Explanation for strange traceroute output.

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Jan 14th, 2010
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Hi,


I've done a traceroute and I get the following output:


traceroute 10.9.64.51


Type escape sequence to abort.
Tracing the route to 10.9.64.51


  1 10.10.51.70 0 msec
    10.10.51.145 0 msec
    10.10.51.70 4 msec
  2 10.10.51.73 104 msec 104 msec 100 msec
  3 10.9.64.51 104 msec 100 msec 104 msec


My question is why does hop 1 have three hops? Can anyone suggest a reason.


Thanks

Dan

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Jon Marshall Thu, 01/14/2010 - 09:06
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dan_track wrote:


Hi,


I've done a traceroute and I get the following output:


traceroute 10.9.64.51


Type escape sequence to abort.
Tracing the route to 10.9.64.51


  1 10.10.51.70 0 msec
    10.10.51.145 0 msec
    10.10.51.70 4 msec
  2 10.10.51.73 104 msec 104 msec 100 msec
  3 10.9.64.51 104 msec 100 msec 104 msec


My question is why does hop 1 have three hops? Can anyone suggest a reason.


Thanks

Dan


Dan


Because you have 2 equal cost paths to the same destination on that router. So if you do a "sh ip route 10.9.64.51" on the router you should see 2 routes, one with the next-hop of 10.10.51.70 and one with the next-hop of 10.10.51.145.


Jon

dan_track Thu, 01/14/2010 - 09:18
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Jon,


Ahhh ic. a few more questions then,


1) I'm using eigrp how is that I have two routes installed in my routing table, don't I have to activate "variance" before it's used?


2) Since there are two paths with equal costs then why are there three ip's showing up? and how does it decide which of the two ip's it will use to send the data through?


Thanks again

Dan

Jon Marshall Thu, 01/14/2010 - 09:25
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dan_track wrote:


Jon,


Ahhh ic. a few more questions then,


1) I'm using eigrp how is that I have two routes installed in my routing table, don't I have to activate "variance" before it's used?


2) Since there are two paths with equal costs then why are there three ip's showing up? and how does it decide which of the two ip's it will use to send the data through?


Thanks again

Dan


Dan


1) variance is only used for unequal cost paths. If the paths are equal cost they will both be installed in the routing table.


2) As Milan alluded to, it depends on what load-balancing you have ie. per-packet or per-destination (the default). The router simply alternates between the 2 routes either on per-packet or per-destination.


Jon

milan.kulik Thu, 01/14/2010 - 09:21
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Hi,


just for a curiosity:

It would be interesting to compare with a traceroute ouput to the same destination started on a PC withing the LAN connected to the first router.


If I remember my CEF readings correctly, per-session load balancing is applied by default.

But per-packet always on packets sent from a router itself!

So you should see just one hop (either 10.10.51.70 or 10.10.51.145) in the traceroute output when running a traceroute from a PC to the same destination.


BR,

Milan

dan_track Fri, 01/15/2010 - 04:07
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Hi,


Here's the output from a pc connected to the switch:


tracert -d 10.9.64.51


Tracing route to 10.9.64.51 over a maximum of 30 hops


  1    <1 ms    <1 ms    <1 ms  10.45.64.254
  2    <1 ms    <1 ms    <1 ms  10.45.66.253
  3     1 ms    <1 ms    <1 ms  10.10.51.145
  4   102 ms   102 ms   102 ms  10.10.51.73
  5   102 ms   102 ms   102 ms  10.9.64.51


If I keep repeating it I keep seeing the following: 10.10.51.145. Is this to do with Jon's suggestion about per-packet or per-destination? Are there any documents about this feature? Is this feature related to CEF?


Thanks

Dan

milan.kulik Fri, 01/15/2010 - 04:27
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