A mobile router is the all-important "outdoor" link to the Internet for the nodes on its mobile networks. If the mobile router fails, the mobile networks are left in a precarious situationwithout Internet connection and without another first-hop router with which to connect. Thus, it seems prudent that the mobile router should have the capability to be "backed up" in some way. Mind you, it is highly unlikely that a Cisco mobile router will fail, but it's better to be safe than sorry!
The Cisco Mobile Networks feature provides full redundancy capability for its mobile routers, as shown in Figure 7-14. Just like the Home Agent Redundancy feature, the Mobile Router Redundancy feature also uses the Hot Standby Router Protocol (HSRP) to provide the backup mechanism. A brief description of HSRP is given in Chapter 6, "Metro Mobility: Client-Based Mobile IP."
Figure 7-14. Mobile Router Redundancy
Recall that HSRP assigns one router in the HSRP group as the active router, and the other router in the group is considered a standby. The HSRP group attributes are defined on the interfaces of each mobile router in the redundancy group, enabling this selection. The active mobile router is the router responsible for registering with the Home Agent and maintaining an active mobility binding. If the active mobile router fails, the standby mobile router is selected as the new active mobile router. The new active mobile router seamlessly takes over with no loss of connectivity or traffic by the mobile networks. Specifically, upon becoming the active mobile router, a mobile router sends an agent solicitation out of its roaming interfaces to learn about the existing FAs, and it registers with its Home Agent.
The router must first be configured to provide mobile router service, as described in the section "Mobile Router Interface-Level Configuration," earlier in this chapter. While in mobile router configuration mode, the following configuration is needed to provide the redundancy feature:
redundancy group name (state)
This subcommand enables the Mobile Router Redundancy feature.
The HSRP does not need to be configured on both the mobile router's roaming interface and the interface attached to the physical mobile networks. It is enough to configure HSRP on one of the interfaces and the standby track command on the other interface.