ICMP Messages CCNA
ICMP Messages CCNA

ICMP Messages

ICMP Messages


This topic explain how ICMP is used to test network connectivity.. Start learning CCNA 200-301 for free right now!!

Note: Welcome: This topic is part of Chapter 13 of the Cisco CCNA 1 course, for a better follow up of the course you can go to the CCNA 1 section to guide you through an order.

ICMPv4 and ICMPv6 Messages

In this topic, you will learn about the different types of Internet Control Message Protocols (ICMPs), and the tools that are used to send them.

Although IP is only a best-effort protocol, the TCP/IP suite does provide for error messages and informational messages when communicating with another IP device. These messages are sent using the services of ICMP. The purpose of these messages is to provide feedback about issues related to the processing of IP packets under certain conditions, not to make IP reliable. ICMP messages are not required and are often not allowed within a network for security reasons.

ICMP is available for both IPv4 and IPv6. ICMPv4 is the messaging protocol for IPv4. ICMPv6 provides these same services for IPv6 but includes additional functionality. In this course, the term ICMP will be used when referring to both ICMPv4 and ICMPv6.

The types of ICMP messages, and the reasons why they are sent, are extensive. The ICMP messages common to both ICMPv4 and ICMPv6 and discussed in this module include:

  • Host reachability
  • Destination or Service Unreachable
  • Time exceeded

Host Reachability

An ICMP Echo Message can be used to test the reachability of a host on an IP network. The local host sends an ICMP Echo Request to a host. If the host is available, the destination host responds with an Echo Reply. In the figure, click the Play button to see an animation of the ICMP Echo Request/Echo Reply. This use of the ICMP Echo messages is the basis of the ping utility.

Host Reachability
Host Reachability

Destination or Service Unreachable

When a host or gateway receives a packet that it cannot deliver, it can use an ICMP Destination Unreachable message to notify the source that the destination or service is unreachable. The message will include a code that indicates why the packet could not be delivered.

Some of the Destination Unreachable codes for ICMPv4 are as follows:

  • 0 – Net unreachable
  • 1 – Host unreachable
  • 2 – Protocol unreachable
  • 3 – Port unreachable

Some of the Destination Unreachable codes for ICMPv6 are as follows:

  • 0 – No route to destination
  • 1 – Communication with the destination is administratively prohibited (e.g., firewall)
  • 2 – Beyond scope of the source address
  • 3 – Address unreachable
  • 4 – Port unreachable

Note: ICMPv6 has similar but slightly different codes for Destination Unreachable messages.

Time Exceeded

An ICMPv4 Time Exceeded message is used by a router to indicate that a packet cannot be forwarded because the Time to Live (TTL) field of the packet was decremented to 0. If a router receives a packet and decrements the TTL field in the IPv4 packet to zero, it discards the packet and sends a Time Exceeded message to the source host.

ICMPv6 also sends a Time Exceeded message if the router cannot forward an IPv6 packet because the packet has expired. 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.

ICMPv6 Messages

The informational and error messages found in ICMPv6 are very similar to the control and error messages implemented by ICMPv4. However, ICMPv6 has new features and improved functionality not found in ICMPv4. ICMPv6 messages are encapsulated in IPv6.

ICMPv6 includes four new protocols as part of the Neighbor Discovery Protocol (ND or NDP).

Messaging between an IPv6 router and an IPv6 device, including dynamic address allocation are as follows:

  • Router Solicitation (RS) message
  • Router Advertisement (RA) message

Messaging between IPv6 devices, including duplicate address detection and address resolution are as follows:

  • Neighbor Solicitation (NS) 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.

Click each for an illustration and explanation of ICMPv6 messages.

RA messages are sent by IPv6-enabled routers every 200 seconds to provide addressing information to IPv6-enabled hosts. The 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.

RA Message ICMP
RA Message ICMP

R1 sends 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."

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.

RS Message ICMP
RS Message ICMP

R1 replies to the RS with an RA message.

  1. 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.”
  2. 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."

When a device is assigned a global IPv6 unicast or link-local unicast address, it 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, as shown in the figure.

If another device on the network has this address, it will respond with an NA message. This NA message will notify the sending device that the address is in use. If a corresponding NA message is not returned within a certain amount of time, the unicast address is unique and acceptable for use.

Note: DAD is not required, but RFC 4861 recommends that DAD is performed on unicast addresses.

NS Message ICMP
NS Message ICMP

PC1 sends an NS message to check the uniqueness of an address, “Will whoever has the IPv6 address 2001:db8:acad:1::10, send me your MAC address?"

Address resolution is used when a device on the LAN knows the IPv6 unicast address of a destination but does not know its Ethernet MAC address. 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.

NA Message ICMP
NA Message ICMP

  1. R1 sends an address resolution NS message. “Will whoever has the IPv6 address 2001:db8:acad:1::10, send me your MAC address?"
  2. PC1 replies with an NA message. “I’m 2001:db8:acad:1::10 and my MAC address is 00:aa:bb:cc:dd:ee."

Glossary: If you have doubts about any special term, you can consult this computer network dictionary.

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