Dynamic Subscriber Interfaces Using Packet Detection Overview

For GRE tunnel interfaces, the event that triggers dynamic creation of subscriber interfaces occurs when the router receives a packet with a source IP address that is not in the demultiplexer table. In this case, the primary IP interface must be in autoconfiguration mode.

Packet detection is the only method of dynamically creating subscriber interfaces on GRE tunnel interfaces; you cannot use DHCP local server or DHCP external server.

Issuing the ip auto-configure ip-subscriber command configures the primary IP address to enable dynamic configuration of subscriber interfaces. Unlike DHCP configurations, the router creates the dynamic subscriber interface when it receives the first packet that contains the subscriber’s IP address as the source address.

In addition, a dynamic subscriber interface becomes inactive after a period of time in which the router receives no packets that contain the subscriber’s IP address as the source address. You can configure the period of time by issuing the ip inactivity-timer command.

To configure dynamic creation of subscriber interfaces on GRE tunnel interfaces, see Example: Configuring Dynamic Subscriber Interfaces over GRE Tunnels.

Designating Traffic for the Primary IP Interface

When dynamic creation of subscriber interfaces is enabled on the primary IP interface (by means of the ip auto-configure ip-subscriber command), you can use the ip source-prefix command to specify the source address of traffic that is destined for the primary IP interface instead of the subscriber interface. If the DHCP server (for DHCP server configurations) or the router (for packet detection configurations) then assigns a subscriber an IP address matching this source prefix, the router does not create a dynamic subscriber interface for that address.

Using Framed Routes

You can use the ip use-framed-routes ip-subscriber command to enable a primary IP interface to use framed routes as source IP addresses when creating dynamic subscriber interfaces. The framed routes are applied to the dynamic subscriber interface during configuration so traffic from the subsets can traverse the interface. By applying framed routes in this fashion, you can extend the per-subscriber interface management to any subnetworks behind the dynamic subscriber interface. RADIUS includes the Framed-Route attribute [22] in Access-Accept messages to specify the route in the following format: