The -n option in the traceroute command is used to avoid a request to a name server. The Linux traceroute command is similar to the Cisco router implementation. Cisco routers send the ICMP message "time exceeded" back to the source from where the UDP/ICMP packet was received. You can also get one of these messages:Īn ICMP type 3, code 3 ("destination unreachable," "port unreachable") message, which indicates that a host has been reached.Ī "host unreachable," "net unreachable," "maximum TTL exceeded," or a "timeout" type of message, which means that the probe is resent.Ĭisco routers send UDP probe packets with a random source port and an incremental destination port (to distinguish the different probes). TTL is incremented, and this cycle repeats with incremental destination port numbers, if the ICMP "time exceeded" message is received. For each of the probes sent, you receive a "TTL exceeded" message, which is used to build a step-by-step path to the destination host. Step 2 is repeated for up to three probes (or as many times as requested in an extended traceroute command output). The UDP destination port is incremented, the source UDP port is randomized, and the second datagram dispatched. The UDP datagram is dispatched with TTL = 1, destination UDP port= 33434, and the source port randomized. This example starts with n = 1 and finishes with n = 3. These steps illustrate what happens when the UDP datagram is launched: The source UDP port of the initial datagram probe is randomized and has logical operator OR with 0x8000 (ensures a minimum source port of 0x8000). For more information on how to use the extended traceroute command, refer to Using the Extended ping and Extended traceroute Commands. The extended traceroute command is a variation of the ordinary traceroute command which allows the default values of the parameters used by the traceroute operation such as TTL and destination port number to be modified. The destination UDP port of the initial datagram probe is set to 33434 (or as specified in the extended traceroute command output). The TTL for the initial User Datagram Protocol (UDP) datagram probe is set to 1 (or the minimum TTL, as specified by user in the extended traceroute command. There are some differences with the way the traceroute command is implemented in the various operating systems this document discusses. This message lets the source know that the packet traverses that particular router as a hop When a router in the middle of the path finds a packet with TTL = 1, it responds with an Internet Control Message Protocol (ICMP) "time exceeded" message to the source. Typically, each router in the path towards the destination decrements the TTL field by one unit while it forwards these packets. If you execute the traceroute ip-addresscommand on a source device (such as a host, or a router acting as a host), it sends IP packets toward the destination with Time To Live (TTL) values that increment up to the maximum specified hop count. Conventionsįor more information on document conventions, refer to the Cisco Technical Tips Conventions. If your network is live, make sure that you understand the potential impact of any command. All of the devices used in this document started with a cleared (default) configuration. The information in this document was created from the devices in a specific lab environment. The information in this document applies to these software and hardware versions:Ĭisco Router that runs Cisco IOS Software Release 12.2(27) Readers of this document should have basic knowledge of one of these operating systems: This utility comes with your host operating system (for example, Linux or Microsoft (MS) Windows), as well as with Cisco IOS® Software. The traceroute command allows you to determine the path a packet takes in order to get to a destination from a given source by returning the sequence of hops the packet has traversed.
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