Mobile IP Technology and Applications

104 Chapter 5: Campus Mobility: Client-Based Mobile IP

Figure 5-4 ZeCC Call Flow

Home Agent Redundancy

The Home Agent is an important entity in the successful operation of the Mobile IP process.

Recall that upon receiving a successful RRQ from the Mobile Node, the Home Agent creates a

mobility binding entry that tracks the association of the Mobile Node’s home address with its

Example 5-3 Home Agent Configuration for a ZeCC Network

hostname HA

!

aaa new-model

!

interface Loopback1

ip address 192.168.101.1 255.255.255.0

!

router mobile

!

ip mobile home-agent

ip mobile host nai @example address pool dhcp-proxy-client dhcp-server 192.168.2.2

interface Loopback1 aaa

!

radius-server host 172.19.192.100 auth-port 1645 acct-port 1646

radius-server domain-stripping

radius-server key skeleton

Mobile

Node DHCP

Home

Agent

RADIUS

Server

Window Domain

Controller or

Active Directory

DHCP Request

DHCP Reply with

Option 68

Registration

Requst (RRQ)

Authenticate

the reply.

Registration

complete.

Registration Reply (RRP)

with configuration info

and security association.

Relay the info to

Home Agent.

Request to authenticate

the user and acquire a

secure key from an

existing security

infrastructure.

Relay the request.

Reply the

authentication result

and the secure key.

Authenticate the

user.

Generate security

association using

user Window login

username and

password.

Generate security association for

the userand perform the Mobile IP

authentication locally.

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Home Agent Redundancy 105

current CoA. The Home Agent maintains all associations in a mobility binding table, which is

essentially the heart of the Mobile IP process. If the Home Agent fails for any reason, the

mobility binding table is lost, and all Mobile Nodes registered with the Home Agent lose

connectivity. The Mobile Nodes usually experience a noticeable lapse in service during outage,

which can lead to some unhappy customers.

The Cisco Mobile IP Home Agent Redundancy feature is designed to provide scalable high

availability with near-instantaneous failover support. When a Home Agent fails, another Home

Agent takes over immediately to maintain all sessions to the Mobile Nodes. With this

mechanism, the mobility binding table is fully replicated on each Home Agent in the

redundancy group, and thus, no downtime occurs for any Mobile Node. The high availability

not only ensures that all mobility bindings survive a Home Agent failure but also ensures

minimal packet loss, which leads to happy customers!

The Home Agent Redundancy mechanism leverages the Hot Standby Router Protocol (HSRP)

(described in RFC 2281), which designates one router as active and another as standby. HSRP

determines redundancy peers and is used as a first-hop routing protocol for the Home Agent

address. HSRP can be defined as follows:

NOTE HSRP is a router redundancy protocol developed by Cisco that provides network resilience in

a way that ensures that user traffic will immediately and transparently recover from “first hop”

failures in network edge devices and access circuits. By sharing a virtual IP address and a MAC

(Layer 2) address, two or more routers can act as a single “virtual” router or default gateway to

the hosts on a LAN. The members of the router group continually exchange status messages to

detect when a peer goes down. This router group is referred to as the HSRP group, and the

virtual IP address is known as the HSRP group address.

The Cisco Home Agent Redundancy functionality then specifies the behavior of these routers

in the HSRP group to provide backup Home Agent service in Mobile IP. The preemption and

priority capabilities of HSRP can be configured to prefer one Home Agent to another, that is,

one Home Agent can be given preference to become the main Home Agent by configuring these

options appropriately.

The main idea in this mechanism is that the Home Agents in the redundancy group all have the

necessary information to provide Mobile IP service to the Mobile Nodes, and can seamlessly

take over for one another. However, to the Mobile Node, the redundancy group looks like one

Home Agent. The Cisco Home Agent Redundancy feature supports two main configurations:

an active-standby configuration and a peer-peer configuration. Active-standby and peer-peer

only refer to how Mobile IP signaling is handled. Actual forwarding of data traffic to Mobile

Nodes is handled by integration with routing. Both Home Agents in the redundancy group bring

up the Mobile IP tunnels and have a full copy of the mobility binding table. Thus, either one

can forward data traffic to the Mobile Node based on which one receives the Mobile Node’s