Enabling Icmp Redirect Messages; Enabling Icmp Mask Reply Messages - Cisco 4500M Software Manual

Chapter 5 Checking Port Status and Connectivity
To limit the rate that Internet Control Message Protocol (ICMP) destination unreachable messages are
generated, perform this task:
Command
Switch (config)# [no] ip icmp rate-limit
unreachable [df] milliseconds

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/univercd/cc/td/doc/product/software/ios121/121cgcr/ip_c/ipcprt1/1cdip.htm
To enable the sending of ICMP Redirect messages if the Cisco IOS software is forced to resend a packet
through the same interface on which it was received, enter the following command in interface
configuration mode:
Command
Switch (config-if)# [no] ip redirects

Enabling ICMP Mask Reply Messages

Occasionally, network devices must know the subnet mask for a particular subnetwork in the
internetwork. To obtain this information, devices can send ICMP Mask Request messages. These
messages are responded to by ICMP Mask Reply messages from devices that have the requested
information. The Cisco IOS software can respond to ICMP Mask Request messages if the ICMP Mask
Reply function is enabled.
OL-6696-01
Purpose
Limits the rate that ICMP destination messages are
generated.
Use the no keyword to remove the rate limit and
reduce the CPU usage.
Purpose
Enables ICMP Redirect messages.
Use the no keyword to disable the ICMP Redirect
messages and reduce CPU usage.
Software Configuration Guide—Release 12.2(25)EW
Configuring ICMP
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