The ping command allows you to verify connectivity to remote hosts. If you attempt to ping a host in a different IP subnetwork, you must define a static route to the network or configure a router to route between those subnets.

The ping command is configurable from normal executive and privileged EXEC mode. A ping returns one of the following responses:

• Normal response—The normal response (hostname is alive) occurs in 1 to 10 seconds, depending on the network traffic.

• Destination does not respond—If the host does not respond, a No Answer message is returned.

• Unknown host—If the host does not exist, an Unknown Host message is returned.

• Destination unreachable—If the default gateway cannot reach the specified network, a Destination Unreachable message is returned.

• Network or host unreachable—If there is no entry in the route table for the host or network, a Network or Host Unreachable message is returned.

To stop a ping in progress, press Ctrl-C.

To ping another device on the network from the switch, enter this command in normal executive and privileged EXEC mode:

Switch# ping host

Checks connectivity to a remote host.

This example shows how to ping a remote host from normal executive mode:

Switch# ping labsparc
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms
Switch#

Switch# ping 72.16.10.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 72.16.10.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms
Switch#

This example shows how to use a ping command in privileged EXEC mode to specify the number of packets, the packet size, and the timeout period:

Switch# ping
Protocol [ip]: ip
Target IP address: 1.1.1.1
Repeat count [5]: 10
Datagram size [100]: 100
Timeout in seconds [2]: 10
Extended commands [n]: n
Sweep range of sizes [n]: n
Type escape sequence to abort.
Sending 10, 100-byte ICMP Echos to 1.1.1.1, timeout is 10 seconds:
!!!!!!!!!!
Success rate is 100 percent (10/10), round-trip min/avg/max = 1/1/1 ms
Switch#

IP traceroute allows you to identify the path that packets take through the network on a hop-by-hop basis. The command output displays all network layer (Layer 3) devices, such as routers, that the traffic passes through on the way to the destination.

Layer 2 switches can participate as the source or destination of the trace command but does not appear as a hop in the trace command output.

The trace command uses the time to live (TTL) field in the IP header to cause routers and servers to generate specific return messages. Traceroute starts by sending a User Datagram Protocol (UDP) datagram to the destination host with the TTL field set to 1. If a router finds a TTL value of 1 or 0, it drops the datagram and sends back an Internet Control Message Protocol (ICMP) Time-Exceeded message to the sender. Traceroute determines the address of the first hop by examining the source address field of the ICMP Time-Exceeded message.

To identify the next hop, traceroute sends a UDP packet with a TTL value of 2. The first router decrements the TTL field by 1 and sends the datagram to the next router. The second router sees a TTL value of 1, discards the datagram, and returns the Time-Exceeded message to the source. This process continues until the TTL is incremented to a value large enough for the datagram to reach the destination host or until the maximum TTL is reached.

To determine when a datagram reaches its destination, traceroute sets the UDP destination port in the datagram to a large value that the destination host is unlikely to be using. When a host receives a datagram with an unrecognized port number, it sends an ICMP Port Unreachable error message to the source. The Port Unreachable error message indicates to traceroute that the destination has been reached.

These are the Layer 2 traceroute usage guidelines:

• CDP must be enabled on all the devices in the network. For Layer 2 traceroute to function properly, do not disable CDP.

  If any devices in the physical path are transparent to CDP, the switch cannot identify the path through these devices.

• All switches in the physical path must have IP connectivity. When a switch is reachable from another switch, you can test connectivity by using the ping command in privileged EXEC mode.

• The maximum number of hops identified in the path is ten.

• You can enter the traceroute mac or the traceroute mac ip command in privileged EXEC mode on a switch that is not in the physical path from the source device to the destination device. All switches in the path must be reachable from this switch.

• The traceroute mac command output shows the Layer 2 path only when the specified source and destination MAC addresses belong to the same VLAN. If you specify source and destination MAC addresses that belong to different VLANs, the Layer 2 path is not identified, and an error message appears.

• If you specify a multicast source or destination MAC address, the path is not identified, and an error message appears.

• If the source or destination MAC address belongs to multiple VLANs, you must specify the VLAN to which both the source and destination MAC addresses belong. If the VLAN is not specified, the path is not identified, and an error message appears.

• The traceroute mac ip command output shows the Layer 2 path when the specified source and destination IP addresses belong to the same subnet. When you specify the IP addresses, the switch uses Address Resolution Protocol (ARP) to associate the IP address with the corresponding MAC address and the VLAN ID.

  • If an ARP entry exists for the specified IP address, the switch uses the associated MAC address and identifies the physical path.

  • If an ARP entry does not exist, the switch sends an ARP query and tries to resolve the IP address. If the IP address is not resolved, the path is not identified, and an error message appears.

• When multiple devices are attached to one port through hubs (for example, multiple CDP neighbors are detected on a port), the Layer 2 traceroute feature is not supported. When more than one CDP neighbor is detected on a port, the Layer 2 path is not identified, and an error message appears.

• This feature is not supported in Token Ring VLANs.

To display the physical path that a packet takes from a source device to a destination device, enter either one of these commands:

Switch# traceroute mac source-mac-address destination-mac-address    
 

OR

Switch# traceroute mac ip source-ip destination-ip 

The following examples show how to use the traceroute mac and traceroute mac ip commands to display the physical path that a packet takes through the network to reach its destination:

Switch# traceroute mac cc16.7eaa.7203 188b.45eb.cc64
Source cc16.7eaa.7203 found on Switch
1 Switch (1.1.1.1) : Vl1 => Gi1/0/1
Destination 188b.45eb.cc64 found on Switch
Layer 2 trace completed.
Switch#

Switch# traceroute mac ip 1.1.1.1 1.1.1.2 detail
Translating IP to mac ..... 
1.1.1.1 => cc16.7eaa.7203
1.1.1.2 => 188b.45eb.cc64

Source cc16.7eaa.7203 found on Switch[C9410R] (1.1.1.1)
1 Switch / C9410R / 1.1.1.1 :Gi1/0/1 [auto, auto]
Destination 188b.45eb.cc64 found on Switch[C9410R] (1.1.1.1)
Layer 2 trace completed.
Switch#

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:

Switch (config-if)# [no] ip unreachables

Use the no keyword to disable the ICMP destination unreachable messages.

Note	If you enter the no ip unreachables command, you will break the path MTU discovery functionality. Routers in the middle of the network might be forced to fragment packets.

To limit the rate that Internet Control Message Protocol (ICMP) destination unreachable messages are generated, enter the following command:

Switch (config)# [no] ip icmp rate-limit unreachable [df]  milliseconds

Use the no keyword to remove the rate limit and reduce the CPU usage.

Occasionally, network devices must know the subnet mask for a particular subnetwork in the internetwork. To obtain this information, devices can send ICMP Mask Request messages. These messages are responded to by ICMP Mask Reply messages from devices that have the requested information. The Cisco IOS software can respond to ICMP Mask Request messages if the ICMP Mask. Reply function is enabled.

To have the Cisco IOS software respond to ICMP mask requests by sending ICMP Mask Reply messages, enter the following command:

Switch (config-if)# [no] ip mask-reply

Use the no keyword to disable this functionality.