Graceful shutdown provides a method to bulk re-route transit LSPs away from the node during software upgrade of a node. A solution is described in RFC 5817, Graceful Shutdown in MPLS and Generalized MPLS Traffic Engineering Networks. This is achieved in this RFC by using a PathErr message with a specific error code Local Maintenance on TE link required flag. When a LER gets this message, it performs a make-before-break on the LSP path to move the LSP away from the links/nodes which IP addresses were indicated in the PathErr message.
Graceful shutdown can flag the affected link/node resources in the TE database so other routers signal LSPs using the affected resources only as a last resort. This is achieved by flooding an IGP TE LSA/LSP containing link TLV for the links under graceful shutdown with the TE metric set to 0xffffffff and 0 as unreserved bandwidth.