Figure 27 shows the interface
stack built for this configuration.
Figure 27: GRE Tunnel Dynamic Subscriber Interface
Dynamic Subscriber Interface Configuration Example
The procedure in this section shows how to configure
dynamic subscriber interfaces by using the same loopback interface
referenced by multiple unnumbered IP interfaces. Instead of assigning
a different IP address to each physical interface, this example assigns
an IP address to a loopback interface (loopback 0). Each physical
interface is then configured as an unnumbered IP interface, referencing
the same loopback interface. This example uses a DHCP local server.
This approach has the following benefits:
A loopback interface provides a stable IP address that
can minimize the impact if a physical interface in the network goes
Unnumbered IP interfaces preserve valuable IP address
To configure dynamic subscriber interfaces, perform
the following steps:
Enable the DHCP local server for standalone mode.
host1(config)#service dhcp-local standalone
Access DHCP Local Pool Configuration mode for the local
host1(config)#ip dhcp-local pool ispWestford
Specify the enduring IP addresses that the DHCP local
server can assign from the local address pool.
Use to select a Fast Ethernet (FE) interface on a line
module or an SRP module.
host1(config)#interface fastEthernet 1/0
Use the no version to remove
IP from an interface or subinterface. You must issue the no version from the highest level down; you cannot
remove an interface or a subinterface if the one above it still exists.
You can use a loopback interface to provide a stable IP
address that can minimize the impact if a physical interface goes
You cannot shut down a loopback interface.
We recommend that you configure a 32-bit subnet mask for the
loopback interface. For example, if you configure a loopback interface
with the IP address and mask as 126.96.36.199/16, the 188.8.131.52/16 route entry
is entered on the line module and all traffic destined to the to 184.108.40.206/16
subnet is forwarded to the SRP module by the line module. Although
the SRP module responds only to traffic destined to the 220.127.116.11 subnet
and discards traffic to all other host IP addresses within that subnet
(18.104.22.168/16), if no specific or longer route entry is found or if
the SRP module receives too much traffic from subnets other than 22.214.171.124,
the CPU utilization on the SRP module reaches the saturation level.
If you use a subnet mask other than a /32 mask for the IP address
configured on the loopback interface, traffic from the entire subnet
is routed to the loopback interface. Therefore, that subnet cannot
be routed through any other interface on the router, unless a more
specific route points to another interface.
Use to configure an IP interface to support creation of
dynamic subscriber interfaces. The specified IP interface is considered
the primary interface.
The router creates the required dynamic subscriber interfaces
when the IP address is assigned to the associated subscriber. The
address might be assigned by an external DHCP server, the DHCP local
server, or the packet detect feature.
Use the include-primary keyword
to specify that the primary interface can be assigned to a subscriber.
Use the exclude-primary keyword to specify
that the primary interface is not used for subscribers. The primary
interface is not assigned to a subscriber by default.
You can issue this command from Interface Configuration
mode, Subinterface Configuration mode, or Profile Configuration mode.
Use the no version to disable
creation of dynamic subscriber interfaces associated with this primary
IP interface. Use the no version with the include-primary keyword to specify that the primary
interface is not assigned to a subscriber.
Use to set the router’s packet detect feature and
specify that IP automatically detect packets that do not match any
entries in the demultiplexer table. When an unmatched packet is detected,
an event is generated that determines whether to create a dynamic
host1(config-if)#ip auto-detect ip-subscriber
Use the no version to restore
the default, in which packet detection is disabled.
Use to configure a static primary IP interface to use
framed routes as source IP addresses when creating dynamic subscriber
interfaces. The router uses the Framed-Route RADIUS attribute 
sent in Access-Accept messages to apply framed routes to subscriber
interfaces associated with the primary interface.