RFC 9917 PROPOSED STANDARD ROUTING · IGP · FLEX-ALGORITHM
Flex-Algo rev. affinity: IGP Flexible Algorithms Reverse Affinity Constraint
Lets Flex-Algorithm prune links using the Admin Groups set on the reverse direction of a link.
RFC EDITOR
Psenak, P., Horn, J., and A. Dhamija, "IGP Flexible Algorithms Reverse Affinity Constraint", RFC 9917, DOI 10.17487/RFC9917, January 2026, <https://www.rfc-editor.org/info/rfc9917>.
BIBTEX
@misc{rfc9917,
series = {Request for Comments},
number = 9917,
howpublished = {RFC 9917},
publisher = {RFC Editor},
doi = {10.17487/RFC9917},
url = {https://www.rfc-editor.org/info/rfc9917},
author = {P. Psenak and J. Horn and A. Dhamija},
title = {{IGP Flexible Algorithms Reverse Affinity Constraint}},
pagetotal = 12,
year = 2026,
month = jan,
} LINEAGE · FROM THE RFC INDEXOpen full graph →
RFC text
Contents
Internet Engineering Task Force (IETF) P. Psenak
Request for Comments: 9917 J. Horn
Updates: 9350, 9843 Cisco Systems
Category: Standards Track A. Dhamija
ISSN: 2070-1721 Arrcus
January 2026
IGP Flexible Algorithms Reverse Affinity Constraint
Abstract
An IGP Flexible Algorithm (Flex-Algorithm) enables the computation of
constraint-based paths within an IGP domain, allowing operators to
influence path selection according to administrative policies. This
document defines an extension to Flex-Algorithm that allows the
inclusion or exclusion of links from path computation based on
Administrative Groups (also known as link affinities) associated with
the reverse direction of the path under computation.
This document updates RFCs 9350 and 9843 by introducing the new IANA
registry that specifies the ordered set of rules that are used to
prune links from the topology during the Flex-Algorithm path
computation.
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/rfc9917.
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
2. Requirements Language
3. Use Case Example
4. Deployment Consideration
5. IS-IS Flexible Algorithm Exclude Reverse Admin Group Sub-TLV
6. IS-IS Flexible Algorithm Include-Any Reverse Admin Group
Sub-TLV
7. IS-IS Flexible Algorithm Include-All Reverse Admin Group
Sub-TLV
8. OSPF Flexible Algorithm Exclude Reverse Admin Group Sub-TLV
9. OSPF Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV
10. OSPF Flexible Algorithm Include-All Reverse Admin Group Sub-TLV
11. Calculation of Flexible Algorithm Paths
12. IANA Considerations
12.1. Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV
12.2. OSPF Flexible Algorithm Definition TLV Sub-TLV Registry
12.3. IGP Flex-Algorithm Path Computation Rules Registry
12.3.1. Guidance for Designated Experts
13. Security Considerations
14. References
14.1. Normative References
14.2. Informative References
Authors' Addresses
1. Introduction
IGP protocols historically computed the best paths over the network
solely based on the IGP metric assigned to the links. An IGP Flex-
Algorithm as specified in [RFC9350] allows IGPs to compute
constraint-based paths. Several mechanisms to include or exclude the
link during the Flex-Algorithm path calculation have been defined
already:
* link inclusion or exclusion based on the presence of a specific
Admin Group(s) [RFC9350]
* link exclusion based on the presence of a specific Shared Risk
Link Group (SRLG) [RFC9350]
* link exclusion based on minimum bandwidth [RFC9843]
* link exclusion based on maximum delay [RFC9843]
This document extends IGP Flex-Algorithm with additional constraints
for inclusion or exclusion of links in the path based on Admin Groups
associated with the reverse direction of the Shortest Path First
(SPF) computation.
This document updates [RFC9350] and [RFC9843] by creating the new
IANA registry that specifies the ordered set of rules that are used
to prune links from the topology during the Flex-Algorithm path
computation (refer to Section 12.3.1).
The term OSPF in this document is used to cover both OSPFv2 and
OSPFv3 protocols.
2. 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.
3. Use Case Example
The Flexible Algorithm Definition (FAD) can specify Extended
Administrative Groups that are used by the operator to include or
exclude links during the Flex-Algorithm path computation [RFC9350].
These link Extended Administrative Groups are checked in the path
direction of the SPF computation, e.g., in the direction from the
root vertex toward vertices of increasing distance.
In certain scenarios, it is beneficial to evaluate the Extended
Administrative Groups associated with the reverse direction of a
link, rather than solely those in the direction of path computation.
Consider a point-to-point link represented as a pair of directed
edges between two nodes, A and B. When computing a path from A to B,
issues such as input errors on the link, detectable only at the
receiving node B, may be operationally significant. An operator
might monitor metrics like Cyclic Redundancy Check (CRC) errors or
other input-related faults at node B and apply thresholds over a
defined observation period. If such a threshold is exceeded, node B
may locally assign specific Extended Administrative Groups to the
link in the direction from B to A.
To accommodate this operational intent, the Flex-Algorithm can be
extended to inspect the Extended Administrative Groups of the
reverse-direction edge (from B to A) when evaluating the forward-
direction edge (from A to B) during path computation. This enables
the exclusion of links from the computed topology based on conditions
detected at the far end of the link, improving network reliability
and policy control.
4. Deployment Consideration
Threshold-based setting of the link Extended Administrative Groups
must be done with care to avoid excessive flooding and constant Flex-
Algorithm path recalculation.
One possible approach is to use two different thresholds when setting
and unsetting the Extended Administrative Group on the link. For
example, when setting the Extended Administrative Group on the link
based on the percentage of certain incoming errors, higher threshold
is used for setting and lower threshold is used for unsetting the
Extended Administrative Group on the link.
Many implementations provide a throttling mechanism that limits the
rate at which the Link State PDU (LSP) in case of IS-IS, or Link
State Advertisement (LSA) in case of OSPFv2 and OSPFv3, is updated at
the originator. Such mechanism is typically not specific to any
particular link attribute, but rather considers any change in the LSP
or LSA. Application of such throttling mechanism can also be used to
avoid frequent changes in the setting of the Extended Administrative
Group on a link to affect the stability of the receivers.
5. IS-IS Flexible Algorithm Exclude Reverse Admin Group Sub-TLV
The IS-IS Flexible Algorithm Exclude Reverse Admin Group (FAERAG)
Sub-TLV is used to advertise the exclude rule that is used during the
Flex-Algorithm path calculation as specified in Section 11.
The IS-IS FAERAG Sub-TLV is a sub-TLV of the IS-IS FAD Sub-TLV. It
has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Admin Group |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type (1 octet): 10
Length (1 octet): Variable, dependent on the size of the Extended
Admin Group. The length MUST be a multiple of 4 octets. If the
length is not a multiple of 4 octets, the entire sub-TLV MUST be
ignored by the receiver.
Extended Administrative Group: Extended Administrative Group as
defined in [RFC7308].
The IS-IS FAERAG Sub-TLV MUST NOT appear more than once in a single
IS-IS FAD Sub-TLV. If it appears more than once, the IS-IS FAD Sub-
TLV MUST be ignored by the receiver.
The IS-IS FAERAG Sub-TLV MUST NOT appear more than once in the set of
FAD Sub-TLVs for a given Flex-Algorithm from a given IS. If it
appears more than once in such set, the IS-IS FAERAG Sub-TLV in the
first occurrence in the lowest-numbered LSP from a given IS MUST be
used and any other occurrences MUST be ignored by the receiver.
6. IS-IS Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV
The IS-IS Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV
is used to advertise the include-any rule that is used during the
Flex-Algorithm path calculation as specified in Section 11.
The format of the IS-IS Flexible Algorithm Include-Any Reverse Admin
Group Sub-TLV is identical to the format of the FAERAG Sub-TLV in
Section 5.
IANA has assigned Type value of 11 for the IS-IS Flexible Algorithm
Include-Any Reverse Admin Group Sub-TLV.
The IS-IS Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV
MUST NOT appear more than once in a single IS-IS FAD Sub-TLV. If it
appears more than once, the IS-IS FAD Sub-TLV MUST be ignored by the
receiver.
The IS-IS Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV
MUST NOT appear more than once in the set of FAD Sub-TLVs for a given
Flex-Algorithm from a given IS. If it appears more than once in such
set, the IS-IS Flexible Algorithm Include-Any Reverse Admin Group
Sub-TLV in the first occurrence in the lowest-numbered LSP from a
given IS MUST be used and any other occurrences MUST be ignored by
the receiver.
7. IS-IS Flexible Algorithm Include-All Reverse Admin Group Sub-TLV
The IS-IS Flexible Algorithm Include-All Reverse Admin Group Sub-TLV
is used to advertise the include-all rule that is used during the
Flex-Algorithm path calculation as specified in Section 11.
The format of the IS-IS Flexible Algorithm Include-All Reverse Admin
Group Sub-TLV is identical to the format of the FAERAG Sub-TLV in
Section 5.
IANA has assigned Type value of 12 for the IS-IS Flexible Algorithm
Include-All Reverse Admin Group Sub-TLV.
The IS-IS Flexible Algorithm Include-All Reverse Admin Group Sub-TLV
MUST NOT appear more than once in a single IS-IS FAD Sub-TLV. If it
appears more than once, the IS-IS FAD Sub-TLV MUST be ignored by the
receiver.
The IS-IS Flexible Algorithm Include-All Reverse Admin Group Sub-TLV
MUST NOT appear more than once in the set of FAD Sub-TLVs for a given
Flex-Algorithm from a given IS. If it appears more than once in such
set, the IS-IS Flexible Algorithm Include-All Reverse Admin Group
Sub-TLV in the first occurrence in the lowest-numbered LSP from a
given IS MUST be used and any other occurrences MUST be ignored by
the receiver.
8. OSPF Flexible Algorithm Exclude Reverse Admin Group Sub-TLV
The OSPF Flexible Algorithm Exclude Reverse Admin Group (FAERAG) Sub-
TLV is used to advertise the exclude rule that is used during the
Flex-Algorithm path calculation as specified in Section 11.
The OSPF FAERAG Sub-TLV is a sub-TLV of the OSPF FAD TLV. It has the
following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Admin Group |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type (2 octets): 10
Length (2 octets): Variable, dependent on the size of the Extended
Admin Group. The length MUST be a multiple of 4 octets. If the
length is not a multiple of 4 octets, the entire sub-TLV MUST be
ignored by the receiver.
Extended Administrative Group: Extended Administrative Group as
defined in [RFC7308].
The OSPF FAERAG Sub-TLV MUST NOT appear more than once in an OSPF FAD
TLV. If it appears more than once, the OSPF FAD TLV MUST be ignored
by the receiver.
9. OSPF Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV
The OSPF Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV
is used to advertise the include-any rule that is used during the
Flex-Algorithm path calculation as specified in Section 11.
The format of the OSPF Flexible Algorithm Include-Any Reverse Admin
Group Sub-TLV is identical to the format of the OSPF FAERAG Sub-TLV
in Section 8.
IANA has assigned Type value of 11 for the OSPF Flexible Algorithm
Include-Any Reverse Admin Group Sub-TLV.
The OSPF Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV
MUST NOT appear more than once in an OSPF FAD TLV. If it appears
more than once, the OSPF FAD TLV MUST be ignored by the receiver.
10. OSPF Flexible Algorithm Include-All Reverse Admin Group Sub-TLV
The OSPF Flexible Algorithm Include-All Reverse Admin Group Sub-TLV
is used to advertise the include-all rule that is used during the
Flex-Algorithm path calculation as specified in Section 11.
The format of the OSPF Flexible Algorithm Include-All Reverse Admin
Group Sub-TLV is identical to the format of the OSPF FAERAG Sub-TLV
in Section 8.
IANA has assigned Type value of 12 for the OSPF Flexible Algorithm
Include-All Reverse Admin Group Sub-TLV.
The OSPF Flexible Algorithm Include-All Reverse Admin Group Sub-TLV
MUST NOT appear more than once in an OSPF FAD TLV. If it appears
more than once, the OSPF FAD TLV MUST be ignored by the receiver.
11. Calculation of Flexible Algorithm Paths
The following procedures augment the rules defined in Section 13 of
[RFC9350] by introducing additional constraints based on
Administrative Groups (AGs) associated with the reverse direction of
a link.
* Check if any exclude reverse Admin Group rule is part of the Flex-
Algorithm definition. If such exclude rule exists, check if any
Admin Group that is part of the exclude rule is also set on the
link in the reverse direction. If such Admin Group is set on the
link in the reverse direction, the link MUST be pruned from the
computation.
* Check if any include-any reverse Admin Group rule is part of the
Flex-Algorithm definition. If such include-any rule exists, check
if any Admin Group that is part of the include-any rule is also
set on the link in the reverse direction. If no such Admin Group
is set on the link in the reverse direction, the link MUST be
pruned from the computation.
* Check if any include-all reverse Admin Group rule is part of the
Flex-Algorithm definition. If such include-all rule exists, check
if all Admin Groups that are part of the include-all rule are also
set on the link in the reverse direction. If all such Admin
Groups are not set on the link in the reverse direction, the link
MUST be pruned from the computation.
Please see rules 8, 9, and 10 in Section 12.3 for these added rules.
12. IANA Considerations
12.1. Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV
IANA has registered the following in the "IS-IS Sub-Sub-TLVs for
Flexible Algorithm Definition Sub-TLV" registry under the "IS-IS TLV
Codepoints" registry group:
Type: 10
Description: Flexible Algorithm Exclude Reverse Admin Group
MP: n
Reference: RFC 9917, Section 5
Type: 11
Description: Flexible Algorithm Include-Any Reverse Admin Group
MP: n
Reference: RFC 9917, Section 6
Type: 12
Description: Flexible Algorithm Include-All Reverse Admin Group
MP: n
Reference: RFC 9917, Section 7
12.2. OSPF Flexible Algorithm Definition TLV Sub-TLV Registry
This document makes the following registration in the "OSPF Flexible
Algorithm Definition TLV Sub-TLVs" registry under the "Open Shortest
Path First (OSPF) Parameters" registry group:
Type: 10
Description: Flexible Algorithm Exclude Reverse Admin Group
Reference: RFC 9917, Section 8
Type: 11
Description: Flexible Algorithm Include-Any Reverse Admin Group
Reference: RFC 9917, Section 9
Type: 12
Description: Flexible Algorithm Include-All Reverse Admin Group
Reference: RFC 9917, Section 10
12.3. IGP Flex-Algorithm Path Computation Rules Registry
IANA has created a new registry called "IGP Flex-Algorithm Path
Computation Rules" within the "Interior Gateway Protocol (IGP)
Parameters" registry group. The registration procedure for the new
registry is Expert Review [RFC8126]. Section 12.3.1 provides
guidance for designated experts.
The "IGP Flex-Algorithm Path Computation Rules" registry specifies
the ordered set of rules that are used to prune links from the
topology during the Flex-Algorithm path computation.
There is no upper bound on the number of rules that the registry
supports.
In Table 1, "FAEMB" means "Flex-Algorithm Exclude Minimum Bandwidth",
and "FAEMD" means "Flex-Algorithm Exclude Maximum Delay".
+==========+===========================================+===========+
| Sequence | Description | Reference |
| Number | | |
+==========+===========================================+===========+
| 1 | Check if any exclude Administrative Group | Section |
| | rule is part of the Flex-Algorithm | 13 of |
| | Definition. If such exclude rule exists, | [RFC9350] |
| | check if any color that is part of the | |
| | exclude rule is also set on the link. If | |
| | such a color is set, the link MUST be | |
| | pruned from the computation. | |
+----------+-------------------------------------------+-----------+
| 2 | Check if any exclude SRLG rule is part of | Section |
| | the Flex-Algorithm Definition. If such | 13 of |
| | exclude rule exists, check if the link is | [RFC9350] |
| | part of any SRLG that is also part of the | |
| | SRLG exclude rule. If the link is part | |
| | of such SRLG, the link MUST be pruned | |
| | from the computation. | |
+----------+-------------------------------------------+-----------+
| 3 | Check if any include-any Administrative | Section |
| | Group rule is part of the Flex-Algorithm | 13 of |
| | Definition. If such include-any rule | [RFC9350] |
| | exists, check if any color that is part | |
| | of the include-any rule is also set on | |
| | the link. If no such color is set, the | |
| | link MUST be pruned from the computation. | |
+----------+-------------------------------------------+-----------+
| 4 | Check if any include-all Administrative | Section |
| | Group rule is part of the Flex-Algorithm | 13 of |
| | Definition. If such include-all rule | [RFC9350] |
| | exists, check if all colors that are part | |
| | of the include-all rule are also set on | |
| | the link. If all such colors are not set | |
| | on the link, the link MUST be pruned from | |
| | the computation. | |
+----------+-------------------------------------------+-----------+
| 5 | If the Flex-Algorithm Definition uses | Section |
| | something other than the IGP metric | 13 of |
| | (Section 5 of [RFC9350]), and such metric | [RFC9350] |
| | is not advertised for the particular link | |
| | in a topology for which the computation | |
| | is done, such link MUST be pruned from | |
| | the computation. A metric of value 0 | |
| | MUST NOT be assumed in such a case. | |
+----------+-------------------------------------------+-----------+
| 6 | Check if any exclude FAEMB rule is part | Section 6 |
| | of the Flex-Algorithm definition. If | of |
| | such exclude rule exists and the link has | [RFC9843] |
| | Maximum Link Bandwidth advertised, check | |
| | if the link bandwidth satisfies the FAEMB | |
| | rule. If the link does not satisfy the | |
| | FAEMB rule, the link MUST be pruned from | |
| | the Flex-Algorithm computation. | |
+----------+-------------------------------------------+-----------+
| 7 | Check if any exclude FAEMD rule is part | Section 6 |
| | of the Flex-Algorithm definition. If | of |
| | such exclude rule exists and the link has | [RFC9843] |
| | Min Unidirectional link delay advertised, | |
| | check if the link delay satisfies the | |
| | FAEMD rule. If the link does not satisfy | |
| | the FAEMD rule, the link MUST be pruned | |
| | from the Flex-Algorithm computation. | |
+----------+-------------------------------------------+-----------+
| 8 | Check if any exclude reverse Admin Group | Section |
| | rule is part of the Flex-Algorithm | 11 of RFC |
| | definition. If such exclude rule exists, | 9917 |
| | check if any Admin Group that is part of | |
| | the exclude rule is also set on the link | |
| | in the reverse direction. If such Admin | |
| | Group is set on the link in the reverse | |
| | direction, the link MUST be pruned from | |
| | the computation. | |
+----------+-------------------------------------------+-----------+
| 9 | Check if any include-any reverse Admin | Section |
| | Group rule is part of the Flex-Algorithm | 11 of RFC |
| | definition. If such include-any rule | 9917 |
| | exists, check if any Admin Group that is | |
| | part of the include-any rule is also set | |
| | on the link in the reverse direction. If | |
| | no such Admin Group is set on the link in | |
| | the reverse direction, the link MUST be | |
| | pruned from the computation. | |
+----------+-------------------------------------------+-----------+
| 10 | Check if any include-all reverse Admin | Section |
| | Group rule is part of the Flex-Algorithm | 11 of RFC |
| | definition. If such include-all rule | 9917 |
| | exists, check if all Admin Groups that | |
| | are part of the include-all rule are also | |
| | set on the link in the reverse direction. | |
| | If all such Admin Groups are not set on | |
| | the link in the reverse direction, the | |
| | link MUST be pruned from the computation. | |
+----------+-------------------------------------------+-----------+
Table 1: IGP Flex-Algorithm Path Calculation Rules
12.3.1. Guidance for Designated Experts
Since the original Flex-Algorithm specification [RFC9350], many Flex-
Algorithm extensions have been proposed and standardized. Many of
them added additional rules to the Flex-Algorithm path computation.
Maintaining an IANA registry for these rules allows specification
across multiple documents progressing independently. The new "IGP
Flex-Algorithm Path Computation Rules" registry has been created and
is specified in Section 12.3.
This section provides the guidance for designated experts on
evaluating new registrations in the "IGP Flex-Algorithm Path
Computation Rules" registry:
1. When a new constraint is defined, the rule associated with that
constraint MAY be inserted at any position. Backwards
compatibility is guaranteed because nodes that don't support the
new constraint will not participate in an algorithm where the FAD
specifies a constraint they don't support.
2. The relative ordering of existing rules MUST NOT be altered.
Doing so has the potential to create backwards compatibility
issues.
3. Deletion of the rules MUST NOT be done. Given that the rules are
only used conditionally based on the information carried in the
winning FAD, deletion of the rule is not necessary.
4. Merging or repeating of the rules MUST NOT be done.
13. Security Considerations
This document inherits security considerations from [RFC9350].
14. References
14.1. Normative References
[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>.
[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>.
[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>.
[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>.
[RFC9843] Hegde, S., Britto, W., Shetty, R., Decraene, B., Psenak,
P., and T. Li, "IGP Flexible Algorithms: Bandwidth, Delay,
Metrics, and Constraints", RFC 9843, DOI 10.17487/RFC9843,
September 2025, <https://www.rfc-editor.org/info/rfc9843>.
14.2. Informative References
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
Authors' Addresses
Peter Psenak
Cisco Systems
Apollo Business Center
Mlynske nivy 43
82109 Bratislava
Slovakia
Email: ppsenak@cisco.com
Jakub Horn
Cisco Systems
Milpitas, CA 95035
United States of America
Email: jakuhorn@cisco.com
Amit Dhamija
Arrcus
Email: amitd@arrcus.com
AI ELI5
On this page
PROBLEMFlex-Algorithm checks link Admin Groups only in the direction of the path computation, so problems visible only at the far end of a link, such as input errors, cannot be used to exclude that link.
MECHANISMThree new sub-TLVs per protocol (exclude, include-any, include-all) carry reverse Admin Group rules in the Flexible Algorithm Definition, and three new pruning rules check the Admin Groups set on the reverse-direction edge.
WHY IT MATTERSOperators can steer traffic away from a link based on conditions detected at its far end. The RFC also creates an IANA registry of ordered path computation rules, and it updates RFC 9350 and RFC 9843.
§1Introduction
IGPs historically computed best paths using only the IGP metric on links. An IGP Flex-Algorithm, specified in RFC 9350, allows constraint-based paths. Existing constraints include link inclusion or exclusion by Admin Group, exclusion by Shared Risk Link Group (SRLG), exclusion based on minimum bandwidth and exclusion based on maximum delay (the last two from RFC 9843).
This document adds constraints that include or exclude links based on Admin Groups associated with the reverse direction of the Shortest Path First (SPF) computation. It also updates RFC 9350 and RFC 9843 by creating a new IANA registry for the ordered set of rules that prune links. The term OSPF here covers both OSPFv2 and OSPFv3.
Until now, path planning only looked at a road's rating in the direction you were traveling. This RFC adds a check of the rating on the way back too.
§2The Use Case
The Flexible Algorithm Definition (FAD) can name Extended Administrative Groups that include or exclude links. Normally these are checked in the path direction, from the root toward vertices of increasing distance.
Consider a point-to-point link as two directed edges between nodes A and B. When computing a path from A to B, input errors that only node B can detect may matter. An operator might watch Cyclic Redundancy Check (CRC) errors at B and, when a threshold is exceeded over an observation period, have B assign Extended Administrative Groups to the link in the B to A direction.
The Flex-Algorithm can then inspect the reverse edge (B to A) while evaluating the forward edge (A to B). This excludes links based on conditions detected at the far end.
Imagine the end of a hallway notices the floor is slippery, but the person at the start cannot tell. This lets the far end put up a sign that the planner reads for the whole hallway.
§3Deployment Consideration
Setting Admin Groups from thresholds must be done with care, to avoid excessive flooding and constant Flex-Algorithm path recalculation.
One approach is to use two thresholds: a higher one to set the Extended Administrative Group on the link, and a lower one to unset it. The RFC also notes that many implementations throttle how fast an IS-IS LSP, or an OSPFv2 or OSPFv3 LSA, can be updated at the originator. That throttling can also keep frequent changes to the Admin Group from destabilizing receivers.
If a sign flips on and off every second, everyone gets confused. Using one level to turn it on and a lower level to turn it off keeps it from flickering.
§4The Reverse Admin Group Sub-TLVs
There are three new sub-TLVs for each protocol: Exclude Reverse Admin Group (FAERAG), Include-Any Reverse Admin Group and Include-All Reverse Admin Group. In IS-IS they are sub-TLVs of the IS-IS FAD Sub-TLV, with a 1-octet Type and 1-octet Length. In OSPF they are sub-TLVs of the OSPF FAD TLV, with a 2-octet Type and 2-octet Length. The Type values are 10, 11 and 12 respectively in both protocols.
The body is an Extended Administrative Group as defined in RFC 7308. The Length is variable and MUST be a multiple of 4 octets. Otherwise the receiver MUST ignore the entire sub-TLV.
Each sub-TLV MUST NOT appear more than once in a single FAD. If it does, the receiver MUST ignore the FAD. For IS-IS, if one appears more than once across the set of FAD Sub-TLVs for a Flex-Algorithm from one IS, the first occurrence in the lowest-numbered LSP is used and the others are ignored.
Each rule is written on a small card with a type number and a length. The length has to come out in neat chunks of four, or the card gets thrown away.
- 10Exclude Reverse Admin Group — Prune a link if any listed group is set on the reverse direction.
- 11Include-Any Reverse Admin Group — Prune a link unless at least one listed group is set on the reverse direction.
- 12Include-All Reverse Admin Group — Prune a link unless all listed groups are set on the reverse direction.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | Type | 1 byte | 10 for Exclude; 11 and 12 for the include variants. | |
LENGTH | Length | 1 byte | Variable; MUST be a multiple of 4 octets. | |
EXTENDED-ADMIN-GROUP | Extended Admin Group | 2 bytes | Extended Administrative Group as defined in RFC 7308; the field continues in 4-octet units. |
§5Calculating Flex-Algorithm Paths
The new procedures add to the rules in Section 13 of RFC 9350. Each looks at the Admin Groups set on the link in the reverse direction:
- Exclude: if any Admin Group in the exclude reverse rule is set on the reverse link, the link MUST be pruned.
- Include-any: if none of the Admin Groups in the rule is set on the reverse link, the link MUST be pruned.
- Include-all: if not all of the Admin Groups in the rule are set on the reverse link, the link MUST be pruned.
These are rules 8, 9 and 10 in the new registry.
For each road, look at the sign on the way back. If it has a banned label, skip the road. If you require a label and it is missing, skip the road too.
§6IANA Registrations and the Rules Registry
IANA registered Types 10, 11 and 12 in the IS-IS Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV registry and in the OSPF Flexible Algorithm Definition TLV Sub-TLVs registry.
IANA also created the IGP Flex-Algorithm Path Computation Rules registry, with Expert Review as the registration procedure and no upper bound on the number of rules. It lists, in order, the existing rules (1 to 5 from RFC 9350 and 6 and 7 from RFC 9843) and the three new ones (8 to 10). Guidance for designated experts: a new rule MAY be inserted at any position, because nodes that do not support a constraint will not take part in an algorithm whose FAD uses it. The relative order of existing rules MUST NOT be altered, rules MUST NOT be deleted, and rules MUST NOT be merged or repeated.
There is now an official list of the checks, in order. New checks can be added, but old ones can never be removed or reshuffled.
§7Security Considerations
The document says only that it inherits the security considerations of RFC 9350 and adds no further text on the subject.
It follows the same safety rules as the document it builds on and adds no new ones.
§8References
[RFC9350]RFC 9350Psenak, P., Ed., et al., "IGP Flexible Algorithm", RFC 9350, February 2023.February 2023[RFC9843]RFC 9843Hegde, S., et al., "IGP Flexible Algorithms: Bandwidth, Delay, Metrics, and Constraints", RFC 9843, September 2025.September 2025[RFC7308]RFC 7308Osborne, E., "Extended Administrative Groups in MPLS Traffic Engineering (MPLS-TE)", RFC 7308, July 2014.July 2014[RFC8126]RFC 8126Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, June 2017.June 2017