[c-nsp] OSPF neighbor up on only one end

[Rick Ernst]

I ran into a situation where members of an OSPF area all saw a (pair of)
neighbors up, but those routers didn't show any of the other area members
up.  Unable to ping to the weird routers.

Several Cisco 7500s and Cat5500/multi-RSM routers in the area.  VLAN 2 and
3 (on different physical links and intervening switches) on all devices
for redundancy.  Link state up on all interfaces.  The two RSMs  (the
"weird" routers) in one 5500 chassis did not show any VLAN 3 neighbors
other than within the same chassis.  All other routers see each other and
the weird routers.

I moved the fiber uplink on the weird 5500 chassis to the other port on
the same Sup-III (standby Supervisor) and everything came up.

Any idea on the cause?  More importantly, how was an OSPF adjaceny able to
come up unidirectionally?

Thanks!