looping traceroutes

while playing around with traceroutes for a measurement study of bgp, i
often come across the traceroute outputs similar to those appended below.
i don't have a reasonable explanation of such behaviour (i don't think
these are routing loops; they are not transient in any case). i am hoping
someone on the list has a hang of traceroute implementations, and provide
insights into under what circumstances this might happen.

  -- ratul

the most interesting thing is that the rtt increases..

and it does so by regular amounts, about 30ms in each hop

i'd guess there is a routing loop going on there for sure, i'd also guess
its between two routers that are 30ms apart!

perhaps it could be being caused by some sort of nat on the router so
there are actually two routers looping but one or both of the addresses
get nat'd as the replies come to you