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Tracert routing loop?

Posted on 2010-08-31
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Last Modified: 2012-05-10
Our T1 went down.  Doing a tracert from a remote location to the gateway of the problem network provides the following.

Tracing route to 216.174.x.x over a maximum of 30 hops

  1    <1 ms    <1 ms    <1 ms  10.200.x.x
  2    38 ms    40 ms    38 ms  68.178.x.x
  3    39 ms    39 ms    39 ms  ge2-24--3.cr01.sttlwatw.integra.net [209.63.115.
1]
  4    41 ms    42 ms    40 ms  ge7-0-0.ar01.evrtwakq.integra.net [209.63.99.98]

  5    63 ms    56 ms    61 ms  mu3.bothwaxb.integra.net [209.63.99.118]
  6    55 ms    51 ms    56 ms  mu3.ar01.evrtwakq.integra.net [209.63.99.117]
  7    61 ms    71 ms    79 ms  mu3.bothwaxb.integra.net [209.63.99.118]
  8    68 ms    59 ms    83 ms  mu3.ar01.evrtwakq.integra.net [209.63.99.117]
  9    89 ms    92 ms    74 ms  mu3.bothwaxb.integra.net [209.63.99.118]
 10   118 ms    98 ms    72 ms  mu3.ar01.evrtwakq.integra.net [209.63.99.117]
 11    89 ms    86 ms   100 ms  mu3.bothwaxb.integra.net [209.63.99.118]
 12    95 ms    97 ms    85 ms  mu3.ar01.evrtwakq.integra.net [209.63.99.117]
 13   179 ms    84 ms    91 ms  mu3.bothwaxb.integra.net [209.63.99.118]
 14   100 ms   117 ms   111 ms  mu3.ar01.evrtwakq.integra.net [209.63.99.117]
 15   143 ms   125 ms   112 ms  mu3.bothwaxb.integra.net [209.63.99.118]
 16   115 ms   106 ms   121 ms  mu3.ar01.evrtwakq.integra.net [209.63.99.117]
 17   220 ms   113 ms   139 ms  mu3.bothwaxb.integra.net [209.63.99.118]
 18   140 ms   143 ms   120 ms  mu3.ar01.evrtwakq.integra.net [209.63.99.117]
 19   156 ms   122 ms   208 ms  mu3.bothwaxb.integra.net [209.63.99.118]
 20   160 ms   121 ms   157 ms  mu3.ar01.evrtwakq.integra.net [209.63.99.117]
 21   270 ms   157 ms   129 ms  mu3.bothwaxb.integra.net [209.63.99.118]
 22   133 ms   143 ms   230 ms  mu3.ar01.evrtwakq.integra.net [209.63.99.117]
 23   162 ms   123 ms   126 ms  mu3.bothwaxb.integra.net [209.63.99.118]
 24   142 ms   136 ms   125 ms  mu3.ar01.evrtwakq.integra.net [209.63.99.117]
 25   164 ms   217 ms   180 ms  mu3.bothwaxb.integra.net [209.63.99.118]
 26   176 ms   155 ms   186 ms  mu3.ar01.evrtwakq.integra.net [209.63.99.117]
 27   126 ms   163 ms   196 ms  mu3.bothwaxb.integra.net [209.63.99.118]
 28   136 ms   143 ms   251 ms  mu3.ar01.evrtwakq.integra.net [209.63.99.117]
 29   201 ms   167 ms   176 ms  mu3.bothwaxb.integra.net [209.63.99.118]
 30   157 ms   188 ms   146 ms  mu3.ar01.evrtwakq.integra.net [209.63.99.117]

Trace complete.

Would this be considered a routing loop, or could it be caused by the gateway router failing?
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Question by:B1izzard
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by:rfc1180
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>Would this be considered a routing loop, or could it be caused by the gateway router failing?
This is a routing loop, and caused because the T1 went down. There is probably some static and dynamic routing occurring based on how the carrier had the T1 configured for IP services. Once your T1 comes back up, you should see normal traceroute results.

Billy
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by:Kvistofta
Kvistofta earned 125 total points
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This is definately a routing loop. The only reason that your tracert stops at 30 hops is that this is the timeout, without that your packer would loop even more times.

/Kvistofta
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by:davorin
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I agree with previous posts. I wanted only to add that it wold be good to inform your ISP about this - if routers *.integra.net belongs to your ISP.
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by:B1izzard
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Just curious what's happening under the hood.   It appears to me that normally the last router in the mix would be  

5    63 ms    56 ms    61 ms  mu3.bothwaxb.integra.net [209.63.99.118]

at which point it would go to the customers router.  Since it times out trying to connect to the customer router, what does it do at this point?  Does it look in it's routing table for a backup route, then send it there, which happens to be the 209.63.99.118?  Then it times-out at 209.63.99.117, looks up a backup route a finds the 209.63.99.118?    

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by:davorin
davorin earned 125 total points
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Yes, it could look for backup route. Or routing table on one of two routers (117, 118) is wrong.
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rfc1180 earned 250 total points
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>Just curious what's happening under the hood
no telling

One possibility:

Assumptions: T1 WAN IP: 216.174.200.100
209.63.99.117 and 209.63.99.118  are neighbors (Assuming a /30)

209.63.99.117 is more than likely an aggregation router (based on the name: ar01) for customer T1s 209.63.99.117 could be advertising a summary route (Say: 216.174.200.0/23)
209.63.99.118 is more than likely a path to the default route and is being used
209.63.99.117 more than likely does not have a null0 route for the summary network
Because the network 216.174.200.100 is not directly connected (Because the T1 is down), there is no route for 216.174.200.100 so it sends the route to the next hop which is more than likely 209.63.99.118 but 209.63.99.118 has a match for 216.174.200.100 via the summary route (216.174.200.0/230 from 209.63.99.117, and there you have a routing loop.

Billy




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by:B1izzard
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That helps to clarify this.  Thanks for all your responses.
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by:B1izzard
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It's now back up.  This is what it should have looked like.  

Tracing route to 216.174.x.x over a maximum of 30 hops

  1    <1 ms    <1 ms    <1 ms  10.200.x.x
  2    38 ms    38 ms    38 ms  68.178.x.x
  3    39 ms    42 ms    39 ms  ge2-24--3.cr01.sttlwatw.integra.net [209.63.115.
1]
  4    41 ms    40 ms    41 ms  ge7-0-0.ar01.evrtwakq.integra.net [209.63.99.98]

  5    43 ms    43 ms    44 ms  mu3.bothwaxb.integra.net [209.63.99.118]
  6    51 ms    51 ms    55 ms  216.174.213.217
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