RFC 10041
Advertising Unreachable Links in OSPF
RFC 10041 is a Proposed Standard published in September 2026 in the IETF stream by the lsr working group.
Proposed Standard IETF stream lsr WG Routing
- Updates
- RFC 5443, RFC 6987, RFC 8379, RFC 8770
- Citations
- Cites 27 RFCs
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).
Plain text 51 KB · rfc-editor.org · Datatracker · doi:10.17487/RFC10041 · draft-ietf-lsr-ospf-ls-link-infinity-25
Internet Engineering Task Force (IETF) L. Gong
Request for Comments: 10041 W. Cheng
Updates: 5443, 6987, 8379, 8770 China Mobile
Category: Standards Track C. Lin
ISSN: 2070-1721 New H3C Technologies
A. Lindem
Arrcus, Inc.
R. Chen
ZTE Corporation
September 2026
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.
Copyright Notice
Copyright (c) 2026 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Requirements Language
2. Use Cases
2.1. Case 1: Traffic Engineering
2.2. Case 2: Flexible Algorithm
3. LSLinkInfinity-Based Solution
3.1. Unreachable Link Advertisement
3.2. Unreachable Link Backward Compatibility
3.3. Stub Router Advertisement Backward Compatibility
3.4. Label Distribution Protocol (LDP) IGP Synchronization
Backward Compatibility
3.5. OSPF Graceful Link Shutdown Backward Compatibility
4. Operational Considerations
4.1. Configuration Parameters
4.2. YANG Data Model
4.2.1. Tree for OSPF Functional Capability
4.2.2. Tree for OSPF Advertising Unreachable Links
4.2.3. IANA Module for OSPF Functional Capability Bits
4.2.4. YANG Module for OSPF Functional Capability
4.2.5. YANG Module for OSPF Advertising Unreachable Links
5. Security Considerations
6. IANA Considerations
6.1. Registering OSPF Router Functional Capability Bits
6.2. Registering YANG Modules
6.3. IANA Module for OSPF Functional Capability Bits
7. References
7.1. Normative References
7.2. Informative References
Acknowledgments
Contributors
Authors' Addresses
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
3.1. Unreachable Link Advertisement
This document specifies that if the OSPF metric of a link is
advertised as LSLinkInfinity (0xffff), it MUST NOT be considered
during the associated SPF computation. This applies to both the
Flex-Algorithm SPF [RFC9350] and the base SPF as long as
LSLinkInfinity is specified for the OSPF metric.
While the interpretation of LSLinkInfinity is only required in the
base topology as other topologies are optional [RFC4915], OSPF
routers supporting this specification MUST consistently interpret
LSLinkInfinity as unreachable during the associated SPF computation.
Interpretation of LSLinkInfinity as unreachable is also applicable to
Flex-Algorithm SPF computations [RFC9350] that use the OSPF link
metric.
An OSPF metric with LSLinkInfinity indicating a link is unreachable
is applicable to the following TLVs/LSAs:
* The Router-LSA [RFC2328] [RFC5340]
* The Router-Link TLV of OSPFv3 E-Router-LSA [RFC8362]
3.2. Unreachable Link Backward Compatibility
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.
+=====+==================+
| Bit | Capabilities |
+=====+==================+
| 0 | Unreachable Link |
+-----+------------------+
Table 1
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.
3.5. OSPF Graceful Link Shutdown Backward Compatibility
OSPF Graceful Link [RFC8379] specifies OSPF advertisement of
MaxLinkMetric (0xffff) to indicate that the link is going to be taken
down and transit 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 a
link prior to it being taken out of service 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
prior to planned shutdown. This document updates [RFC8379] 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>:
+============+==================+===========+
| Bit Number | Capability Name | Reference |
+============+==================+===========+
| 0 | Unreachable Link | RFC 10041 |
+------------+------------------+-----------+
Table 2
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, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
DOI 10.17487/RFC3688, January 2004,
<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, June 2007,
<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, July 2008,
<https://www.rfc-editor.org/info/rfc5340>.
[RFC5443] Jork, M., Atlas, A., and L. Fang, "LDP IGP
Synchronization", RFC 5443, DOI 10.17487/RFC5443, March
2009, <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, October 2010,
<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, September 2013,
<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,
February 2016, <https://www.rfc-editor.org/info/rfc7770>.
[RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, DOI 10.17487/RFC7950, August 2016,
<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,
May 2017, <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, March 2018,
<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, March 2018,
<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, April
2018, <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, May 2018,
<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, June 2018,
<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, April 2020,
<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, October 2022,
<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, February 2023,
<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,
January 2006, <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, June 2011,
<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, July 2014,
<https://www.rfc-editor.org/info/rfc7308>.
[RFC8040] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
<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, March 2018,
<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, May 2021,
<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, July 2026,
<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
Email: chen.mengxiao@h3c.com
Yanrong Liang
Ruijie Networks Co., Ltd.
China
Email: liangyanrong@ruijie.com.cn
Authors' Addresses
Liyan Gong
China Mobile
China
Email: gongliyan@chinamobile.com
Weiqiang Cheng
China Mobile
China
Email: chengweiqiang@chinamobile.com
Changwang Lin
New H3C Technologies
China
Email: linchangwang.04414@h3c.com
Acee Lindem
Arrcus, Inc.
United States of America
Email: acee.ietf@gmail.com
Ran Chen
ZTE Corporation
China
Email: chen.ran@zte.com.cn
Contents 34
- Abstract
- Status of This Memo
- Copyright Notice
- Table of Contents
- 1 Introduction
- 1.1 Requirements Language
- 2 Use Cases
- 2.1 Case 1: Traffic Engineering
- 2.2 Case 2: Flexible Algorithm
- 3 LSLinkInfinity-Based Solution
- 3.1 Unreachable Link Advertisement
- 3.2 Unreachable Link Backward Compatibility
- 3.3 Stub Router Advertisement Backward Compatibility
- 3.4 Label Distribution Protocol (LDP) IGP Synchronization Backward
- 3.5 OSPF Graceful Link Shutdown Backward Compatibility
- 4 Operational Considerations
- 4.1 Configuration Parameters
- 4.2 YANG Data Model
- 4.2.1 Tree for OSPF Functional Capability
- 4.2.2 Tree for OSPF Advertising Unreachable Links
- 4.2.3 IANA Module for OSPF Functional Capability Bits
- 4.2.4 YANG Module for OSPF Functional Capability
- 4.2.5 YANG Module for OSPF Advertising Unreachable Links
- 5 Security Considerations
- 6 IANA Considerations
- 6.1 Registering OSPF Router Functional Capability Bits
- 6.2 Registering YANG Modules
- 6.3 IANA Module for OSPF Functional Capability Bits
- 7 References
- 7.1 Normative References
- 7.2 Informative References
- Acknowledgments
- Contributors
- Authors' Addresses