Single IP routing problems through Level3

Rubens Kuhl Jr. rubensk at
Sun Jun 15 14:12:25 UTC 2008

1) I've seen this behavior before; you are not alone in the universe.
2) Most likely there is a balanced channel on the path, either L3 or
L2, and one of the links in the bundle is dead but has not been
detected as such.


On Sun, Jun 15, 2008 at 11:01 AM, Matt Palmer <mpalmer at> 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 (we don't have all of it, just most of it),
> while the far end is
> - Matt

More information about the NANOG mailing list