Which statement about the overload bit in IS-IS is true?
A.
The IS-IS adjacencies on the links for which the overload bit is set are brought down.
B.
Routers running SPF ignore LSPs with the overload bit set and hence avoid blackholing traffic.
C.
A router setting the overload bit becomes unreachable to all other routers in the IS-IS area.
D.
The overload bit in IS-IS is used only for external prefixes.
The OL bit is used to prevent unintentional blackholing of packets in BGP transit networks. Due to
the nature of these protocols, IS-IS and OSPF converge must faster than BGP. Thus there is a
possibility that while the IGP has converged, IBGP is still learning the routes. In that case if other
IBGP routers start sending traffic towards this IBGP router that has not yet completely converged it
will start dropping traffic. This is because it isnt yet aware of the complete BGP routes. OL bit
comes handy in such situations. When a new IBGP neighbor is added or a router restarts, the ISIS
OL bit is set. Since directly connected (including loopbacks) addresses on an “overloaded”
router are considered by other routers, IBGP can be bought up and can begin exchanging routes.
Other routers will not use this router for transit traffic and will route the packets out through an
alternate path. Once BGP has converged, the OL bit is cleared and this router can begin
forwarding transit traffic.
Reference. https://routingfreak.wordpress.com/category/ospf-vs-is-is/
In Cisco implementation, when overload bit set is set, the directly connected interfaces/IP prefixes are reachable.
Setting the overload bit for a fixed amount of time right after a reload ensures that the router does not receive transit traffic while the routing protocol is still converging.
http://www.cisco.com/c/en/us/support/docs/ip/integrated-intermediate-system-to-intermediate-system-is-is/24509-set-overload-bit.html