OSPF Graceful Link Shutdown
Extracted elements (25)
An explicit Graceful-Link-Shutdown sub-TLV indication is needed for controllers rather than relying solely on metric increases, because metric increases can occur for other reasons such as LDP-IGP synchronization and do not unambiguously signal maintenance activity.
Increasing the metric on only one side of a link is insufficient to divert traffic in both directions; the Graceful-Link-Shutdown sub-TLV allows the remote node to also raise its metric, providing bidirectional traffic diversion while still permitting last-resort use of the link.
OSPFv3 does not require a separate remote IPv6 address sub-TLV because the OSPFv3 Router-Link TLV (RFC 8362) already contains the Interface ID and neighbor Interface ID, which uniquely identify the connecting interface on the remote side.
The Extended Link Opaque LSAs and Extended Link TLV are not scoped for multi-topology (RFC 4915); in multi-topology deployments, the Graceful-Link-Shutdown sub-TLV corresponds to all topologies including the link, and receivers SHOULD change the metric in the reverse direction for all affected topologies.
The mechanism is fully backward compatible. If the remote node does not support Graceful-Link-Shutdown, it will continue to use the graceful-shutdown link without adverse effects; for broadcast links using two-part metrics, backward-compatibility procedures from RFC 8042 apply.
For broadcast links, the node originating the Graceful-Link-Shutdown sub-TLV MUST set the metric in the Network-to-Router Metric sub-TLV to MaxLinkMetric (0xffff) for OSPFv2 and OSPFv3 and reoriginate the corresponding LSAs.
For hybrid broadcast and P2MP interfaces receiving a Graceful-Link-Shutdown sub-TLV, the remote node MUST identify the neighbor corresponding to the graceful-shutdown link and set its metric to MaxLinkMetric; all remote nodes connected to the originator MUST reoriginate the Router-LSA.
For point-to-multipoint links, when a Graceful-Link-Shutdown sub-TLV is received, the remote node MUST identify the neighbor corresponding to the graceful-shutdown link, set its metric to MaxLinkMetric (0xffff), and MUST reoriginate the Router-LSA.
For point-to-point links, the node initiating graceful shutdown MUST set the metric to MaxLinkMetric (0xffff) and reoriginate its Router-LSA; the Traffic Engineering metric SHOULD be set to 0xffffffff.
For unnumbered interfaces with multiple parallel links between two nodes, the Local/Remote Interface ID sub-TLV MUST be advertised to distinguish the interfaces.
The node that has the link to be taken out of service MUST advertise the Graceful-Link-Shutdown sub-TLV in the Extended Link TLV of the Extended Link Opaque LSA for OSPFv2 and in the Router-Link TLV of E-Router-LSA for OSPFv3.
When a Graceful-Link-Shutdown sub-TLV is received for a point-to-point link, the remote node MUST identify the corresponding local link, set its metric to MaxLinkMetric (0xffff), and MUST reoriginate its Router-LSA with the changed metric.
When a link is ready to carry traffic, the Graceful-Link-Shutdown sub-TLV MUST be removed from the Extended Link TLV/Router-Link TLV and the corresponding LSAs MUST be readvertised; similarly, the metric MUST be set to original values.
The BGP-LS Graceful-Link-Shutdown TLV (Type 1121, Length 0) is a new link attribute TLV for BGP Link NLRI that distributes graceful-link-shutdown state to external controllers via BGP-LS.
The Local/Remote Interface ID sub-TLV (Type 9, Length 8) carries 4 octets of Local Interface ID followed by 4 octets of Remote Interface ID, used to identify parallel unnumbered interfaces between two nodes.
The OSPFv2 Graceful-Link-Shutdown sub-TLV (Type 7, Length 0) is advertised in the Extended Link TLV of the Extended Link Opaque LSA to identify a link as being gracefully shutdown.
The OSPFv3 Graceful-Link-Shutdown sub-TLV (Type 8, Length 0) is carried in the Router-Link TLV as defined in RFC 8362; the Router-Link TLV already contains Neighbor Interface ID enabling unique link identification on the remote node.
The Remote IPv4 Address sub-TLV (Type 8, Length 4) is an optional sub-TLV advertised in the Extended Link TLV to identify the remote endpoint when multiple parallel links exist between two nodes.
IANA registered three values in the 'OSPFv2 Extended Link TLV Sub-TLVs' registry: 7 (Graceful-Link-Shutdown Sub-TLV), 8 (Remote IPv4 Address Sub-TLV), and 9 (Local/Remote Interface ID Sub-TLV).
IANA registered value 1121 in the 'BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs' registry for the Graceful-Link-Shutdown TLV.
IANA registered value 8 in the 'OSPFv3 Extended-LSA Sub-TLVs' registry for the Graceful-Link-Shutdown sub-TLV.
This document uses OSPF packets and LSAs from RFC 2328, RFC 3630, RFC 5329, and RFC 5340; the authentication procedures from RFC 2328 for OSPFv2 and RFC 4552 for OSPFv3 are applicable and no further security issues beyond those in RFC 2328 and RFC 5340 are introduced.
The Local/Remote Interface ID sub-TLV is encoded as Type (2 octets, value 9), Length (2 octets, value 8), a 4-octet Local Interface ID, and a 4-octet Remote Interface ID.
The OSPFv2 Graceful-Link-Shutdown sub-TLV is encoded as a 2-octet Type field (value 7) and a 2-octet Length field (value 0), with no Value field.
The Remote IPv4 Address sub-TLV is encoded as Type (2 octets, value 8), Length (2 octets, value 4), and a 4-octet Remote IPv4 Address value field.