Internet Protocol (IP) Addressing and Protocols
©2008 Allied Telesis Inc. All rights reserved.
22.8
AlliedWare Plus
TM
Operating System Software Reference C613-50003-00 REV E
Software Version 5.2.1
Internet Control Message Protocol
(ICMP)
The Internet Control Message Protocol (ICMP) allows networking devices to send information
and control messages to other devices or hosts. Your device implements all non-obsolete
ICMP functions.
The following table lists the ICMP messages implemented by your device.
ICMP messages are enabled on all interfaces by default. You can control the flow of ICMP
messages across different interfaces using the
access-list
commands. See
Chapter 38, Access
Control List (ACL) Commands
.
ICMP Message Type
Device Response
Echo reply (0)
This is used to implement the ping command. Your device
sends out an echo reply in response to an echo request.
Destination unreachable (3)
This message is sent when your device drops a packet
because it did not have a route to the destination.
Source Quench (4)
Your device sends this message when it must drop a packet
due to limited internal resources. This could be because the
source was sending data too fast to be forwarded.
Redirect (5)
Your device issues this message to inform a local host that its
target is located on the same LAN (no routing is required) or
when it detects a host using a non-optimal route (usually
because a link has failed or changed its status).
For example, if your device receives a packet destined to its
own MAC address, but with a destination IP address of
another host in the local subnet, it returns an ICMP redirect to
the originating host.
ICMP redirects are disabled on interfaces on which local proxy
ARP is enabled.
Echo request (8)
This is related to echo replies. If your device receives an echo
request, it sends an echo reply. If you enter the ping
command, your device generates echo requests.
Router Advertisements (10)
These are Router Discovery Protocol messages. If Router
Discovery is enabled, your device sends these to announce
the IP addresses of the sending interface.
Time to Live Exceeded (11)
If the TTL field in a packet falls to zero, your device sends this
message.This occurs when there are too many hops in the
path that a packet is traversing.
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