Help me make sense of these traceroutes please

Sam Moats sam at
Wed Dec 25 00:03:02 UTC 2013

On 2013-12-24 18:55, Jeroen Massar wrote:
> On 2013-12-25 00:16, Sam Moats wrote:
>> Hello Nanog community,
>> I would like to enlist your help with understanding this latency I'm
>> seeing.
> You are likely seeing the effects of asymmetric routing.

That's what I was thinking to.
> [..]
>> Tracing route to [193.1.x.x]
> by chance? :)

Yes they were the owners of the IP I used for the example case and the 
heanet folks are actually totally awesome :-)

> Though you could use for instance:
> to do a reverse traceroute, do make sure you force your connectivity 
> to
> IPv4 as that host will do IPv6 too. (locally nullrouting the 
> destination
> /128 is the trick I use for 'disabling' IPv6 temporarily).
> Otherwise the HEANET folks are extremely helpful and clued in, you 
> can
> always ask them for help with issues. It is the end-of-year though 
> and
> those Irish folks have lots of really good whiskey, Guinness thus you
> might have to be patient till the new year.

Also you'd be amazed how many network issues can be solved with a bunch 
of IT folks and an ample supply of Guinness

> Alternatively, you could use a tool like 'tracepath' or 'mtr' as 
> those
> reports multiple answers to a response and also check for the TTL on 
> the
> return packets.
> Greets,
>  Jeroen

Thanks, this isn't affecting my service now I've worked around it so 
it's more a curiosity than anything. It seems really odd to me that the 
same L3 edge router would route the ICMP unreachable back to me via 
different paths based on the final destination IP of the of the ICMP 
echo packet.

Well its Christmas eve here and the customers are happy so Guinness 
seems like the best approach now :-)

Thanks and have a good Holiday,
Sam Moats

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