0

In traceroute,

The sender expects a reply within a specified number of seconds. If a packet is not acknowledged within the expected interval, an asterisk is displayed.

Pasting here the example trace route output from linked Wikipedia page:

$ traceroute -w 3 -q 1 -m 16 example.com
traceroute to example.com (93.184.216.34), 16 hops max, 52 byte packets
 1  192.x.x.x (192.x.x.x)  5.152 ms
 2  10.x.x.x (10.x.x.x)  12.767 ms
 3  172.x.x.x (172.x.x.x)  11.638 ms
 4  172.x.x.x (172.x.x.x)  13.193 ms
 5  xxx.x.x.x.cox.net (68.x.x.x)  20.624 ms
 6  xxx.xxx.xxx.edgecastcdn.net (192.x.x.x)  56.205 ms
 7  xxx.xxx.xxx.edgecastcdn.net (192.x.x.x)  24.573 ms
 8  *
 9  *
10  93.x.x.x (93.x.x.x)  22.810 ms
11  93.x.x.x (93.x.x.x)  20.235 ms

On line 8 and 9 (TTLs 8 and 9) it shows asterisks where the router did not respond within the timeout.

Now I have a question on this and about trace route. Routers 8 and 9 did not respond within the timeout that's why we see asterisks. But still, packets are able reach the destination because we have lines 10 and 11. Is it because routers 8 and 9 still forward packets even though they do not respond? If not, then how are packets able to reach routers 10 and 11?

1 Answer 1

4

Answering ICMP requests sent to the router is a function typically performed by a router's management plane. Replying to ICMP (and at what rate) is optional.

Forwarding packets is done by the forwarding plane, and unrelated to what happens to packets sent to the router itself.

So what you're seeing is nothing out of the ordinary, the routers of hops 8 and 9 just have ICMP filtered on their management plane.

6
  • 1
    "the routers of hops 8 and 9 just have ICMP filtered on their management plane. Or they are just too busy forwarding packets to respond in time...
    – Ron Maupin
    Jan 8 at 0:36
  • ok so bottomline, they still forward those packets, correct?
    – Noob_Guy
    Jan 8 at 11:31
  • yes, since the traceroute continues after those hops. the bottomline is: if a router is replying to pings it unrelated to if it's forwarding packets.
    – Teun Vink
    Jan 9 at 6:24
  • ok thank you...
    – Noob_Guy
    Jan 9 at 15:54
  • wait just one more question please.. replying to icmp is optional, what about forwarding packets? can it be optional too?
    – Noob_Guy
    Jan 9 at 15:57

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.