Home Agent Enhancements


The Home Agent receives and processes Mobile IP RRQs from the mobile router in the same way that it does for standard Mobile Nodes. Specifically, the Home Agent validates the RRQ and, upon validation, it establishes a mobility binding. In addition to the standard Mobile IP processing, the Home Agent must also "process" the mobile networks associated with the mobile router. Recall that a mobile network can either be statically configured on the Home Agent or dynamically learned in the RRQ. In either case, the Home Agent injects the mobile networks into its forwarding table so that routing protocols configured on it can redistribute these mobile routes.

If the mobile router registered dynamic mobile networks with the Home Agent, the Home Agent acknowledges proper processing of all the mobile networks to the mobile router. This is accomplished by the Home Agent appending the Mobile Router Dynamic Mobile Network NVSE to convey dynamic mobile networks and Mobile Router Static Mobile Network NVSE to convey static mobile networks to its RRP to the mobile router. If the Home Agent shares a security association with the FA, it also appends the FAHome Agent Authentication Extension (FHAE) to the RRP. The Home Agent does not append all the NVSEs in a RRP for a deregistration message.

In addition to injecting the mobile networks, the mobile router also creates an additional tunnel to the mobile router's Home Address and adds routes to the mobile networks through this tunnel. The Home Agent then advertises reachability to these mobile networks through the Interior Gateway Routing Protocol (IGRP), thereby attracting packets that are destined to nodes on the mobile networks, as shown in Figure 7-8.

Figure 7-8. Home Agent Advertises Reachability


Thus, a key feature of network mobility is that inside the Home Agent CoA tunnel is another tunnel from the Home Agent to the mobile router. This second tunnel is required because if a FA is in use, it has a route only to the Home Address. Packets destined to the mobile networks would follow the standard routing back to the Home Agent and end up in a routing loop. Pictorially, this can be seen in Figure 7-9 and is described as follows. Consider a packet that is sent from a Correspondent Node (CN) to a node on one of the mobile router's networks. Because the Home Agent is injecting the mobile networks into the IGP, the packet is routed to the Home Agent using standard IP routing. The Home Agent receives the packet and realizes that it is destined for a node on a mobile network. Thus, the Home Agent encapsulates the packet in a tunnel from itself to the mobile router. For this tunneled packet to reach the mobile router, the packet must be encapsulated again to the mobile router's CoA. Figure 7-10 shows the IP header of a double-encapsulated packet from the Home Agent to the mobile router.

Figure 7-9. Dual Tunnels Deliver Traffic to the Mobile Networks


Figure 7-10. Two Layers of Encapsulation


NOTE

The Cisco Home Agent Redundancy feature, which provides fault tolerance on the Home Network is enhanced so that the Home Agents in the redundancy group are kept in sync with respect to dynamic mobile networks. Specifically, the active Home Agent updates the standby Home Agent with mobile networks that are registered dynamically.


Home Agent Configuration for Network Mobility

The Home Agent must first be configured to provide Home Agent services, as described in Chapter 4. In addition, the Home Agent must be configured appropriately for it to support the mobile networks associated with mobile routers. This includes configuring the Home Agent with the following command:

 ip mobile host lower [upper] {interface name | virtual-network net mask}[lifetime   number] 

This configures the mobile router as a mobile host. This is the same configuration that would be used for a Mobile Node with a static Home Address. lower and upper are a range of IP addresses on the Home Network that are allowed to register as Mobile Nodes. The interface name option configures a physical connection from the Home Agent to the mobile router. The following command establishes that the mobile host (or range of hosts) is a mobile router:

 ip mobile mobile-networks lower [upper] 

The command enters the Home Agent into mobile networks configuration mode, where details about the mobile networks associated with the mobile router are configured. The upper range can be used only with dynamically registered networks and allows multiple mobile routers to be configured at once. This range does not need to match the range in the ip mobile host statement. This is useful if the network includes Mobile Nodes and mobile routers.

Within the mobile networks configuration mode on the Home Agent, the following configuration commands can be used to configure the mobile networks:

  • description string This optional command adds a description to the mobile router configuration.

  • network net mask This optional command statically configures a mobile network on the mobile router. Specifically, it configures a network that is attached to the mobile router as a mobile network.

  • register This optional command allows the mobile router to dynamically register mobile networks with the Home Agent. That is, the Home Agent is configured to accept RRQs with a Mobile Network Prefix Critical Vendor Specific Extension (CVSE) from the mobile router to learn about the mobile networks. When the mobile router registers its mobile networks on the Home Agent, the Home Agent looks up the mobile network configuration and verifies that the register command is configured before adding forwarding entries to the mobile networks. If the register command is not configured, the Home Agent rejects an attempt by the mobile router to dynamically register its mobile networks.

The Home Agent must be configured to share a security association with the mobile router and redistribute routes into its routing protocol.

Priority Home Agent Assignment

Although not an enhancement to the Home Agent per se, the Cisco Mobile NetworksPriority Home Agent Assignment feature allows a mobile router to select a "closer" Home Agent when it is roaming. However, the mobile router doesn't just choose a random Home Agent. Rather, the mobile router can select a preferred Home Agent from a set of configured Home Agents based on a combination of existing Home Agent priority configurations on the mobile router and CoA access lists configured on the Home Agent, as shown in Figure 7-11. Although this feature might sound like a dream come true, some caveats also need to be discussed.

Figure 7-11. Priority Home Agent Assignment


Each participating Home Agent is configured with an access list that contains the FA CoAs in its region. When a mobile router sends a RRQ to the selected Home Agent, the Home Agent consults its access list and either accepts or denies the request based on the CoA in the request. If the Home Agent denies the request because the CoA is outside its "domain," that is, the CoA is not in the access list, the mobile router tries to register with the next-best Home Agent, and so on. By doing so, the mobile router is able to register with a geographically closer Home Agent, thereby improving latency on the network.

A number of things must be considered when deploying priority Home Agents. Routing updates don't come free. When the mobile router registers with the new Home Agent, the new Home Agent injects the mobile networks associated with the mobile router into the IGP.Following the successful registration with the new Home Agent, the mobile router attempts to deregister from the old Home Agent so that the old Home Agent can withdraw the redistributed mobile network routes from the IGP. At this point, the IGP must converge before the forwarding path is optimized. While the IGP is reconverging, packets can bounce around the network following stale routes. In some cases, these packets can be lost.

With priority Home Agents, the Home Address of the mobile router must also not be used. The same Home Network needs to be configured on all Home Agents. However, you cannot make the Home Address reachable through the Home Agent that is in use without injecting host routes into the network. Host routing would defeat the purpose of Mobile IP. Use a route map to prevent the Home Network from even being redistributed into the IGP, and use an address from the mobile network for management of the mobile router.

Deployment of priority Home Agents on a large scale also requires careful evaluation of the impacts to the IGP. When large numbers of mobile routers are used in conjunction with many Home Agents, frequent routing updates can cause instability in the IGP. Try to avoid changing Home Agents too frequently. For example, if a train operator has many trains in several cities, but only a few trains ever cross from city to city, having one Home Agent per city would be ideal. In the same example, if multiple Home Agents were used in the same city, frequent Home Agent changes would force the IGP to reconverge frequently.



    Mobile IP Technology and Applications
    Mobile IP Technology and Applications
    ISBN: 158705132X
    EAN: 2147483647
    Year: 2005
    Pages: 124

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