed on a number of entities, such as "planes, trains, and automobiles." Hey, sounds like a movie! Seriously though, because the range of deployment scenarios for mobile routers varies, some can find themselves in varied environments, including an asymmetric link situation in which one link sends traffic and another link receives traffic. This arises, for example, with some satellite links in which an uplink transmits traffic while a downlink receives traffic. The implication is that each link supports only one-way traffic, as shown in Figure 7-15.

Figure 7-15. Asymmetric Links

If a mobile router and FA communicate through a satellite, this means that the FA receives the mobile router's RRQ on an interface that is different from the one on which it sent its agent advertisement. Also, it means that the FA forwards the RRP to the mobile router on an interface that is different from the one on which it received the RRQ. Because this is a departure from the way that Mobile IP normally works, modifications to the mobile router and FA are necessary to support these asymmetrical links. The modifications are what comprise the Cisco Asymmetric Link Support feature for network mobility.

The first issue to decide in designing the Asymmetric Link Support feature is where the FA should be located: on the path from the mobile router or on the path to the mobile router. Going back to the roots of network mobility and Mobile IP makes this question easy to answer. Because the Home Agent tunnels the mobile router's traffic to the FA, which in turn forwards the traffic to the mobile router, it's seams clear that the FA should be on the receiving path of the mobile router. In other words, because the FA is the tunnel endpoint with the Home Agent, it should be on the uplink path to the mobile router. The FA does not need to be on the mobile router's sending path, because the downlink router can always forward the packets to the FA.

The FA maps a CoA to the interface that is connected to an uplink, and sends its periodic agent advertisements on the uplink. Note that a FA can have multiple uplinks. If a mobile router hears the agent advertisements on its uplink, it registers with that (CoA (FA) by sending a RRQ on its downlink. The downlink router then forwards the RRQ to the FA. Because the FA maps the CoA to the advertising interface, it treats the RRQ as if it received the request on that interface. The FA then processes the RRQ and forwards it to the Home Agent as normal. Upon receiving a RRP from the Home Agent, the FA forwards the reply to the mobile router out the proper CoA interface (or, uplink).

The special behavior just described does not preclude the standard Mobile IP and network mobility behavior. This means that the FA and mobile router can support any combination of symmetric (bidirectional) and asymmetric (unidirectional) links.

Configuration Needed for Asymmetric Links

The FA and mobile router must be configured appropriately to support asymmetric link behavior. This is largely accomplished with existing serial interface commands to configure unidirectional interfaces.

On the mobile router, the serial interfaces for the uplink (receive-only) and downlink (transmit-only) are configured as follows:

• interface type number This command configures an interface type for the uplink. It enters interface configuration mode, where the matching downlink interface is specified.

• transmit-interface type number This interface subcommand assigns a transmit interface (downlink) to the receive-only interface (uplink) to use.

• ip address ip-address mask This command sets the primary IP address for the interface for the uplink. This is the IP address of the roaming interface.

• ip mobile router-service roam This command enables the mobile router to discover FAs on this interface for the uplink.

The receive-only interface (uplink) is not configured to solicit for agent advertisements. The transmit-interface command must then be configured with the usual interface commands, for example, IP address and so on.

A new interface-level configuration option is introduced on the FA to map the CoA to the advertising uplink interface, as follows:

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

This interface subcommand enables FA service on the interface.

The interface-only keyword causes the interface type specified in the interface argument to advertise only its own address as the CoA. Other interfaces configured for FA service do not advertise this CoA. The transmit-only keyword informs Mobile IP that the interface acts as an uplink. Thus, for the registration process, RRQs received for this CoA are treated as having arrived on the transmit-only interface. Note that any CoA can be configured as interface-only, but only serial interfaces can be configured as transmit-only.

Typically, the advertising interface on the FA is configured to advertise more often to reduce latency when the mobile router roams to a new FA. This is accomplished with the IRDP interface-level commands detailed in the section "Agent DiscoveryTuning IRDP Options," earlier in this chapter. This becomes more important because the mobile router is configured to not solicit for advertisements in this environment.

Example of Asymmetric Link Behavior

Although the asymmetric link support might sound confusing and visions of a ball bouncing up and down might be floating in your head, an example should help to clarify things. Review Figure 7-16 to see how the Cisco Asymmetric Link Support feature for network mobility works. The mobile router and FA in Figure 7-16 are configured as described in Example 7-1.

Figure 7-16. Asymmetric Link Example

Example 7-1. Mobile Router Example Configuration

 ! interface Loopback1  ip address 20.0.4.1 255.255.255.0 ! interface Serial3/0 ! Uplink interface  transmit-interface Serial3/1  ! Specifies the matching transmit interface as 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 Foreign Agent Example 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 

In this example, the FA sends agent advertisements with CoA 11.0.0.2 out the uplink interface 11.0.0.2. The mobile router receives the advertisement on interface 11.0.0.1. The mobile router sends its RRQ with CoA 11.0.0.2 out interface 12.0.0.1, which is then received by the downlink router. The downlink router forwards the RRQ using normal IP routing to the FA. The FA treats the RRQ as if it were received on the CoA interface 11.0.0.2 and then forwards the request to the Home Agent as usual. Upon receiving the RRP from the Home Agent, the FA forwards the reply to the mobile router on the uplink 11.0.0.2.