overload bit

A flag in an IS-IS router's LSP that tells other routers not to use it for transit, while still reaching it as a destination.

ISO 10589 defined it for a router whose link-state database had overflowed. Operators now also set it on purpose for a set time after a reboot, so traffic avoids a router whose BGP routes have not yet converged.

As RFC 3787 restates ISO 10589, a router in the LSP Database Overload condition sets the bit in its non-pseudonode LSP number zero, and other systems must not use it as a transit router. It may still be used to reach systems directly attached to it, since it may be their only path. In FRRouting's show isis database the bit is the OL in the ATT/P/OL column.

Its meaning has since been, in RFC 3787's word, overloaded. When a router running BGP and IS-IS reboots, IS-IS converges first and the router attracts traffic it cannot yet deliver, discarding packets for destinations learned only through BGP. RFC 3277 describes that transient black hole and the cure, setting the bit until BGP has converged, a mechanism it says had been deployed in several large IS-IS networks for years. FRRouting offers set-overload-bit, and set-overload-bit on-startup with a duration in seconds.

OSPF reaches the same end differently: under RFC 6987, a router advertises its transit links at the maximum metric, so others avoid paths through it while still reaching its own networks.

Also known as: ol bit, set-overload-bit, lsp database overload, is-is overload bit

Sources

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