[nSLUG] eastlink packets going in circles!?!?

Robert McKay robert at mckay.com
Tue Jul 26 19:43:03 ADT 2011

On Tue, Jul 26, 2011 at 11:13 PM, George N. White III <gnwiii at gmail.com>wrote:

> traceroute to mirror.its.dal.ca (, 30 hops max, 60 byte
> packets
>  1 (  0.847 ms  2.166 ms  2.489 ms
>  2  blk-11-32-1.eastlink.ca (  30.005 ms  30.016 ms  30.229 ms
>  3  hlfx-asr1.eastlink.ca (  12.320 ms  19.991 ms  20.620
> ms
>  4  ns-hlfx-asr002.ns.eastlink.ca (  20.306 ms  21.017
> ms  21.210 ms
>  5  ns-hlfx-asr001.ns.eastlink.ca (  21.688 ms  21.921
> ms  22.181 ms
>  6  ns-hlfx-asr002.ns.eastlink.ca (  21.334 ms  10.367
> ms  12.395 ms
>  7  ns-hlfx-asr001.ns.eastlink.ca (  12.677 ms  13.781

> [snip]

Dalhousie has internet connections via both eastlink and Internetworking
Atlantic Inc. What could be happening is that the two eastlink routers have
a different setup and therefore make different decisions about which path to
use and each one seeing a more preferable path via the other.

The specific route that mirror.its.dal.ca is on is being advertised with
three prepends on the eastlink path. This is a traffic engineering scheme
that is designed to make less traffic choose the eastlink path, forcing more
inbound traffic to go via the internetworking atlantic path.

There isn't anything wrong with Dal doing this, but it is different than
Dal's other routes ( for example) which is not similarly
prepended and eastlink seems to be able to connect just fine to hosts in
those other Dal networks.. so it may be that the two eastlink routers are
handling the prepending differently, causing a path preference mismatch with
both routers preferring to send to the other one.

Ultimately it's something that eastlink will have to look at.

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