Cisco Systems 78-11741-02 Home Agent Redundancy

Configuring Mobile IP

How Mobile IP Works

IPC-165

Cisco IOS IP Configuration Guide

A AAA server can store a large number of SAs and scale well for future SA storage. It can accommodate

not only the SAs for MN-HA authorization, but SAs for authorization between other Mobile IP

components as well. Storing all SAs in a centralized location can streamline administrative and

maintenance tasks related to the SAs.

When an MN is registering with an HA, keys are needed for the MN-HA authorization process, which

requires AAA authorization for Mobile IP. If SAs are stored on a AAA server, the HA must retrieve the

appropriate SA from the server. The SA is downloaded to the HA, and the HA caches the SA and reuses

it when necessary rather than retrieving it from the AAA server again.

Home Agent Redundancy

During the Mobile IP registration process, an HA creates a mobility binding table that maps the home

IP address of an MN to the current care-of address of the MN. If the HA fails, the mobility binding table

will be lost and all MNs registered with the HA will lose their connectivity. To reduce the impact of an

HA failure, Cisco IOS software supports the HA redundancy feature.

The functionality of HA redundancy runs on top of the Hot Standby Router Protocol (HSRP). HSRP is

a protocol developed by Cisco that provides network redundancy in a way that ensures that user traffic

will immediately and transparently recover from failures.

Before configuring HA redundancy, you must understand the concept of HSRP groups.

An HSRP group is composed of two or more routers that share an IP address and a MAC (Layer 2)

address and act as a single virtual router. For example, your Mobile IP topology can include one active

HA and one or more standby HAs that the rest of the topology view as a single virtual HA.

You must define certain HSRP group attributes on the interfaces of the HAs so that Mobile IP can

implement the redundancy. You can use the groups to provide redundancy for MNs with a home link on

either the interface of the group (a physical network) or on virtual networks. Virtual networks are logical

circuits that are programmed and share a common physical infrastructure.

The HA redundancy feature enables you to configure an active HA and one or more standby HAs.

HA functionality is a service provided by the router and is not interface specific. Therefore, the HA and

the MN must agree on which HA interface the MN should send its registration requests, and conversely,

on which HA interface the HA should receive the registration requests. This agreement must factor in

the following two scenarios:

•An MN that has an HA interface (HA IP address) that is not on the same subnet as the MN

•An MN that requires the HA interface to be on the same subnet as the MN, that is, the HA and the

MN must be on the same home network

For MNs on physical networks, an active HA accepts registration requests from the MN and sends

binding updates to the standby HA. This process keeps the mobility binding table on the active and

standby HAs synchronized. See (a) in Figure29 for an example of this process.