ICMP Redirect

ICMP redirect messages are used by the gateway to inform the source host of a better route to the destination is available. For these messages to be sent by the gateway, the below conditions have to be met:

• The interface on which the packet comes into the router is the same interface on which the packet gets routed out.
  
• The subnet or network of the source IP address is on the same subnet or network of the next-hop IP address of the routed packet.
  
• The datagram is not source-routed.
  
• The kernel is configured to send redirects. (By default, Cisco routers send ICMP redirects. The interface subcommand no ip redirects can be used to disable ICMP redirects.) 
  
  
  Below an important fact to remember when configuring HSRP on a cisco router:
  
  ICMP redirects are disabled by default if Hot Standby Router Protocol (HSRP) is configured on the interface. In Cisco IOS Software Release 12.1(3)T and later, ICMP Redirect is allowed to be enabled on interfaces configured with HSRP.