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  1. Hi Rene,
    Good stuff as always.
    I have a questions regarding Probe keyword. How router calculating 3 rtt. Could you Pls make it clear once again.Thx

  2. Hello Mohammad

    The probe keyword in Cisco IOS just tells the router how many packets to send. A probe is essentially one ICMP packet. The router responds with three lines as shown below:

    R1#traceroute probe 1
    Type escape sequence to abort.
    Tracing the route to
    VRF info: (vrf in name/id, vrf out name/id)
      1 0 msec
      2 4 msec
      3 0 msec

    There are three lines because traceroute provides a response from each router that is traversed.

    I hope this has been helpful!


  3. Hi Laz ,
    Still I am confused regarding Probe 1 Keyword that from the capture we saw sender sending 3 ICMP request packet (for Windows) while we used probe 1 keyword .So what is the meaning your sentence “The probe keyword in Cisco IOS just tells the router how many packets to send” . Actually I want to know how the 3 rtt value coming …

    1. 1 ms 1 ms <1 ms

  4. Hi team,

    Can you please explain why we send incremental port number (dst port ) in the UDP traceroute ?


  5. Hello Sameer.

    Looking over the cloudshark capture, it simply looks like the UDP port 33437 at the destination device of is just not listening. You get a Type 3 Code 3 result which means, according to RFC 792, that the UDP port is blocked, possibly by a firewall.


    This drop of the packet is different than the drop of the packet that occurs due to TTL going to 0. In such a case, you have different Type and Code numbers such as is the case when the

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