traceroute from router to pc on router on stick

Unanswered Question
Mar 13th, 2009

Please see the attached diagram from one of cisco traing videos

My question is if we do traceroute from router R1 to user IP 192.168.10.50 it goes to router R2 fa0/0 interface as next hop which has IP 192.168.1.2 but not to users default gateway

fa0/0.10 which is gateway for user 192.168.10.50.

Also if we do traceroute from user PC IP 192.168.10.50 to Router R1 then it shows traceroute as follows

192.168.10.1

192.168.1.1

Here it does not show Router R2 intergace fa0/0 as next hop why?

Can someone explain me this please

thanks

mahesh

I have this problem too.
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Giuseppe Larosa Fri, 03/13/2009 - 12:29

Hello Mahesh,

what is shown derives from the way the traceroute works:

traceroute sends UDP probes to an high UDP port.

First series of probes is sent out with TTL =1

3 probes are sent with TTL=1

the first router in the path to the traceroute destination receives the packet decrements the TTL finds out that the new TTL is zero and the packet cannot be forwarded.

So an ICMP message saying TTL exceeed is sent back to origina traceback sender.

In this scenario the following happens:

udp probes with TTL=1 reach R2 but they cannot go out R2 fas0/0.10

this allows R1 to say that first hop in traceroute is 191.168.1.2

(in sending back the icmp message R2 uses its nearest ip address to traceroute source ip address)

When R1 sends out UDP probes with TTL 2 the following happens:

the packet can be sent out with TTL=1 and reaches the final destination that answers with icmp port unreachable (the high UDP port used is not a well-known service)

Something similar happens if you start the traceroute from PC 192.168.10.50:

first probe hits R2 fas0/0.10 the PC default gateway

second series of probes with TTL=2 arrive at R1 but they cannot go out to final destination .

R1 sends back to 192.168.10.50 an icmp ttl exceed using its nearest ip address that is 192.168.1.1

third series of probes reach the final destination

R2 fas0/0 is the outgoing interface to R1 so it cannot appear in the traceroute for the way it works

Hope to help

Giuseppe

viyuan700 Sat, 03/14/2009 - 12:24

Hi Mahesh,

We dont set IP address for interface if you are using subinterface. for example R2 F0/0 have IP and then subinterface.

Tracert from R1 is right since it is the next hop for R1 but confused with tracert from PC why it is showing 192.168.10.1 as subinterface IP dont work if there is IP for that interface.

As once my setup was not working for subinterface since there was IP set for that interface. Saw similar message few days back where intervlan setup was not working as he was using subinterface and setIP for that interface.

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