Cisco ICMP Redirect

ICMP redirect messages are used by routers to notify the hosts on the data link that a better route is available for a particular destination.

Here is the following network topology:

All routers are configured in the same broadcast domain – 10.1.123.0/24

R1 MAC Address – c0:00:14:98:00:00

R2 MAC Address – c0:01:14:98:00:00

R3 MAC Address – c0:02:14:98:00:00

I have disabled R3 routing capabilities (no ip routing) and configured a default static route to R1,

R1 is configured with default static route to R2 while R2 is configured with default static route to R1.

ICMP redirect is enabled:

When trying to send ICMP request from R3 to 192.168.21.1 the following will happen:

      1.       R3 will send an ARP request message for 10.1.123.1 (his default gateway)

      2.       R1 will answer with an ARP replay message to R3

      3.       R3 will send tan ICMP request for IP 192.168.21.1 to R1

      4.       R1 will send an ICMP redirect to R3

      5.       R3 will send an ARP request message for 10.1.123.2

      6.       R2 will answer with an ARP replay message to R3

      7.       R3 will send an ICMP request for IP 192.168.21.1 to R2

      8.       R2 will send an ICMP replay message to R3

In the ICMP redirect we can see the source and destination and the new gateway address which will send in response:

What will happen if I will disable IP redirect?

ICMP redirect is disabled:

I have configured R1 Fa0/0 with no ip redirect command and clear the arp table of R3 then I sent another ICMP request to 192.168.21.1 and this is what happens:

      1.       R3 will send an ARP request message for 10.1.123.1 (his default gateway)

      2.        R1 will answer with an ARP replay message to R3

      3.       R3 will send to R1 an ICMP request for IP 192.168.21.1

      4.       R1 will send an ARP request message for IP 10.1.123.2

      5.       R2 will answer with an ARP replay message to R1

      6.       R1 will send an ICMP request to R2 with source MAC address of R1 destination MAC address of R2, source IP address of R3 and destination IP address of R2

      7.       R2 will send an ARP request message for 10.1.123.3

      8.       R3 will answer with an ARP replay message to R2

 9.       R2 will send an ICMP replay to R3 

The first ICMP request is from R3 to R1:

note the MAC addresses - source is R3 destination is R1

The second ICMP request is from R1 to R2:

Note the MAC addresses - source is R1 destination is R2,

and the IP addresses - source is R3 and destination is R2

This capture has been made on R1 Fa0/0 link and that's the reason we can't see R2 replies.

When Are ICMP Redirects Sent? 

Cisco routers send ICMP redirects when all of these conditions are 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.)

Note: 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. For more information, refer to HSRP Support for ICMP Redirects section of Hot Standby Router Protocol Features and Functionality.

 

For more information from Cisco web site:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094702.shtml