Even more interesting, I think, is the fact that the first example (with the same repeating host address) increases RTT while the second (with the alternating address) does not. It seems counter-intuitive based on the alternating versus non-alternating host addresses, but it may be that the first is a route loop, while the second is actually the filter, as described by Patrick, but between two load-balancing interfaces. As for the route-loop (?) with the same repeating address... maybe the addresses are NATed? - Jeb -----Original Message----- From: owner-nanog@merit.edu [mailto:owner-nanog@merit.edu]On Behalf Of Stephen J. Wilcox Sent: Wednesday, September 19, 2001 5:16 AM To: Ratul Mahajan Cc: nanog@merit.edu Subject: Re: looping traceroutes 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 Steve On Tue, 18 Sep 2001, Ratul Mahajan wrote:

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.

thanks, -- ratul

--------------

$ traceroute 200.13.39.1 traceroute to 200.13.39.1 (200.13.39.1), 30 hops max, 38 byte packets 1 regina-GE1-2.cac.washington.edu (128.95.219.100) 1.098 ms 1.233 ms 2 uwbr2-GE3-0.cac.washington.edu (140.142.153.24) 0.349 ms 0.377 ms 3 cnsp1-wes-GE1-0.pnw-gigapop.net (198.107.150.1) 1.561 ms 1.005 ms 4 so-1-2-2.a07.sttlwa01.us.ra.verio.net (204.203.3.1) 21.690 ms 6.076 5 ge-6-2-0.r03.sttlwa01.us.bb.verio.net (129.250.28.1) 8.481 ms 8.756 6 p4-7-2-0.r06.plalca01.us.bb.verio.net (129.250.3.138) 29.934 ms 7 p16-3-0-0.r00.plalca01.us.bb.verio.net (129.250.2.162) 26.740 ms 8 p4-0.francetelecom.plalca01.us.bb.verio.net (129.250.9.134) 23.058 ms 9 P4-3.DALBB1.Dallas.opentransit.net (193.251.132.221) 84.523 ms 10 P5-1.FWHBB2.Fort-worth.opentransit.net (193.251.132.213) 86.653 ms 11 Sr9-0-0.FWHAR1.Fort-worth.opentransit.net (193.251.132.227) 88.507 ms 12 193.251.133.117 (193.251.133.117) 115.472 ms * 116.554 ms 13 193.251.133.117 (193.251.133.117) 145.386 ms 147.224 ms 148.333 ms 14 193.251.133.117 (193.251.133.117) 178.751 ms 172.224 ms * 15 193.251.133.117 (193.251.133.117) 208.640 ms 203.841 ms 199.946 ms 16 193.251.133.117 (193.251.133.117) 231.618 ms 233.234 ms 229.844 ms 17 193.251.133.117 (193.251.133.117) 259.227 ms 258.488 ms 262.034 ms 18 193.251.133.117 (193.251.133.117) 288.661 ms 292.598 ms 288.129 ms 19 193.251.133.117 (193.251.133.117) 321.982 ms 320.806 ms 318.364 ms 20 193.251.133.117 (193.251.133.117) 350.245 ms 349.834 ms 540.883 ms 21 193.251.133.117 (193.251.133.117) 539.971 ms 410.838 ms 419.743 ms 22 193.251.133.117 (193.251.133.117) 468.291 ms 409.761 ms 406.256 ms 23 193.251.133.117 (193.251.133.117) 432.990 ms 439.589 ms 434.184 ms 24 * 193.251.133.117 (193.251.133.117) 466.399 ms 465.287 ms 25 193.251.133.117 (193.251.133.117) 500.719 ms 498.601 ms 499.957 ms 26 193.251.133.117 (193.251.133.117) 530.089 ms 527.642 ms 528.999 ms 27 193.251.133.117 (193.251.133.117) 556.208 ms 555.552 ms 557.387 ms 28 193.251.133.117 (193.251.133.117) 586.776 ms 584.171 ms 586.944 ms 29 193.251.133.117 (193.251.133.117) 608.677 ms 610.789 ms 618.232 ms 30 193.251.133.117 (193.251.133.117) 641.141 ms 643.149 ms 639.050 ms

----------

$ traceroute 198.146.21.26 traceroute to 198.146.21.26 (198.146.21.26), 30 hops max, 38 byte packets 1 regina-GE1-2.cac.washington.edu (128.95.219.100) 4.395 ms 0.986 ms 2 uwbr2-GE2-0.cac.washington.edu (140.142.155.24) 0.326 ms 0.310 ms 3 cnsp1-wes-GE1-0.pnw-gigapop.net (198.107.150.1) 1.305 ms 1.113 ms 4 so-1-2-2.a07.sttlwa01.us.ra.verio.net (204.203.3.1) 4.219 ms 4.308 5 ge-6-2-0.r03.sttlwa01.us.bb.verio.net (129.250.28.1) 4.096 ms 6.462 6 p4-5-0-0.r06.plalca01.us.bb.verio.net (129.250.3.89) 25.469 ms 7 p4-0-0.r05.plalca01.us.bb.verio.net (129.250.2.129) 23.593 ms 22.371 8 svl-brdr-01.inet.qwest.net (205.171.4.45) 24.093 ms 25.967 ms 9 svl-core-03.inet.qwest.net (205.171.14.109) 27.487 ms 22.669 ms 10 svl-core-01.inet.qwest.net (205.171.14.121) 22.500 ms 24.719 ms 11 chi-core-03.inet.qwest.net (205.171.8.50) 62.208 ms 55.153 ms 12 chi-core-01.inet.qwest.net (205.171.20.173) 57.742 ms 54.672 ms 13 chi-edge-01.inet.qwest.net (205.171.20.34) 54.261 ms 58.971 ms 14 208.46.63.202 (208.46.63.202) 88.583 ms 69.619 ms 67.605 ms 15 208.63.128.3 (208.63.128.3) 70.551 ms 67.581 ms 69.375 ms 16 208.63.128.1 (208.63.128.1) 65.115 ms 70.951 ms 66.083 ms 17 208.63.128.3 (208.63.128.3) 67.628 ms 65.488 ms 68.418 ms 18 208.63.128.1 (208.63.128.1) 72.197 ms 71.422 ms 69.517 ms 19 208.63.128.3 (208.63.128.3) 68.539 ms 68.976 ms 67.911 ms 20 208.63.128.1 (208.63.128.1) 72.373 ms 67.838 ms 66.468 ms 21 208.63.128.3 (208.63.128.3) 67.040 ms 69.136 ms 67.631 ms 22 208.63.128.1 (208.63.128.1) 66.317 ms 83.947 ms 70.693 ms 23 208.63.128.3 (208.63.128.3) 66.865 ms 69.521 ms 69.760 ms 24 208.63.128.1 (208.63.128.1) 69.604 ms 67.534 ms 70.447 ms 25 208.63.128.3 (208.63.128.3) 68.285 ms 69.772 ms 67.721 ms 26 208.63.128.1 (208.63.128.1) 71.915 ms 69.530 ms 72.403 ms 27 208.63.128.3 (208.63.128.3) 111.475 ms 69.670 ms 69.267 ms 28 208.63.128.1 (208.63.128.1) 68.883 ms 67.147 ms 72.106 ms 29 208.63.128.3 (208.63.128.3) 69.842 ms 67.889 ms 66.944 ms 30 208.63.128.1 (208.63.128.1) 70.986 ms 73.124 ms 68.452 ms

-- Stephen J. Wilcox IP Services Manager, Opal Telecom http://www.opaltelecom.co.uk/ Tel: 0161 222 2000 Fax: 0161 222 2008