Dynamic cellular interface IPv4 mode

In the dynamic cellular interface IPv4 mode of operation, the unnumbered interface configured under the PDN router interface is a loopback interface that has no IP address assigned to it. When the cellular port associated with the PDN interface attaches to the cellular network, the cellular network assigns a dynamic IP address to the cellular port, which is then used as the IP address for the loopback interface under the PDN router interface.

Because cellular IP address allocation is dynamic, the address changes during every PDN attachment. Because the loopback interface associated with the PDN router interface is not configured with any IP address, this allows the node to learn the IP address assigned during the PDN attachment process and then assign that address to the loopback interface. The PDN router interface remains fixed to that address until the cellular port goes down and another PDN attachment is performed. This mode of operation is useful in applications where using dynamic address pools simplifies management and deployment of large numbers of nodes.

In this mode, the PDN router interface is operationally up when the system verifies that the IP address assigned to the interface does not conflict with any other IP address configured on the system. If there is a conflict, the PDN router interface is kept down.

The CLI output below shows an example of a PDN router interface configured for dynamic cellular interface IPv4 mode.

*A:DUT# config# router
                interface "pdn1-loopback"
                   loopback
                   no shutdown
                exit
                interface "pdn1-sim1" pdn
                   port 1/1/1
                   unnumbered "pdn-loopback"
                   no shutdown
                exit
           exit
      exit

When using dynamic cellular interface IPv4 mode, consider the points listed below.

IP/MPLS services cannot be anchored to a fixed address on the node. Instead, only those IP/MPLS services that support dynamic IP address learning and behaviors are supported, such as VPRNs with auto-bind or Layer 2 services using pseudowire templates configured with auto-gre-sdp. See Services overview for more information.
Some wireless service providers require that all packets entering their network from UE attached to their network have a source IP address that matches the IP address that the cellular network assigned to the UE. When this is a requirement, dynamic cellular interface IPv4 mode should not be used; instead, static cellular interface IPv4 mode should be used. When dynamic cellular interface IPv4 mode is used, there is no way to ensure all packets will meet the source IP address requirement as the node cannot filter a dynamically changing source IP address.
The system IP address used by the NSP NFM-P to manage the node is a private IP address. An in-band management VPRN service is required for the NSP NFM-P to reach the node.