Configuring IP Addressing

Enabling IP Routing

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:

 

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.

 

 

 

The tunnel key should correspond to the NHRP network identifier specified in the ip nhrp network-idinterface 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-cachingkeyword. 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:

Command

Purpose

 

 

Router(config-if)#ip nhrp server-only [non-caching]

Configures NHRP server-only 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:

Command

Purpose

 

 

Router(config)# ip routing

Enables IP routing.

 

 

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.

Cisco IOS IP Configuration Guide

IPC-27

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Cisco Systems 78-11741-02 manual Enabling IP Routing, Configuring a GRE Tunnel for Multipoint Operation, IPC-27