Module 13: ICMP
Introduction of Networks v7.0
(ITN)
13.1 ICMP Messages
© 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 2
ICMP Messages
ICMPv4 and ICMPv6 Messages
• Internet Control Message Protocol (ICMP) provides feedback about issues related to the
processing of IP packets under certain conditions.
• ICMPv4 is the messaging protocol for IPv4. ICMPv6 is the messaging protocol for IPv6 and
includes additional functionality.
• The ICMP messages common to both ICMPv4 and ICMPv6 include:
• Host reachability
• Destination or Service Unreachable
• Time exceeded
Note: ICMPv4 messages are not required and are often not allowed within a network for
security reasons.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 3
ICMP Messages
Host Reachability
ICMP Echo Message can be used to
test the reachability of a host on an IP
network.
In the example:
• The local host sends an ICMP Echo
Request to a host.
• If the host is available, the
destination host responds with an
Echo Reply.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 4
ICMP Messages
Destination or Service Unreachable
• An ICMP Destination Unreachable message can be used to notify the source that a
destination or service is unreachable.
• The ICMP message will include a code indicating why the packet could not be delivered.
A few Destination Unreachable A few Destination Unreachable codes for
codes for ICMPv4 are as follows: ICMPv6 are as follows:
• 0 - Net unreachable • 0 - No route to destination
• 1 - Host unreachable • 1 - Communication with the destination is
administratively prohibited (e.g., firewall)
• 2 - Protocol unreachable
• 2 – Beyond scope of the source address
• 3 - Port unreachable
• 3 - Address unreachable
• 4 - Port unreachable
Note: ICMPv6 has similar but slightly different codes for Destination Unreachable messages.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 5
ICMP Messages
Time Exceeded
• When the Time to Live (TTL) field in a packet is decremented to 0, an ICMPv4 Time
Exceeded message will be sent to the source host.
• ICMPv6 also sends a Time Exceeded message. Instead of the IPv4 TTL field, ICMPv6 uses
the IPv6 Hop Limit field to determine if the packet has expired.
Note: Time Exceeded messages are used by the traceroute tool.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 6
ICMP Messages
ICMPv6 Messages
ICMPv6 has new features and improved functionality not found in ICMPv4, including four new
protocols as part of the Neighbor Discovery Protocol (ND or NDP).
Messaging between an IPv6 router and an Messaging between IPv6 devices, including
IPv6 device, including dynamic address duplicate address detection and address
allocation are as follows: resolution are as follows:
• Router Solicitation (RS) message • Neighbor Solicitation (NS) message
• Router Advertisement (RA) message • Neighbor Advertisement (NA) message
Note: ICMPv6 ND also includes the redirect message, which has a similar function to the redirect
message used in ICMPv4.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 7
ICMP Messages
ICMPv6 Messages (Cont.)
• RA messages are sent by IPv6-enabled
routers every 200 seconds to provide
addressing information to IPv6-enabled
hosts.
• RA message can include addressing
information for the host such as the prefix,
prefix length, DNS address, and domain
name.
• A host using Stateless Address
Autoconfiguration (SLAAC) will set its
default gateway to the link-local address of
the router that sent the RA.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 8
ICMP Messages
ICMPv6 Messages (Cont.)
• An IPv6-enabled router will also send out
an RA message in response to an RS
message.
• In the figure, PC1 sends a RS message to
determine how to receive its IPv6 address
information dynamically.
• R1 replies to the RS with an RA message.
• PC1 sends an RS message, “Hi, I just booted up.
Is there an IPv6 router on the network? I need to
know how to get my IPv6 address information
dynamically.”
• R1 replies with an RA message. “Hi all IPv6-
enabled devices. I’m R1 and you can use SLAAC
to create an IPv6 global unicast address. The
prefix is 2001:db8:acad:1::/64. By the way, use my
link-local address fe80::1 as your default gateway."
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 9
ICMP Messages
ICMPv6 Messages (Cont.)
• A device assigned a global IPv6 unicast or
link-local unicast address, may perform
duplicate address detection (DAD) to
ensure that the IPv6 address is unique.
• To check the uniqueness of an address, the
device will send an NS message with its
own IPv6 address as the targeted IPv6
address.
• If another device on the network has this
address, it will respond with an NA
message notifying to the sending device Note: DAD is not required, but RFC 4861
that the address is in use. recommends that DAD is performed on
unicast addresses.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 10
ICMP Messages
ICMPv6 Messages (Cont.)
• To determine the MAC address for the
destination, the device will send an NS
message to the solicited node address.
• The message will include the known
(targeted) IPv6 address. The device that
has the targeted IPv6 address will
respond with an NA message containing
its Ethernet MAC address.
• In the figure, R1 sends a NS message to
2001:db8:acad:1::10 asking for its MAC
address.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 11
13.2 Ping and Traceroute Tests
© 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 12
Ping and Traceroute Tests
Ping – Test Connectivity
• The ping command is an IPv4 and IPv6 testing
utility that uses ICMP echo request and echo
reply messages to test connectivity between
hosts and provides a summary that includes the
success rate and average round-trip time to the
destination.
• If a reply is not received within the timeout, ping
provides a message indicating that a response
was not received.
• It is common for the first ping to timeout if address
resolution (ARP or ND) needs to be performed
before sending the ICMP Echo Request.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 13
Ping and Traceroute Tests
Ping the Loopback
Ping can be used to test the internal
configuration of IPv4 or IPv6 on the local
host. To do this, ping the local loopback
address of 127.0.0.1 for IPv4 (::1 for
IPv6).
• A response from 127.0.0.1 for IPv4, or
::1 for IPv6, indicates that IP is properly
installed on the host.
• An error message indicates that TCP/IP
is not operational on the host.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 14
Ping and Traceroute Tests
Ping the Default Gateway
The ping command can be used to test the
ability of a host to communicate on the local
network.
The default gateway address is most often used
because the router is normally always
operational.
• A successful ping to the default gateway
indicates that the host and the router
interface serving as the default gateway
are both operational on the local network.
• If the default gateway address does not
respond, a ping can be sent to the IP
address of another host on the local
network that is known to be operational.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 15
Ping and Traceroute Tests
Ping a Remote Host
Ping can also be used to test the ability of a
local host to communicate across an
internetwork.
A local host can ping a host on a remote
network. A successful ping across the
internetwork confirms communication on
the local network.
Note: Many network administrators limit or
prohibit the entry of ICMP messages therefore,
the lack of a ping response could be due to
security restrictions.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 16
Ping and Traceroute Tests
Traceroute – Test the Path
• Traceroute (tracert) is a utility that is used
to test the path between two hosts and
provide a list of hops that were
successfully reached along that path.
• Traceroute provides round-trip time for
each hop along the path and indicates if a
hop fails to respond. An asterisk (*) is
used to indicate a lost or unreplied
packet. Note: Traceroute makes use of a function of the
• This information can be used to locate a TTL field in IPv4 and the Hop Limit field in IPv6
problematic router in the path or may in the Layer 3 headers, along with the ICMP
indicate that the router is configured not to Time Exceeded message.
reply.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 17
Ping and Traceroute Tests
Traceroute – Test the Path (Cont.)
• The first message sent from traceroute will
have a TTL field value of 1. This causes the
TTL to time out at the first router. This router
then responds with a ICMPv4 Time
Exceeded message.
• Traceroute then progressively increments
the TTL field (2, 3, 4...) for each sequence
of messages. This provides the trace with
the address of each hop as the packets time
out further down the path.
• The TTL field continues to be increased until
the destination is reached, or it is
incremented to a predefined maximum.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 18
Ping and Traceroute Tests
Packet Tracer – Verify IPv4 and IPv6 Addressing
In this Packet Tracer, you will do the following:
• Complete the Addressing Table Documentation
• Test Connectivity Using Ping
• Discover the Path by Tracing the Route
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 19
Ping and Traceroute Tests
Packet Tracer – Use Ping and Traceroute to Test Network
Connectivity
In this Packet Tracer, you will do the following:
• Test and Restore IPv4 Connectivity
• Test and Restore IPv6 Connectivity
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 20
