Determining Reachability of IP Destinations in the Network

Sending Echo Request Packets to the IP Address

You can send an Internet Control Message Protocol (ICMP) or ICMPv6 echo request packet to a specific IP address using the ping command.

The following characters can appear in the display after issuing the ping command:

• !—Reply received
• .—Timed out while waiting for a reply
• ?—Unknown packet type
• A—Address mask request message
• a—Address mask reply message
• D—Router discovery advertisement message
• d—Router discovery request message
• H—Host unreachable
• I—Information request message
• i—Information reply message
• L—TTL expired message
• M—Could not fragment, DF bit set
• m—Parameter problem message
• N—Network unreachable
• P—Protocol unreachable
• Q—Source quench
• r—Redirect message
• T—Timestamp request message
• t —Timestamp reply message
• U—Destination unreachable

To send an ICMP echo request packet to the IP address that you specify:

• Issue the ping command in Privileged Exec mode.
  host1#ping 172.16.1.1 extended interface serial 5/2:1/1

Discovering the Routes Followed by Router Packets when Traveling to the IP Destination

You can discover routes that router packets follow when traveling to their destination using the traceroute command.

You can specify the following:

• A VRF context
• Destination IP or IPv6 address
• Source interface for each of the transmitted packets
• Source address for each of the transmitted packets
• Maximum number of hops of the trace and a timeout value
• Size of the IP packets (not the ICMP payload) in the range 0–64000 bytes sent with the traceroute command. Including a size might help locate any MTU problems that exist between your router and a particular device.
• Extended IP header attributes, including the ToS byte (IP only), whether to set the DF bit for the transmitted packets (IP only), the traffic class (IPv6 only), and flow label (IPv6 only).

You can also force transmission of the packets on a specified interface regardless of what the IP address lookup indicates.

To discover the routes that router packets follow when traveling to their destination:

• Issue the traceroute command in Privileged Exec mode.
  host1#traceroute 172.20.13.1 20 timeout 10