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
router mobile
ip mobile home-agent
ip mobile host nai @example address pool dhcp-proxy-client dhcp-server
interface Loopback1 aaa
radius-server host auth-port 1645 acct-port 1646
radius-server domain-stripping
radius-server key skeleton
Window Domain
Controller or
Active Directory
DHCP Request
DHCP Reply with
Option 68
Requst (RRQ)
the reply.
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
Relay the request.
Reply the
authentication result
and the secure key.
Authenticate the
Generate security
association using
user Window login
username and
Generate security association for
the userand perform the Mobile IP
authentication locally.
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

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