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Configuring IP Addressing
Enabling IP Routing
IPC-27
Cisco IOS IP Configuration Guide
Configuring a GRE Tunnel for Multipoint Operation
You can enable a generic routing encapsulation (GRE) tunnel to operate in multipoint fashion. A tunnel
network of multipoint tunnel interfaces can be thought of as an NBMA network. To configure the tunnel,
use the following commands in interface configuration mode:
The tunnel key should correspond to the NHRP network identifier specified in the ip nhrp network-id
interface configuration command. See the “NHRP on a Multipoint Tunnel Example” section at the end
of this chapter for an example of NHRP configured on a multipoint tunnel.
Configuring NHRP Server-Only Mode
You can configure an interface so that it cannot initiate NHRP requests or set up NHRP shortcut SVCs
but can only respond to NHRP requests. Configure NHRP server-only mode on routers you do not want
placing NHRP requests.
If an interface is placed in NHRP server-only mode, you have the option to specify the non-caching
keyword. In this case, NHRP does not store information in the NHRP cache, such as NHRP responses
that could be used again. To save memory, the non caching option is generally used on a router located
between two other routers.
To configure NHRP server-only mode, use the following command in interface configuration mode:
Enabling IP Routing
IP routing is automatically enabled in the Cisco IOS software. If you choose to set up the router to bridge
rather than route IP datagrams, you must disable IP routing. To re-enable IP routing if it has been
disabled, use the following command in global configuration mode:
When IP routing is disabled, the router will act as an IP end host for IP packets destined for or sourced
by it, whether or not bridging is enabled for those IP packets not destined for the device. To re-enable IP
routing, use the ip routing command.
Command Purpose
Step 1
Router(config-if)# tunnel mode gre ip multipoint
Enables a GRE tunnel to be used in multipoint fashion.
Step 2
Router(config-if)# tunnel key key-number
Configures a tunnel identification key.
Command Purpose
Router(config-if)# ip nhrp server-only [non-caching]
Configures NHRP server-only mode.
Command Purpose
Router(config)# ip routing
Enables IP routing.