North American Network Operators Group
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Re: Single IP routing problems through Level3

  • From: Tim Peiffer
  • Date: Sun Jun 15 12:34:52 2008

Matt Palmer wrote: 
We're seeing some really weird issues with connections that go through / to
Level3 IP space.  Basically, certain "pairs" of IPs (particular L3 IPs
coupled with particular IPs of ours) have dodgy/nonexistent connectivity,
but if you change the IP at either end everything's hunky dory.

I've sniffed (from both ends) pings going from a host in L3 space to our end
and seen the pings arrive at our end and head back in the direction of L3,
but they never get to their destination.  Traceroutes from L3 stop at the
next-to-last hop, while traceroutes back get to the hop before L3 space and
stop.

All of this behaviour is source/dest *pair* specific -- if I ping/traceroute
from another address (in the same netblock as the problematic IP, so all the
same equipment is involved) at either end, or to another address (again,
same netblock) at either end, it all works again.

I've got two questions:

1) Has anyone else seen similar behaviour from L3 (or other providers,
even), so I know I'm not going mad?
2) What sort of configuration problem or software bug would cause this sort
of problem to occur? If it was an IP blacklist (or even a block routing
issue) anywhere along the line, surely it wouldn't be sensitive to
changing the other end's address to another one in the same /24?

Any insight/anecdotes/etc would be greatly appreciated, as it's starting to
do my head in.  Just knowing I'm not alone with this insanity would be nice
at this point.  <grin>

If it makes any difference, the blocks I'm working from at my end are
Internap, in 74.201.254.0/23 (we don't have all of it, just most of it),
while the far end is 8.12.35.0/24.

- Matt

We commonly see this sort of problem with Layer2 or Layer3 bonded etherchannel (LACP also). One member of the channel is failing for one reason or another and dropping traffic. The channel is really not a load balance mechanism, but is a frame distribution mechanism. The distribution of frames uses the source and destination IP addresses to hash out to a particular channel member, and that distribution provides a rough balance. The problems noted affect traffic in one direction differently as it is likely assymetric across the channel. Only traffic across the ailing member will be impacted.

The above can present itself anywhere in the path if channeling is used.


Regards,
Tim Peiffer
Networking and Telecommunications Services
University of Minnesota/NorthernLights GigaPOP