using Overload Bit to avoid BGP black hole when router reboots

From RFC3787

4.  Overload Bit

  To deal with transient problems that prevent an IS from storing all
  the LSPs it receives, ISO 10589 defines an LSP Database Overload
  condition in section 7.3.19.  When an IS is in Database Overload
  condition, it sets a flag called the Overload Bit in the non-
  pseudonode LSP number Zero that it generates.  Section 7.2.8.1 of ISO
  10589 instructs other systems not to use the overloaded IS as a
  transit router.  Since the overloaded IS does not have complete
  information, it may not be able to compute the right routes, and
  routing loops could develop.  However, an overloaded router may be
  used to reach End Systems directly attached to the router, as it may
  provide the only path to an End System.

  The ability to signal reduced knowledge is so useful that the meaning
  of this flag has been overloaded.  In a Service Provider's network,
  when a router running BGP and IS-IS reboots, BGP might take more time
  to converge than IS-IS.  Thus the router may drop traffic for
  destinations not yet learned via BGP.  It is convenient to set the
  Overload Bit until BGP has converged, as described in "Intermediate
  System to Intermediate System (IS-IS) Transient Blackhole Avoidance"
  [6].

  An implementation SHOULD use the Overload Bit to signal that it is
  not ready to accept transit traffic.


router isis
set-overload-bit on-startup wait-for-bgp