154 Chapter 7: Metro Mobility: Cisco Mobile Networks
hold-down seconds—This specifies the time (in seconds) that the mobile router should
wait before registering to agents that are heard on an interface.
roam—This enables the mobile router to use the interface on which it is configured to
establish a registration with its Home Agent. This is the key piece necessary to make an
interface a roaming interface.
priority value—This sets the priority value that is compared among multiple configured
interfaces to select the interface on which to send the RRQ.
solicit—This instructs the mobile router to send agent solicitation messages periodically.
interval seconds—This specifies the interval (in seconds) that the mobile router waits
before it sends the next agent solicitation message after an advertisement is received on
an interface.
retransmit initial—This specifies the wait period (in seconds) before a RRQ is
retransmitted when no reply is received.
maximum seconds—This is the maximum wait period (in seconds) before a RRQ is
retransmitted when no reply is received. Each successive retransmission timeout period is
twice the previous period, as long as that is less than the maximum value.
retry number—This is the number of times to retry sending the RRQ. Retransmission
stops after the maximum number of retries are attempted. The range is from 0 to 10; the
default is 3. A value of 0 means no retransmission.
Agent Selection and Mobile IP Registration
A mobile router learns about Mobility Agents on its interfaces configured for roaming through
the same agent discovery process as for Mobile Nodes. It is highly conceivable that the mobile
router can hear advertisements from multiple agents at the same time, as depicted in Figure 7-6.
If the mobile router does not choose its agent in an intelligent manner, it could continually
toggle between different agents, leading to inefficient communication, not to mention excess
overhead and processing.
So, what happens when a mobile router hears multiple FA advertisements? How does it select
which of these agents to register through? For FAs heard on the same interface, the mobile
router selects the Mobility Agent from which it has most recently learned about. That is not to
say that if it is currently hearing from two FAs, it can flip-flop between them each time it hears
an advertisement. Changes only occur when a new FA is heard (and its hold down timer has
expired.) For FAs heard on different interfaces, the mobile router selects the agent on the
preferred interface, that is, the preferred path as described previously. Keep in mind that a FA
is selected only after the hold down period, as described in the previous section. If the mobile
router does change FAs, it sends a Mobile IP RRQ to its Home Agent.
Mobile Router 155
Figure 7-6 Mobile Router with Multiple Active Foreign Agents
In addition to moving from one FA to another, and the typical reregistration to extend a mobility
binding’s lifetime, several other events trigger a mobile router to send a Mobile IP registration
to its Home Agent. If the mobile router detects that its FA has rebooted, as described in Chapter
2, and thus has lost all its visitor information, the mobile router initiates a RRQ. If the mobile
router doesn’t hear from its current FA for a period of time and thus ages out, the mobile router
registers through another FA, if one exists. Similarly, if the interface that is connected to the
current FA goes down, the mobile router registers with another FA, if one exists. If the mobile
router does not have a usable CoA, it transitions to an isolated state. If the mobile router is in
an isolated state and then hears an advertisement from a FA, it also initiates a RRQ. The final
way that registration is triggered is with the clear ip mobile router registration exec
command. The following events trigger a RRQ:
Roam to new FA
FA ages out
FA is rebooted
Interface goes down
Isolated MR hears FA
clear ip mobile router registration
Regardless of the scenario, it is the mobile router’s responsibility to maintain a valid mobility
binding with its Home Agent whenever possible. Note that just as in standard Mobile IP, when
the mobile router roams back home, it deregisters its mobility binding with its Home Agent and
resumes standard operation.
Three FAs…which
one do I choose?
I'll choose FA3 since I
hear it on my preferred
roaming interface.
Mobile Router Roaming Interface
Preferred Roaming Interface
CoA 1
CoA 2
CoA 3

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