OSPF Routing Bit
The routing bit is an internal implementation variable that is used only by Cisco. It is not part of the official definition of OSPF. It is a bit that is set on an LSA only if the LSA should be considered during the SPF calculation. The bit is set only after the LSA has passed all necessary checks. This makes it more efficient so that these checks don’t have to be performed over and over. If the routing bit is set, the router knows that the LSA is eligible for processing. Note that this is a bit in the options field of the LSA and is not propagated from router to router.