comcast routing issue question

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1

Question: What could cause the first trace below to succeed, but the
second trace to fail?

$ mtr 69.61.40.35
HOST: blue Loss% Snt Last Avg Best Wrst
  1. 192.168.3.1 0.0% 1 4.3 4.3 4.3 4.3
  2. 73.62.48.1 0.0% 1 10.6 10.6 10.6 10.6
  3. 68.86.108.25 0.0% 1 11.4 11.4 11.4 11.4
  4. 68.86.106.54 0.0% 1 9.8 9.8 9.8 9.8
  5. 68.86.106.9 0.0% 1 20.5 20.5 20.5 20.5
  6. 68.86.90.121 0.0% 1 11.3 11.3 11.3 11.3
  7. 68.86.84.70 0.0% 1 27.7 27.7 27.7 27.7
  8. 64.213.76.77 0.0% 1 24.5 24.5 24.5 24.5
  9. 208.50.254.150 0.0% 1 39.4 39.4 39.4 39.4
10. 208.49.83.237 0.0% 1 46.6 46.6 46.6 46.6
11. 208.49.83.234 0.0% 1 40.7 40.7 40.7 40.7
12. 69.61.40.35 0.0% 1 43.9 43.9 43.9 43.9

$ mtr 69.61.40.34
HOST: blue Loss% Snt Last Avg Best Wrst
  1. 192.168.3.1 0.0% 1 1.1 1.1 1.1 1.1
  2. 73.62.48.1 0.0% 1 9.9 9.9 9.9 9.9
  3. 68.86.108.25 0.0% 1 9.3 9.3 9.3 9.3
  4. 68.86.106.54 0.0% 1 9.6 9.6 9.6 9.6
  5. 68.86.106.9 0.0% 1 9.0 9.0 9.0 9.0
  6. 68.86.90.121 0.0% 1 18.2 18.2 18.2 18.2
  7. 68.86.84.70 0.0% 1 23.9 23.9 23.9 23.9
  8. ??? 100.0 1 0.0 0.0 0.0 0.0

Taking the 69.61.40.33/28 subnet a bit further, .36 drops at 68.86.84.70
but .37 - .39 make it. .40 drops at 68.86.84.70, but .41 makes it.

Crazy.

-Jim P.

2

Btw, the problem has now been resolved, however I'm still curious as to
what scenario could have caused that.

-Jim P.

3

> Question: What could cause the first trace below to succeed, but the
> second trace to fail?
>

[snip]

> Taking the 69.61.40.33/28 subnet a bit further, .36 drops at 68.86.84.70
> but .37 - .39 make it. .40 drops at 68.86.84.70, but .41 makes it.
>
> Crazy.

Btw, the problem has now been resolved, however I'm still curious as to
what scenario could have caused that.

Perhaps CEF-style load balancing over multiple paths, with one of them
down and not properly failing over? Per-flow balancing would decide
which path to use based on source & target IP.

4

You're not the only one who noticed this.

http://www.dslreports.com/forum/remark,17368208

5

Comcast broke themselves doing a maintenance..

6

Not that its probably it; but i've seen some crappy etherchannel links
with IP path selection exhibit this whacky behaviour.

Adrian

7

eBGP multihop peering across a pair of 10 gigE links with
static routes pointing to the remote router loopback; one
link goes south, but the interface still shows as up/up,
and voila, depending upon the hash, your packets may
go across the good link, or they may disappear into the
black hole of oblivion.

This is why multipath is a good thing, and eBGP multihop
with static routes is a Bad Thing(tm).

Matt