Configuring Icmp; Enabling Icmp Protocol Unreachable Messages - Cisco 4500M Software Manual

Configuring ICMP

These examples show how to use the traceroute mac and traceroute mac ip commands to display the
physical path a packet takes through the network to reach its destination:
Switch# traceroute mac 0000.0201.0601 0000.0201.0201
Source 0000.0201.0601 found on con6[WS-C2950G-24-EI] (2.2.6.6)
con6 (2.2.6.6) :Fa0/1 => Fa0/3
con5
con1
con2
Destination 0000.0201.0201 found on con2[WS-C3550-24] (2.2.2.2)
Layer 2 trace completed
Switch#
Switch# traceroute mac ip 2.2.66.66 2.2.22.22 detail
Translating IP to mac .....
2.2.66.66 => 0000.0201.0601
2.2.22.22 => 0000.0201.0201
Source 0000.0201.0601 found on con6[WS-C2950G-24-EI] (2.2.6.6)
con6 / WS-C2950G-24-EI / 2.2.6.6 :
con5 / WS-C2950G-24-EI / 2.2.5.5 :
con1 / WS-C3550-12G / 2.2.1.1 :
con2 / WS-C3550-24 / 2.2.2.2 :
Destination 0000.0201.0201 found on con2[WS-C3550-24] (2.2.2.2)
Layer 2 trace completed.
Switch#
Configuring ICMP
Internet Control Message Protocol (ICMP) provides many services that control and manage IP
connections. ICMP messages are sent by routers or access servers to hosts or other routers when a
problem is discovered with the Internet header. For detailed information on ICMP, refer to RFC 792.

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 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 the generation of ICMP Protocol Unreachable and Host Unreachable messages, enter the
following command in interface configuration mode:
Command
Switch (config-if)# [no] ip unreachables
Software Configuration Guide—Release 12.2(25)EW
5-10
(2.2.5.5
(2.2.1.1
(2.2.2.2
Fa0/1 [auto, auto] => Fa0/3 [auto, auto]
Fa0/3 [auto, auto] => Gi0/1 [auto, auto]
Gi0/1 [auto, auto] => Gi0/2 [auto, auto]
Gi0/2 [auto, auto] => Fa0/1 [auto, auto]
Chapter 5
) : Fa0/3 => Gi0/1
) : Gi0/1 => Gi0/2
) : Gi0/2 => Fa0/1
Purpose
Enables ICMP destination unreachable messages.
Use the no keyword to disable the ICMP destination
unreachable messages.
Checking Port Status and Connectivity
OL-6696-01