IP unnumbered – FAQs

  1. What is APNIC’s position on IP unnumbered?
  2. What is IP unnumbered?
  3. Can you see if the serial interface is up?
  4. Can the “ping” utility still be used to determine whether the interface is up?
  5. Can SNMP be used to remotely monitor interface status?
  6. Can a runnable image be netbooted over an unnumbered serial interface?
  7. Can IP security options be supported on an unnumbered interface?
  8. Where can I find more information about the use of IP unnumbered configuration?

What is APNIC’s position on IP unnumbered?

APNIC encourages the use of the IP unnumbered functionality as it helps conserve IPv4 address space. See section 7 of APNIC guidelines for IPv4 allocation and assignment requests.

What is IP unnumbered?

Depending on the router vendor, IP unnumbered allows you to do IP processing on a serial interface without assigning an explicit IP address for point-to-point links. This is applicable to statically routed, singly homed customers. It is not applicable to those doing BGP.

Can you see if the serial interface is up?

Yes. Please note that while the interface defined by the interface name must be enabled (listed as “up” in the show interfaces command display), the physical state of an interface (layer 1) does not relate to protocol activity (layer 3).

Can the “ping” utility still be used to determine whether the interface is up?

Yes, although the interface has no address, you can still determine whether or not the interface is up by “pinging” the ethernet IP address. If the serial interface is up and the ethernet interface is down, the serial interface “adopts” the ethernet’s IP address, so the router will still respond to pings received on the serial interface.

Can SNMP be used to remotely monitor interface status?

No, it cannot. But many ISPs produce their own software tools for interface monitoring.

Can a runnable image be netbooted over an unnumbered serial interface?

No. But as frequent upgrades interrupt stability, many operators choose to do this once or twice a year.

Can IP security options be supported on an unnumbered interface?

Yes, most security options do support unnumbered interfaces. This includes security options such as filtering, tunnels and IPsec. If you find any that do not work, APNIC would appreciate it if you could email the examples to the APNIC Helpdesk.

Where can I find more information about the use of IP unnumbered configuration?

See Cisco’s Understanding and Configuring the ip unnumbered Command.