Cisco Systems 4500 Enabling ICMP Protocol Unreachable Messages, Enabling ICMP Redirect Messages

7-12
Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 7 Checking Port Status and Connectivity
Configuring ICMP
Enabling ICMP Protocol Unreachable Messages
If the Cisco IOS software receives a nonbroadcast packet 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 deliv er to the ultimate desti nation 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 the generation of ICMP Protocol Unreachable and Host Unreachable messages, enter the
following command in interface configuration mode:
Caution If you enter the no ip unreachables command, you will break the path MTU discovery functionality.
Routers in the middle of the network might be forced to fragment packets.
To limit the rate that Internet Control Message Protocol (ICMP) destination unreachable messages are
generated, enter the following command:
Enabling ICMP Redirect Messages
Data 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 this occurs, 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 packet's
originator 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.
However, when Hot Standby Router Protocol (HSRP) is configured on an interface, ICMP Redirect
messages are disabled (by default) for the interface. For more information on HSRP, refer to the
following URL:
http://www.cisco.com/en/US/docs/ios/ipapp/configuration/guide/ipapp_hsrp_ps6350_TSD_Products_Confi
guration_Guide_Chapter.html
Command Purpose
Switch (config-if)# [no] ip unreachables Enables ICMP destination unreachable messages.
Use the no keyword to disable the ICMP destination
unreachable messages.
Command Purpose
Switch (config)# [no] ip icmp rate-limit
unreachable [df] milliseconds Limits the rate that ICMP destination messages are
generated.
Use the no keyword to remove the rate limit and
reduce the CPU usage.