There are disadvantages to using unnumbered serial interfaces.

For example, you cannot use the Ping command to determine whether the interface is up or down.

You cannot reboot a runable image over an unnumbered interface.

Nor can you support IP security options on an unnumbered interface.
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You have two options when specifying ARP cache entries.

You can specify that the software respond to ARP requests as if it was the owner of the specified IP address.

And you can specify an ARP entry timeout period when you define ARP entries.

The ARP Timeout specifies how long the ARP signal lasts on the network.

The form of the command:

  ARP ip-address hardware-address type

To specify the owner option you use the IP address and the hardware address with the Alias command:

  ARP ip-address hardware-address type alias

You specify the timeout using the Arp Timeout command:

  ARP timeout seconds
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You must explicitly configure all interfaces that will use Probe.

Using Probe, the router can communicate transparently with Ethernet hosts that use this type of encapsulation.
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