Cisco Systems 78-11741-02 manual Enabling Icmp Protocol Unreachable Messages, IPC-82

Configuring IP Services

Managing IP Connections

To manage various aspects of IP connections, perform the optional tasks described in the following sections:

•Enabling ICMP Protocol Unreachable Messages (Optional)

•Enabling ICMP Redirect Messages (Optional)

•Enabling ICMP Mask Reply Messages (Optional)

•Understanding Path MTU Discovery (Optional)

•Setting the MTU Packet Size (Optional)

•Enabling IP Source Routing (Optional)

•Configuring Simplex Ethernet Interfaces (Optional)

•Configuring a DRP Server Agent (Optional)

See the “ICMP Services Example” section at the end of this chapter for examples of ICMP services.

Enabling ICMP Protocol Unreachable Messages

If the Cisco IOS software receives a nonbroadcast packet destined for itself that uses an unknown protocol, it sends an ICMP protocol unreachable message back to the source. Similarly, if the software receives a packet that it is unable to deliver to the ultimate destination because it knows of no route to the destination address, it sends an ICMP host unreachable message to the source. This feature is enabled by default.

To enable this service if it has been disabled, use the following command in interface configuration mode:

Command	Purpose
Router(config-if)# ip unreachables	Enables the sending of ICMP protocol unreachable and host unreachable
	messages.

To limit the rate that ICMP destination unreachable messages are generated, use the following command in global configuration mode:

Command	Purpose
Router(config)# ip icmp rate-limit	Limits the rate that ICMP destination unreachable messages are generated.
unreachable [df] milliseconds

Enabling ICMP Redirect Messages

Routes are sometimes less than optimal. For example, it is possible for the router to be forced to resend a packet through the same interface on which it was received. If the router resends a packet through the same interface on which it was received, the Cisco IOS software sends an ICMP redirect message to the originator of the packet telling the originator that the router is on a subnet directly connected to the receiving device, and that it must forward the packet to another system on the same subnet. The software sends an ICMP redirect message to the originator of the packet because the originating host presumably could have sent that packet to the next hop without involving this device at all. The redirect message instructs the sender to remove the receiving device from the route and substitute a specified device representing a more direct path. This feature is enabled by default.

Cisco IOS IP Configuration Guide

IPC-82