RFC 10041 Advertising Unreachable Links in OSPF September 2026
Gong, et al. Standards Track [Page]
Stream:
Internet Engineering Task Force (IETF)
RFC:
10041
Updates:
5443, 6987, 8379, 8770
Category:
Standards Track
Published:
ISSN:
2070-1721
Authors:
L. Gong
China Mobile
W. Cheng
China Mobile
C. Lin
New H3C Technologies
A. Lindem
Arrcus, Inc.
R. Chen
ZTE Corporation

RFC 10041

Advertising Unreachable Links in OSPF

Abstract

OSPF Router Link State Advertisements (LSAs) use fixed-format encodings that always include advertised links in the default Shortest Path First (SPF) computation. For non-default SPF computations, e.g., Flexible Algorithms as described in RFC 9350, advertised OSPF links are used in the default SPF computation even if this is not intended. In order to advertise these links and not use them in the base SPF calculation, the metric LSLinkInfinity (0xffff) is used to specify that the link is unreachable. If all OSPF routers in an OSPF area support this functionality and have advertised the capability via an area-scoped OSPF Router Information LSA, then links advertised with a metric of LSLinkInfinity are considered unreachable.

MaxReachableLinkMetric (0xfffe) is defined to provide backward compatible reachability in specifications that previously specified advertisement of MaxLinkMetric (0xffff). This document updates RFC 5443, RFC 6987, RFC 8379, and RFC 8770 with respect to the advertisement of MaxReachableLinkMetric (0xfffe) rather than MaxLinkMetric (0xffff).

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc10041.

1. Introduction

OSPF Router Link State Advertisements (LSAs) use fixed-format encodings that always include advertised links in the default Shortest Path First (SPF) computation. For example, a link may be required for Traffic Engineering (TE) paths but not intended for hop-by-hop routing. Another example is an OSPF link used exclusively by a Flexible Algorithm [RFC9350] but excluded from the default algorithm.

In order to advertise these links as unreachable, the metric LSLinkInfinity (0xffff) is used to specify that the link is unreachable and OSPF routers supporting this specification will exclude the link from SPF calculations (subject to backward-compatibility, refer to Section 3.2).

Stub Router Advertisement [RFC6987] defines MaxLinkMetric (0xffff) to indicate a router-LSA link should only be used for transit IP traffic as a last resort. When an OSPF router supports the Unreachable Link capability defined in this document, OSPF stub router links are advertised as MaxReachableLinkMetric (0xfffe) rather than MaxLinkMetric (0xffff). This document updates [RFC6987] and [RFC8770] with respect to the advertisement of MaxReachableLinkMetric rather than MaxLinkMetric.

Similarly, Label Distribution Protocol (LDP) IGP Synchronization [RFC5443] specifies OSPF advertisement of MaxLinkMetric (0xffff) to indicate that while the OSPF adjacency is in FULL state, LDP has not been synchronized between the two neighbors and transit traffic is discouraged. This document updates [RFC5443] with respect to the advertisement of MaxReachableLinkMetric rather than MaxLinkMetric.

Finally, OSPF Graceful Link Shutdown [RFC8379] specifies OSPF advertisement of MaxLinkMetric (0xffff) to indicate that the OSPF link will be taken out of service and transit traffic is discouraged. This document updates [RFC8379] with respect to the advertisement of MaxReachableLinkMetric rather than MaxLinkMetric.

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

2. Use Cases

2.1. Case 1: Traffic Engineering

A network topology is shown in Figure 1. The OSPF link between Node A and E is only to be used for Traffic Engineering. Since the OSPF link is advertised by default, it will be included in the base SPF calculation for the default topology and may be used for hop-by-hop routing in the default topology.

    TE Link
   ---------
  /         \
 /           \
A------C------E
|      |      |
|      |      |
|      |      |
B------D------F
Figure 1: Network Topology

2.2. Case 2: Flexible Algorithm

A network topology is shown in Figure 2. The links between Nodes A and B and between C and D are to be used exclusively for a Flex-Algorithm [RFC9350] devoted to specific traffic. These links have an Extended Administrative Group (EAG) [RFC7308] attribute specifying the "Red" color.

 ******
A------C------E
|*     |*     |
|*     |*     |        ******: "Red" link
|*     |*     |
B------D------F
 ******
Figure 2: Network Topology

Flex-Algorithm 128 is enabled on Nodes A, B, C, and D, with an EAG rule including "Red", and the Metric-Type is designated to be a type other than the OSPF metric. OSPF will compute routes for Flex-Algorithm 128 using these links. The topology associated with Flex-Algorithm 128 is shown in Figure 3.

A******C
*      *
*      *
*      *
B******D
Figure 3: Topology of Flex-Algorithm 128

The "Red" links are used by Flex-Algorithm 128 calculation. However, these "Red" links are also included in the default algorithm calculation [RFC9350] since they are reachable. Note that links used by the default algorithm are omitted from Figure 3 for clarity.

If the OSPF metrics for all the "Red" links are advertised as unreachable, they will be excluded from the default SPF calculation as shown in Figure 4. This allows the "Red" links from A to B and C to D to be used exclusively by the Flex-Algorithm 128 calculation.

A------C------E
              |
              |
              |
B------D------F
Figure 4: Base SPF Topology Excluding Unreachable Links

3. LSLinkInfinity-Based Solution

Prior to this specification, OSPF treated links with an advertised metric of LSLinkInfinity as reachable [RFC2328]. Hence, partial deployment of this specification may result in routing loops due to inconsistent interpretation of LSLinkInfinity. For example, in the network shown in Figure 5, link D-F is advertised with LSLinkInfinity (65535/0xffff). Router B supports LSLinkInfinity as unreachable, but router A doesn't. Router A considers link D-F as reachable, and the shortest path to F is A->B->D->F. Router B considers link D-F as unreachable, and the shortest path to F is B->A->C->E->F. As a result, A forwards the packets to B, but B returns them to A, which results in a routing loop.

      40000  40000      Traffic: A->F
    A------C------E       A considers link D-F as reachable
    |             |         A's shortest path to F: A->B->D->F
   5|             |5      B considers link D-F as unreachable
    |             |         B's shortest path to F: B->A->C->E->F
    B------D------F
        5    65535
Figure 5: Inconsistent LSLinkInfinity Interpretation Causing Loops

To provide backward compatibility, this document specifies that routers supporting LSLinkInfinity for unreachable links support advertisement of an area-scoped Router Information LSA with a Router Functional Capabilities TLV [RFC7770] that includes the following Router Functional Capability Bit. The value of the bit is controlled by configuration as specified in Section 4.1.

Table 1
Bit Capabilities
0 Unreachable Link

OSPF routers MUST NOT treat links with an advertised metric of LSLinkInfinity as unreachable unless all routers in the OSPF area, i.e., all routers with router-LSAs in the area Link State Database (LSDB), have advertised this capability in an OSPFv2 Router Information Opaque LSA or an OSPFv3 Router Information LSA. If all OSPF routers in the area have advertised this capability, then links with an advertised metric of LSLinkInfinity MUST be treated as unreachable. When the number of routers in the area not supporting the Unreachable Link capability changes to 0 or from 0 to greater than 0, all OSPF routers in the area MUST recalculate their routes.

In normal operation, it is possible that an area-scoped OSPF Router Information LSA will fail to reach all routers in an area in a timely manner. For example, if a new router not supporting the Unreachable Link capability joins an area that previously only had OSPF routers advertising support of the Unreachable Link capability, then it may take some time for the OSPF Router Information LSA to propagate to all routers. While it is propagating, the routers in the area will gradually detect the presence of a router that does not support the capability and will revert to treating links with MaxLinkMetric as being reachable. During the propagation time, this inconsistency of MaxLinkMetric interpretation between OSPF routers can result in transient routing loops.

3.3. Stub Router Advertisement Backward Compatibility

Stub Router Advertisement [RFC6987] defines MaxLinkMetric (0xffff) to indicate a router-LSA link should not be used for transit traffic.

When an OSPF router supports the Unreachable Link capability defined in this document, the OSPF stub router MaxLinkMetric (0xffff) MUST be updated to MaxReachableLinkMetric (0xfffe). This document updates [RFC6987] and [RFC8770] with respect to the advertisement of MaxReachableLinkMetric rather than MaxLinkMetric.

When an OSPF router supports OSPF Stub Router Advertisement [RFC6987] and the Unreachable Link capability defined in this document, it MUST support advertisement of all its non-stub links with a link cost of MaxReachableLinkMetric (0xfffe). Since MaxLinkMetric will not be used to indicate that a link is unreachable unless all OSPF routers in the area support this specification as specified in Section 3.2, all routers in the area will also support the usage of MaxReachableLinkMetric to discourage the usage of stub router links for transit traffic. If there are any OSPF routers in the area that do not support the Unreachable Link capability, then all OSPF routers in the area will treat links advertised with a cost MaxLinkMetric as reachable (Section 3.2).

3.4. Label Distribution Protocol (LDP) IGP Synchronization Backward Compatibility

LDP IGP Synchronization [RFC5443] specifies OSPF advertisement of MaxLinkMetric (0xffff) to indicate that while the OSPF adjacency is in FULL state, LDP has not been synchronized between the two neighbors and transit IP traffic is discouraged. When an OSPF router supports the Unreachable Link capability defined in this document, the usage of OSPF MaxLinkMetric (0xffff) to discourage usage of the link until LDP is "fully operational" MUST be updated to MaxReachableLinkMetric (0xfffe). It is important to keep the link in the topology to allow IP traffic to use the link as a last resort in case of LDP packets between OSPF router loopbacks addresses or a network failure. This document updates [RFC5443] with respect to the advertisement of MaxReachableLinkMetric rather than MaxLinkMetric.

4. Operational Considerations

4.1. Configuration Parameters

Support of the Unreachable Link capability MUST be configurable. The default MUST be to not advertise the capability, i.e., the functional capability in the area-scoped OSPF Router Information LSA is false.

In some networks, the operator may still want links with maximum metric (0xffff) to be treated as reachable. For example, when the cost of links is automatically computed based on the inverse of the link's bandwidth and there is a mix of low-speed and high-speed links, the computation may result in the maximum metric. Hence, implementations supporting this document and auto-costing MUST limit the maximum computed cost to MaxReachableLinkMetric (0xfffe). An example of auto-costing would be to automatically set the link metric to be inversely proportional to the link bandwidth (refer to the auto-cost feature in the "ietf-ospf" module [RFC9129]).

4.2. YANG Data Model

This section defines three YANG [RFC7950] modules. The "iana-ospf-functional-cap-bits" module defines the identities for OSPF Router Functional Capabilities as per the "OSPF Router Functional Capability Bits" IANA registry [IANA-OSPF-FC-Bits]. The "ietf-ospf-functional-capability" and "ietf-ospf-unreachable-links" modules can be used to configure and manage the usage of OSPF LSLinkInfinity for unreachable links as defined in this document, which augments the OSPF YANG data model [RFC9129] and the YANG data model for Routing Management [RFC8349].

This document uses the graphical representation of data models per [RFC8340].

4.2.1. Tree for OSPF Functional Capability

The following shows the tree diagram of the module for OSPF Functional Capability:

module: ietf-ospf-functional-capability
  augment /rt:routing/rt:control-plane-protocols
            /rt:control-plane-protocol/ospf:ospf/ospf:areas
            /ospf:area/ospf:interfaces/ospf:interface
            /ospf:database/ospf:link-scope-lsa-type
            /ospf:link-scope-lsas/ospf:link-scope-lsa/ospf:version
            /ospf:ospfv2/ospf:ospfv2/ospf:body/ospf:opaque
            /ospf:ri-opaque/ospf:router-capabilities-tlv:
    +--ro router-functional-capabilities
       +--ro functional-capability*   identityref
  augment /rt:routing/rt:control-plane-protocols
            /rt:control-plane-protocol/ospf:ospf/ospf:areas
            /ospf:area/ospf:database/ospf:area-scope-lsa-type
            /ospf:area-scope-lsas/ospf:area-scope-lsa/ospf:version
            /ospf:ospfv2/ospf:ospfv2/ospf:body/ospf:opaque
            /ospf:ri-opaque/ospf:router-capabilities-tlv:
    +--ro router-functional-capabilities
       +--ro functional-capability*   identityref
  augment /rt:routing/rt:control-plane-protocols
            /rt:control-plane-protocol/ospf:ospf/ospf:database
            /ospf:as-scope-lsa-type/ospf:as-scope-lsas
            /ospf:as-scope-lsa/ospf:version/ospf:ospfv2
            /ospf:ospfv2/ospf:body/ospf:opaque/ospf:ri-opaque
            /ospf:router-capabilities-tlv:
    +--ro router-functional-capabilities
       +--ro functional-capability*   identityref
  augment /rt:routing/rt:control-plane-protocols
            /rt:control-plane-protocol/ospf:ospf/ospf:areas
            /ospf:area/ospf:interfaces/ospf:interface
            /ospf:database/ospf:link-scope-lsa-type
            /ospf:link-scope-lsas/ospf:link-scope-lsa/ospf:version
            /ospf:ospfv3/ospf:ospfv3/ospf:body
            /ospf:router-information/ospf:router-capabilities-tlv:
    +--ro router-functional-capabilities
       +--ro functional-capability*   identityref
  augment /rt:routing/rt:control-plane-protocols
            /rt:control-plane-protocol/ospf:ospf/ospf:database
            /ospf:as-scope-lsa-type/ospf:as-scope-lsas
            /ospf:as-scope-lsa/ospf:version/ospf:ospfv3
            /ospf:ospfv3/ospf:body/ospf:router-information
            /ospf:router-capabilities-tlv:
    +--ro router-functional-capabilities
       +--ro functional-capability*   identityref
  augment /rt:routing/rt:control-plane-protocols
            /rt:control-plane-protocol/ospf:ospf/ospf:areas
            /ospf:area/ospf:database/ospf:area-scope-lsa-type
            /ospf:area-scope-lsas/ospf:area-scope-lsa/ospf:version
            /ospf:ospfv3/ospf:ospfv3/ospf:body
            /ospf:router-information/ospf:router-capabilities-tlv:
    +--ro router-functional-capabilities
       +--ro functional-capability*   identityref

4.2.2. Tree for OSPF Advertising Unreachable Links

The following shows the tree diagram of the module for OSPF Advertising Unreachable Links:

module: ietf-ospf-unreachable-links

  augment /rt:routing/rt:control-plane-protocols
            /rt:control-plane-protocol/ospf:ospf/ospf:areas
            /ospf:area:
    +--rw unreachable-link-advertisement
       +--rw enabled?   boolean
  augment /rt:routing/rt:control-plane-protocols
            /rt:control-plane-protocol/ospf:ospf/ospf:areas
            /ospf:area/ospf:interfaces/ospf:interface:
    +--rw advertise-unreachable-link?   boolean

4.2.3. IANA Module for OSPF Functional Capability Bits

IANA has created the "OSPF Router Functional Capability Bits" registry within the "Open Shortest Path First (OSPF) Parameters" registry group to identify OSPF Router Functional Capabilities. This document defines the initial version of the IANA-maintained "iana-ospf-functional-cap-bits" module, which mirrors the IANA "OSPF Router Functional Capability Bits" registry [IANA-OSPF-FC-Bits]. The module will be updated as the "OSPF Router Functional Capability Bits" registry changes. The latest version of this YANG module is available in the "YANG Module Names” registry <https://www.iana.org/assignments/yang-parameters>.

4.2.4. YANG Module for OSPF Functional Capability

The following is the YANG module for OSPF Functional Capability:

<CODE BEGINS> file "ietf-ospf-functional-capability@2026-09-16.yang"

module ietf-ospf-functional-capability {
  yang-version 1.1;
  namespace "urn:ietf:params:xml:ns:yang:"
          + "ietf-ospf-functional-capability";
  prefix ospf-fc;

  import ietf-routing {
    prefix rt;
    reference
      "RFC 8349: A YANG Data Model for Routing Management
                 (NMDA Version)";
  }
  import ietf-ospf {
    prefix ospf;
    reference
      "RFC 9129: YANG Data Model for the OSPF Protocol";
  }
  import iana-ospf-functional-cap-bits {
    prefix iana-ospf-fc-bits;
    reference
      "https://www.iana.org/assignments/ospf-parameters"
    + "#router-functional-capability";
  }

  organization
    "IETF Link State Routing (LSR) Working Group";
  contact
    "WG Web:   <https://datatracker.ietf.org/wg/lsr/>
     WG List:  <mailto:lsr@ietf.org>

     Author:   Yingzhen Qu
               <mailto:yqu@futurewei.com>
     Author:   Acee Lindem
               <mailto:acee.ietf@gmail.com>
     Author:   Liyan Gong
               <mailto:gongliyan@chinamobile.com>
     Author:   Weiqiang Cheng
               <mailto:chengweiqiang@chinamobile.com>
     Author:   Changwang Lin
               <mailto:linchangwang.04414@h3c.com>
     Author:   Ran Chen
               <mailto:chen.ran@zte.com.cn>";
  description
    "This YANG module defines the operational state for
     Functional Capability in OSPF as defined in RFC 7770.

     Copyright (c) 2026 IETF Trust and the persons identified as
     authors of the code.  All rights reserved.
     Redistribution and use in source and binary forms, with or
     without modification, is permitted pursuant to, and subject
     to the license terms contained in, the Revised BSD License
     set forth in Section 4.c of the IETF Trust's Legal Provisions
     Relating to IETF Documents
     (https://trustee.ietf.org/license-info).

     All revisions of IETF and IANA published modules can be found
     at the 'YANG Parameters' registry group
     (https://www.iana.org/assignments/yang-parameters).

     This version of this YANG module is part of RFC 10041; see
     the RFC itself for full legal notices.";

  revision 2026-09-16 {
    description
      "Initial revision.";
    reference
      "RFC 10041: Advertising Unreachable Links in OSPF";
  }

  grouping router-functional-capabilities {
    description
      "Grouping for OSPF router capabilities TLV types.";
    reference
      "RFC 7770: Extensions to OSPF for Advertising Optional
                 Router Capabilities";
    container router-functional-capabilities {
      leaf-list functional-capability {
        type identityref {
          base iana-ospf-fc-bits:functional-capability;
        }
        description
          "List of Router Functional Capabilities.  This list
           contains the identities for functional capabilities
           supported by the router.";
      }
      description
        "OSPF Router Functional identity definitions.";
    }
  }

  augment "/rt:routing/"
        + "rt:control-plane-protocols/rt:control-plane-protocol/"
        + "ospf:ospf/ospf:areas/ospf:area/"
        + "ospf:interfaces/ospf:interface/ospf:database/"
        + "ospf:link-scope-lsa-type/ospf:link-scope-lsas/"
        + "ospf:link-scope-lsa/ospf:version/ospf:ospfv2/"
        + "ospf:ospfv2/ospf:body/ospf:opaque/ospf:ri-opaque/"
        + "ospf:router-capabilities-tlv" {
    when "derived-from-or-self(/rt:routing/"
       + "rt:control-plane-protocols/"
       + "rt:control-plane-protocol/rt:type, 'ospf:ospfv2')" {
      description
        "This augmentation is only valid for OSPFv2.";
    }
    description
      "OSPFv2 Opaque Link-Scoped Router-Information LSA Router
       Functional Capabilities.";
    uses router-functional-capabilities;
    reference
      "RFC 7770: Extensions to OSPF for Advertising Optional
                 Router Capabilities";
  }

  augment "/rt:routing/"
        + "rt:control-plane-protocols/rt:control-plane-protocol/"
        + "ospf:ospf/ospf:areas/"
        + "ospf:area/ospf:database/"
        + "ospf:area-scope-lsa-type/ospf:area-scope-lsas/"
        + "ospf:area-scope-lsa/ospf:version/ospf:ospfv2/"
        + "ospf:ospfv2/ospf:body/ospf:opaque/"
        + "ospf:ri-opaque/ospf:router-capabilities-tlv" {
    when "derived-from-or-self(/rt:routing/"
       + "rt:control-plane-protocols/"
       + "rt:control-plane-protocol/rt:type, 'ospf:ospfv2')" {
      description
        "This augmentation is only valid for OSPFv2.";
    }
    description
      "OSPFv2 Opaque Area-Scoped Router-Information LSA Router
       Functional Capabilities.";
    uses router-functional-capabilities;
    reference
      "RFC 7770: Extensions to OSPF for Advertising Optional
                 Router Capabilities";
  }

  augment "/rt:routing/"
        + "rt:control-plane-protocols/rt:control-plane-protocol/"
        + "ospf:ospf/ospf:database/"
        + "ospf:as-scope-lsa-type/ospf:as-scope-lsas/"
        + "ospf:as-scope-lsa/ospf:version/ospf:ospfv2/"
        + "ospf:ospfv2/ospf:body/ospf:opaque/"
        + "ospf:ri-opaque/ospf:router-capabilities-tlv" {
    when "derived-from-or-self(/rt:routing/"
       + "rt:control-plane-protocols/"
       + "rt:control-plane-protocol/rt:type, 'ospf:ospfv2')" {
      description
        "This augmentation is only valid for OSPFv2.";
    }
    description
      "OSPFv2 Opaque AS-Scoped Router-Information LSA Router
       Functional Capabilities.";
    uses router-functional-capabilities;
    reference
      "RFC 7770: Extensions to OSPF for Advertising Optional
                 Router Capabilities";
  }

  augment "/rt:routing/"
        + "rt:control-plane-protocols/rt:control-plane-protocol/"
        + "ospf:ospf/ospf:areas/ospf:area/"
        + "ospf:interfaces/ospf:interface/ospf:database/"
        + "ospf:link-scope-lsa-type/ospf:link-scope-lsas/"
        + "ospf:link-scope-lsa/ospf:version/ospf:ospfv3/"
        + "ospf:ospfv3/ospf:body/ospf:router-information/"
        + "ospf:router-capabilities-tlv" {
    when "derived-from-or-self(/rt:routing/"
       + "rt:control-plane-protocols/"
       + "rt:control-plane-protocol/rt:type, 'ospf:ospfv3')" {
      description
        "This augmentation is only valid for OSPFv3.";
    }
    description
      "OSPFv3 Link-Scoped Router-Information LSA Router
       Functional Capabilities.";
    uses router-functional-capabilities;
    reference
      "RFC 7770: Extensions to OSPF for Advertising Optional
                 Router Capabilities";
  }

  augment "/rt:routing/"
        + "rt:control-plane-protocols/rt:control-plane-protocol/"
        + "ospf:ospf/ospf:database/"
        + "ospf:as-scope-lsa-type/ospf:as-scope-lsas/"
        + "ospf:as-scope-lsa/ospf:version/ospf:ospfv3/"
        + "ospf:ospfv3/ospf:body/ospf:router-information/"
        + "ospf:router-capabilities-tlv" {
    when "derived-from-or-self(/rt:routing/"
       + "rt:control-plane-protocols/"
       + "rt:control-plane-protocol/rt:type, 'ospf:ospfv3')" {
      description
        "This augmentation is only valid for OSPFv3.";
    }
    description
      "OSPFv3 AS-Scoped Router-Information LSA Router
       Functional Capabilities.";
    uses router-functional-capabilities;
    reference
      "RFC 7770: Extensions to OSPF for Advertising Optional
                 Router Capabilities";
  }

  augment "/rt:routing/"
        + "rt:control-plane-protocols/rt:control-plane-protocol/"
        + "ospf:ospf/ospf:areas/"
        + "ospf:area/ospf:database/"
        + "ospf:area-scope-lsa-type/ospf:area-scope-lsas/"
        + "ospf:area-scope-lsa/ospf:version/ospf:ospfv3/"
        + "ospf:ospfv3/ospf:body/ospf:router-information/"
        + "ospf:router-capabilities-tlv" {
    when "derived-from-or-self(/rt:routing/"
       + "rt:control-plane-protocols/"
       + "rt:control-plane-protocol/rt:type, 'ospf:ospfv3')" {
      description
        "This augmentation is only valid for OSPFv3.";
    }
    description
      "OSPFv3 Area-Scoped Router-Information LSA Router
       Functional Capabilities.";
    uses router-functional-capabilities;
    reference
      "RFC 7770: Extensions to OSPF for Advertising Optional
                 Router Capabilities";
  }
}


<CODE ENDS>

4.2.5. YANG Module for OSPF Advertising Unreachable Links

The following is the YANG module for OSPF Advertising Unreachable Links:

<CODE BEGINS> file "ietf-ospf-unreachable-links@2026-09-16.yang"

module ietf-ospf-unreachable-links {
  yang-version 1.1;
  namespace "urn:ietf:params:xml:ns:yang:"
          + "ietf-ospf-unreachable-links";
  prefix ospf-unreach-link;

  import ietf-routing {
    prefix rt;
    reference
      "RFC 8349: A YANG Data Model for Routing Management
                 (NMDA Version)";
  }
  import ietf-ospf {
    prefix ospf;
    reference
      "RFC 9129: YANG Data Model for the OSPF Protocol";
  }

  organization
    "IETF Link State Routing (LSR) Working Group";
  contact
    "WG Web:   <https://datatracker.ietf.org/wg/lsr/>
     WG List:  <mailto:lsr@ietf.org>

     Author:   Yingzhen Qu
               <mailto:yqu@futurewei.com>
     Author:   Acee Lindem
               <mailto:acee.ietf@gmail.com>
     Author:   Liyan Gong
               <mailto:gongliyan@chinamobile.com>
     Author:   Weiqiang Cheng
               <mailto:chengweiqiang@chinamobile.com>
     Author:   Changwang Lin
               <mailto:linchangwang.04414@h3c.com>
     Author:   Ran Chen
               <mailto:chen.ran@zte.com.cn>";
  description
    "This YANG module defines the configuration and operational
     state for Advertising Unreachable Links in OSPF as defined
     in RFC 10041.

     Copyright (c) 2026 IETF Trust and the persons identified as
     authors of the code.  All rights reserved.

     Redistribution and use in source and binary forms, with or
     without modification, is permitted pursuant to, and subject
     to the license terms contained in, the Revised BSD License
     set forth in Section 4.c of the IETF Trust's Legal Provisions
     Relating to IETF Documents
     (https://trustee.ietf.org/license-info).

     All revisions of IETF and IANA published modules can be found
     at the 'YANG Parameters' registry group
     (https://www.iana.org/assignments/yang-parameters).

     This version of this YANG module is part of RFC 10041; see
     the RFC itself for full legal notices.";

  revision 2026-09-16 {
    description
      "Initial revision.";
    reference
      "RFC 10041: Advertising Unreachable Links in OSPF";
  }

  augment "/rt:routing/rt:control-plane-protocols/"
        + "rt:control-plane-protocol/ospf:ospf/"
        + "ospf:areas/ospf:area" {
    when
      "derived-from-or-self(../../../rt:type, 'ospf:ospfv2') or "
    + "derived-from-or-self(../../../rt:type, 'ospf:ospfv3')" {
      description
        "This augments the OSPF routing protocol when used.";
    }
    description
      "This augments the OSPF protocol with Unreachable Link
       advertisement.";
    container unreachable-link-advertisement {
      leaf enabled {
        type boolean;
        default "false";
        description
          "Controls the interpretation of MaxLinkMetric as
           unreachable and the advertisement of the Unreachable
           Link capability for the area.  It is enabled when set
           to true and disabled when set to false.";
      }
      description
        "OSPF Unreachable Link advertisement parameters.";
    }
  }

  augment "/rt:routing/rt:control-plane-protocols/"
        + "rt:control-plane-protocol/ospf:ospf/"
        + "ospf:areas/ospf:area/ospf:interfaces/ospf:interface" {
    when "derived-from(/rt:routing/rt:control-plane-protocols/"
       + "rt:control-plane-protocol/rt:type, 'ospf:ospfv2') or "
       + "derived-from(/rt:routing/rt:control-plane-protocols/"
       + "rt:control-plane-protocol/rt:type, 'ospf:ospfv3')" {
      description
        "This augments the OSPF interfaces.";
    }
    leaf advertise-unreachable-link {
      type boolean;
      must "not(../advertise-unreachable-link = 'true' and "
         + "/rt:routing/rt:control-plane-protocols/"
         + "rt:control-plane-protocol/ospf:ospf/ospf:areas/"
         + "ospf:area/"
         + "ospf-unreach-link:unreachable-link-advertisement/"
         + "ospf-unreach-link:enabled = 'false')" {
        error-message
          "The interface cannot be advertised "
        + "as unreachable (true) unless the "
        + "unreachable-link-advertisement capability is "
        + "enabled (true).";
        description
          "Ensures an interface isn't explicitly advertised as
           unreachable unless the OSPF area level capability is
           enabled.";
      }
      default "false";
      description
        "Specifies that the link should be advertised with a metric
         of MaxLinkMetric (0xffff) in the default topology.";
    }
    description
      "Augment OSPF interfaces with an option to advertise the link
       as unreachable.";
  }
}

<CODE ENDS>

5. Security Considerations

A compromised OSPF router could advertise changes to its Unreachable Link capability rapidly resulting in repeated route recalculations on routers in the area supporting this specification (Section 3.2). Hence, it is RECOMMENDED that routers supporting this specification also support the SPF back-off delay algorithm described in [RFC8405].

The security considerations for [RFC2328], [RFC5340], [RFC6987], and [RFC7770] are also applicable to this protocol extension.

The "ietf-ospf-unreachable-links" YANG module and the "ietf-ospf-functional-capability" YANG module each define a data model that is designed to be accessed via YANG-based management protocols, such as NETCONF [RFC6241] and RESTCONF [RFC8040]. These YANG-based management protocols (1) have to use a secure transport layer (e.g., SSH [RFC4252], TLS [RFC9846], and QUIC [RFC9000]) and (2) have to use mutual authentication.

The Network Configuration Access Control Model (NACM) [RFC8341] provides the means to restrict access for particular NETCONF or RESTCONF users to a pre-configured subset of all available NETCONF or RESTCONF protocol operations and content.

There are a number of data nodes defined in the "ietf-ospf-unreachable-links" YANG module that are writable/creatable/deletable (i.e., "config true", which is the default). All writable data nodes are likely to be sensitive or vulnerable in some network environments. Write operations (e.g., edit-config) and delete operations to these data nodes without proper protection or authentication can have a negative effect on network operations. The modification of these data nodes without proper protection can prevent interpretation of the OSPF LSLinkInfinity metric as unreachable or result in a link being advertised as unreachable.

/ospf:ospf/ospf:areas/ospf:area/ospf-unreach-link:unreachable-link-advertisement/ospf-unreach-link:enabled

/ospf:ospf/ospf:areas/ospf:area/ospf:interfaces/ospf:interface/ospf-unreach-link:advertise-unreachable-link

Some of the readable data nodes in the "ietf-ospf-unreachable-links" YANG module may be considered sensitive or vulnerable in some network environments. Exposure of the OSPF unreachable link configuration may be useful in mounting Denial-of-Service (DoS) attacks. These are the readable data nodes:

/ospf:ospf/ospf:areas/ospf:area/ospf-unreach-link:unreachable-link-advertisement/ospf-unreach-link:enabled

/ospf:ospf/ospf:areas/ospf:area/ospf:interfaces/ospf:interface/ospf-unreach-link:advertise-unreachable-link

6. IANA Considerations

6.1. Registering OSPF Router Functional Capability Bits

This document defines a new bit in the "OSPF Router Functional Capability Bits" registry <https://www.iana.org/assignments/ospf-parameters>:

Table 2
Bit Number Capability Name Reference
0 Unreachable Link RFC 10041

6.2. Registering YANG Modules

IANA has registered the following three new URIs in the "ns" registry within the "IETF XML Registry" [RFC3688]:

URI:
urn:ietf:params:xml:ns:yang:iana-ospf-functional-cap-bits
Registrant Contact:
The IESG.
XML:
N/A; the requested URI is an XML namespace.
URI:
urn:ietf:params:xml:ns:yang:ietf-ospf-functional-capability
Registrant Contact:
The IESG.
XML:
N/A; the requested URI is an XML namespace.
URI:
urn:ietf:params:xml:ns:yang:ietf-ospf-unreachable-links
Registrant Contact:
The IESG.
XML:
N/A; the requested URI is an XML namespace.

IANA has registered the following three new YANG module names in the "YANG Module Names" registry [RFC6020] within the "YANG Parameters" registry group:

Name:
iana-ospf-functional-cap-bits
Maintained by IANA?
Y
Namespace:
urn:ietf:params:xml:ns:yang:iana-ospf-functional-cap-bits
Prefix:
iana-ospf-fc-bits
Reference:
RFC 10041
Name:
ietf-ospf-functional-capability
Maintained by IANA?
N
Namespace:
urn:ietf:params:xml:ns:yang:ietf-ospf-functional-capability
Prefix:
ospf-fc
Reference:
RFC 10041
Name:
ietf-ospf-unreachable-links
Maintained by IANA?
N
Namespace:
urn:ietf:params:xml:ns:yang:ietf-ospf-unreachable-links
Prefix:
ospf-unreach-link
Reference:
RFC 10041

6.3. IANA Module for OSPF Functional Capability Bits

This document defines the initial version of the IANA-maintained "iana-ospf-functional-cap-bits" YANG module (Section 4.2). The most recent version of the YANG module is available in the "YANG Parameters" registry [IANA-YANG-Parameters].

IANA has added the following note to the registry:

New values must not be directly added to the "iana-ospf-functional-cap-bits" YANG module. They must instead be added to the "OSPF Router Functional Capability Bits" registry in the "Open Shortest Path First (OSPF) Parameters" registry group [IANA-OSPF-FC-Bits].

When a value is added to the "OSPF Router Functional Capability Bits" registry, a new "identity" statement needs to be added to the "iana-ospf-functional-cap-bits" YANG module. The name of the "identity" is the lowercase name provided in the registry with all spaces replaced with "-". The "identity" statement should have the following sub-statements defined:

"base":
Contains 'functional-capability'.
"description":
Contains the non-abbreviated OSPF capability bit name from the registry.
"reference":
Replicates the reference(s) from the registry with the title of the document(s) added.

IANA has added this note to [IANA-OSPF-FC-Bits]:

When this registry is modified, the YANG module "iana-ospf-functional-cap-bits" must be updated as defined in RFC 10041.

7. References

7.1. Normative References

[IANA-OSPF-FC-Bits]
IANA, "OSPF Router Functional Capability Bits", <https://www.iana.org/assignments/ospf-parameters>.
[IANA-YANG-Parameters]
IANA, "YANG Module Names", <https://www.iana.org/assignments/yang-parameters>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[RFC2328]
Moy, J., "OSPF Version 2", STD 54, RFC 2328, DOI 10.17487/RFC2328, , <https://www.rfc-editor.org/info/rfc2328>.
[RFC3688]
Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, DOI 10.17487/RFC3688, , <https://www.rfc-editor.org/info/rfc3688>.
[RFC4915]
Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P. Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF", RFC 4915, DOI 10.17487/RFC4915, , <https://www.rfc-editor.org/info/rfc4915>.
[RFC5340]
Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF for IPv6", RFC 5340, DOI 10.17487/RFC5340, , <https://www.rfc-editor.org/info/rfc5340>.
[RFC5443]
Jork, M., Atlas, A., and L. Fang, "LDP IGP Synchronization", RFC 5443, DOI 10.17487/RFC5443, , <https://www.rfc-editor.org/info/rfc5443>.
[RFC6020]
Bjorklund, M., Ed., "YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)", RFC 6020, DOI 10.17487/RFC6020, , <https://www.rfc-editor.org/info/rfc6020>.
[RFC6987]
Retana, A., Nguyen, L., Zinin, A., White, R., and D. McPherson, "OSPF Stub Router Advertisement", RFC 6987, DOI 10.17487/RFC6987, , <https://www.rfc-editor.org/info/rfc6987>.
[RFC7770]
Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and S. Shaffer, "Extensions to OSPF for Advertising Optional Router Capabilities", RFC 7770, DOI 10.17487/RFC7770, , <https://www.rfc-editor.org/info/rfc7770>.
[RFC7950]
Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language", RFC 7950, DOI 10.17487/RFC7950, , <https://www.rfc-editor.org/info/rfc7950>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
[RFC8341]
Bierman, A. and M. Bjorklund, "Network Configuration Access Control Model", STD 91, RFC 8341, DOI 10.17487/RFC8341, , <https://www.rfc-editor.org/info/rfc8341>.
[RFC8349]
Lhotka, L., Lindem, A., and Y. Qu, "A YANG Data Model for Routing Management (NMDA Version)", RFC 8349, DOI 10.17487/RFC8349, , <https://www.rfc-editor.org/info/rfc8349>.
[RFC8362]
Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and F. Baker, "OSPFv3 Link State Advertisement (LSA) Extensibility", RFC 8362, DOI 10.17487/RFC8362, , <https://www.rfc-editor.org/info/rfc8362>.
[RFC8379]
Hegde, S., Sarkar, P., Gredler, H., Nanduri, M., and L. Jalil, "OSPF Graceful Link Shutdown", RFC 8379, DOI 10.17487/RFC8379, , <https://www.rfc-editor.org/info/rfc8379>.
[RFC8405]
Decraene, B., Litkowski, S., Gredler, H., Lindem, A., Francois, P., and C. Bowers, "Shortest Path First (SPF) Back-Off Delay Algorithm for Link-State IGPs", RFC 8405, DOI 10.17487/RFC8405, , <https://www.rfc-editor.org/info/rfc8405>.
[RFC8770]
Patel, K., Pillay-Esnault, P., Bhardwaj, M., and S. Bayraktar, "Host Router Support for OSPFv2", RFC 8770, DOI 10.17487/RFC8770, , <https://www.rfc-editor.org/info/rfc8770>.
[RFC9129]
Yeung, D., Qu, Y., Zhang, Z., Chen, I., and A. Lindem, "YANG Data Model for the OSPF Protocol", RFC 9129, DOI 10.17487/RFC9129, , <https://www.rfc-editor.org/info/rfc9129>.
[RFC9350]
Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K., and A. Gulko, "IGP Flexible Algorithm", RFC 9350, DOI 10.17487/RFC9350, , <https://www.rfc-editor.org/info/rfc9350>.

7.2. Informative References

[RFC4252]
Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Authentication Protocol", RFC 4252, DOI 10.17487/RFC4252, , <https://www.rfc-editor.org/info/rfc4252>.
[RFC6241]
Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed., and A. Bierman, Ed., "Network Configuration Protocol (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, , <https://www.rfc-editor.org/info/rfc6241>.
[RFC7308]
Osborne, E., "Extended Administrative Groups in MPLS Traffic Engineering (MPLS-TE)", RFC 7308, DOI 10.17487/RFC7308, , <https://www.rfc-editor.org/info/rfc7308>.
[RFC8040]
Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF Protocol", RFC 8040, DOI 10.17487/RFC8040, , <https://www.rfc-editor.org/info/rfc8040>.
[RFC8340]
Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams", BCP 215, RFC 8340, DOI 10.17487/RFC8340, , <https://www.rfc-editor.org/info/rfc8340>.
[RFC9000]
Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based Multiplexed and Secure Transport", RFC 9000, DOI 10.17487/RFC9000, , <https://www.rfc-editor.org/info/rfc9000>.
[RFC9846]
Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 9846, DOI 10.17487/RFC9846, , <https://www.rfc-editor.org/info/rfc9846>.

Acknowledgments

Thanks to Yingzhen Qu for providing the YANG data models.

Thanks to Dhruv Dhody for OPS Directorate review and comments.

Thanks to Gunter Van de Velde for review and comments.

Thanks to Mohamed Boucadair for review and comments.

Thanks to Mike Bishop, Mahesh Jethanadani, and Ketan Taulaulikar for review and comments.

Contributors

The following individuals have contributed to this document:

Mengxiao Chen
New H3C Technologies
China
Yanrong Liang
Ruijie Networks Co., Ltd.
China

Authors' Addresses

Liyan Gong
China Mobile
China
Weiqiang Cheng
China Mobile
China
Changwang Lin
New H3C Technologies
China
Acee Lindem
Arrcus, Inc.
United States of America
Ran Chen
ZTE Corporation
China