[c-nsp] SXI3 strange issue, Loose mode uRPF jumps to strict by itself

[bas]

Hi All,

Yesterday we had a strange issue.
Our monitoring tool alerted that one of our boxes (SUP720-3BXL - 6506
running SXI3) became unreachable.

When we logged in everything looked ok.
BGP was up, OSPF was up and nothing special in logging.
Still traffic had dropped to near zero.

With "debug ip cef drop" we immediately saw that traffic was dropped
due to uRPF feature.
All upstream interfaces had strict mode uRPF configured, before the
problems started it was loose mode uRPF.

After manually changing them back too loose mode traffic was restored.

A couple of minutes before the problems started an engineer had
configured a customer facing interface with strict mode uRPF.
Apparently this configuration changed triggered a bug that caused
upstream interface loose mode to be automagically turned to strict
mode.

So, hereby a heads up. If your SXI3 boxes show strange behavior,
quickly check uRPF.

Cya,

Bas