Recall from Chapter 2 that two types of CoAs exista FA CoA and a CCoA. Thus far, this text has presented the network mobility model in which the mobile router roams with a FA CoA with the support of a FA. This section explores the situation in which FAs are not available, and thus, the mobile router can either wait for a FA or roam with a CCoA.
A CCoA is an IP address assigned to the interface of the mobile router itself. This address is usually dynamically obtained, for example, using Dynamic Host Configuration Protocol (DHCP) or Point-to-Point Protocol (PPP)/IP Control Protocol (IPCP), but can also be statically configured in special cases. The benefit of using a CCoA is that the mobile router can use Foreign Networks, where FAs are not available.
Static Colocated Care-of Address
The static CCoA is a special case and applies to networks in which the endpoint IP address is always fixed. Sometimes a static IP address can be acquired from mobile cellular service providers.
If you are using a static CCoA, the mobile router is configured with a static IP address (CoA) in the Foreign Network and a default gateway in the Foreign Network, if needed. This is accomplished with additional interface-level configuration on the interface, after the interface is configured to roam, as specified in the section "Mobile Router Interface-Level Configuration," earlier in this chapter. The following interface commands are needed on the roaming interface:
Dynamic Colocated Care-of Address
This feature allows a mobile router to not only roam to a Foreign Network, where FAs are not available, but also allows the mobile router to dynamically obtain a CCoA. To accomplish this, the roaming interface on the mobile router is configured to allow the IPCP negotiation of the CCoA.
Dynamic CCoAing on Ethernet using DHCP is supported on the Cisco Mobile Access Router (MAR) 3200 Series routers when using the WLAN Mobile Interface Card (WMIC). Generic support for DHCP CCoA is not available because, in most cases, Ethernet interfaces attach wireless bridging devices. When using bridging devices, no move detection information is available, and the router does not know when it needs to acquire a new address. The implementation on the 3200 Series routers uses Simple Network Management Protocol (SNMP) traps sent by the bridge for move detection. Documentation on this feature can be found in the 3200 section of Cisco.com.
The interface is first configured to roam as specified in the section "Mobile Router Interface-Level Configuration," earlier in this chapter, with additional subcommands as follows:
Dynamic CCoA using DHCP is a much harder problem to solve because it is often difficult for the router to determine when it has changed links. As discussed previously, when using many Ethernet-configured bridges, the interface is always up and the mobile router does not know when the bridge has associated with a new subnet. DHCP-based dynamic CCoA is supported on the Cisco 3200 Series routers when using the Wireless LAN Mobile Interface Card. The dynamic CCoA uses SNMP traps from the bridge to monitor the state of the radio interface. This feature receives linkUp and linkDown traps on the roaming interface from the Dot11Radio0 interface. A linkUp trap triggers to the interface to restart DHCP; a linkDown indicates that Mobile IP should select another interface. The following interface-level command, when coupled with the ip address dhcp command, causes the DHCP client to respond to Layer 2 information received through SNMP:
ip dhcp client mobile renew count count interval msec
Behavior Using Colocated Care-of Addresses
The mobile router can be configured to ignore FA advertisements on an interface with a CCoA, or it can be configured to use a CCoA only when a FA is not available.
In the latter case, when an interface comes up, the mobile router attempts to discover a FA on the link through the normal agent discovery process. If it succeeds in finding a FA, the mobile router registers using the advertised FA CoA and continues to do so as long as a FA is heard.
On the other hand, if a FA is not available, the mobile router enters into CCoA mode and the roaming interface registers its CCoA. If a FA is heard again, the mobile router reregisters with the FA CoA.
The desired behavior is accomplished through configuration. The roaming interface on the mobile router can be configured to register only its CCoA and ignore FA advertisements by using the following subinterface command:
ip mobile router-service colocated ccoa-only
This option defers to CCoA mode regardless of the availability of a FA.
The default behavior is for the mobile router to defer to a FA CoA, if one is available. Explicit configuration is required for the mobile router to ignore the FA advertisements.
When the mobile router registers back to its Home Agent with a CCoA, it registers in the same way that a Mobile Node would register. Specifically, it sets the D bit in its RRQ, as specified in Chapter 2.
When a mobile router is using a CCoA, only one tunnel transports all traffic, as opposed to the two tunnels needed when using a FA. The CCoA is then the remote endpoint of the tunnel between the Home Agent and mobile router. Upon decapsulation of a tunneled packet, the mobile router retrieves the original packet destined for a node on one of its mobile networks and forwards the packet. The mobile router uses this same tunnel to reverse-tunnel packets to the Home Agent, if configured.
Configuration Examples Using Colocated Care-of Addresses
Because examples serve us so well, we look at three sample configurations to obtain the desired CCoA behavior.
Example One: Mobile Networks with Static Colocated Care-of Address Only
In this configuration example, the mobile router is configured with a static CCoA. Furthermore, the mobile router is configured to use only the static CCoA and ignore FA advertisements heard on the interface.
! Static CCoA with CCoA-only option interface Ethernet 1/0 ip address 10.0.1.1 255.255.255.0 ip mobile router-service roam ip mobile router-service colocated gateway 10.0.1.2 ccoa-only ip mobile router-service colocated registration retry 30
Example Two: Mobile Networks with Dynamic CCoA
In this configuration example, the mobile router is configured to dynamically obtain a CCoA through IPCP. However, the mobile router listens to FA advertisements heard on the interface and prefers a FA CoA, if one is available.
! Dynamic CCoA. interface Serial 3/1 ip address negotiated encapsulation ppp ip mobile router-service roam ip mobile router-service colocated
Example Three: Mobile Networks with Dynamic CCoA Only
In this example, the mobile router is configured to dynamically obtain a CCoA. Furthermore, the mobile router is configured to ignore FA advertisements and use only the dynamic CCoA.
! Dynamic CCoA with CCoA-only option interface Serial 2/0 ip address negotiated encapsulation ppp ip mobile router-service roam ip mobile router-service colocated ccoa-only ip mobile router-service colocated registration retry 30