Internet Engineering Task Force (IETF) K. Talaulikar, Ed.
Request for Comments: 9294
Category: Standards Track P. Psenak
ISSN: 2070-1721 Cisco Systems
Application-Specific Link Attributes Advertisement Using the Border
Gateway Protocol - Link State (BGP-LS)
Extensions have been defined for link-state routing protocols that
enable distribution of application-specific link attributes for
existing as well as newer applications such as Segment Routing (SR).
This document defines extensions to the Border Gateway Protocol -
Link State (BGP-LS) to enable the advertisement of these application-
specific attributes as a part of the topology information from the
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/rfc9294
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Table of Contents 1.
Requirements Language 2.
Application-Specific Link Attributes TLV 3.
Application-Specific Link Attributes 4.
Illustration for IS-IS 5.
Deployment Considerations 6.
IANA Considerations 7.
Manageability Considerations 8.
Security Considerations 9.
Normative References 9.2.
The Border Gateway Protocol - Link State (BGP-LS) [RFC7752
the distribution of the link-state topology information from link-
state routing protocols (viz., IS-IS [RFC1195
], OSPFv2 [RFC2328
]) to an application like a controller or Path
Computation Engine (PCE) via BGP. The controller or PCE gets the
end-to-end topology information across IGP domains so it can perform
path computations for use cases like end-to-end traffic engineering
The link-state topology information distributed via BGP-LS includes
link attributes that were originally defined for MPLS TE (i.e., using
] or GMPLS [RFC4202
] applications). In recent years,
applications, such as Segment Routing (SR) Policy [RFC8402
] and Loop-
Free Alternates (LFA) [RFC5286
], which also make use of link
attributes, have been introduced. [RFC8919
] and [RFC8920
extensions for IS-IS and OSPF, respectively, that enable advertising
application-specific link attributes for these and other future
applications. This has resulted in the need for a similar BGP-LS
extension to include this additional link-state topology information
from the link-state routing protocols.
This document defines the BGP-LS extensions for the advertisement of
application-specific link attributes. It describes the advertisement
of these link attributes as top-level TLVs (i.e., as TLVs of the BGP-
LS Attribute) and as sub-TLVs of the (top-level) Application-Specific
Link Attributes (ASLA) TLV. The document also describes the
procedures for the advertisement of these attributes from the
underlying IGPs and discusses their deployment aspects.
1.1. Requirements Language
The key words "MUST
", "MUST NOT
", "SHALL NOT
", "SHOULD NOT
", "NOT RECOMMENDED
" in this document are to be interpreted as described in
BCP 14 [RFC2119
] when, and only when, they appear in all
capitals, as shown here.
2. Application-Specific Link Attributes TLV
] specifies the Link Network Layer Reachability
Information (NLRI) for the advertisement of links and their
attributes using the BGP-LS Attribute. The ASLA TLV is an optional
top-level BGP-LS Attribute TLV that is introduced for Link NLRIs. It
is defined such that it may act as a container for certain existing
and future link attributes that require application-specific
The format of this TLV is as follows and is similar to the
corresponding ASLA sub-TLVs defined for OSPF and IS-IS in [RFC8920
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 |
| SABM Length | UDABM Length | Reserved |
| Standard Application Identifier Bit Mask (variable) //
| User-Defined Application Identifier Bit Mask (variable) //
| Link Attribute sub-TLVs //
Figure 1: Application-Specific Link Attributes TLV
SABM Length: 1-octet field that carries the Standard Application
Identifier Bit Mask Length in octets as defined in [RFC8920
UDABM Length: 1-octet field that carries the User-Defined
Application Identifier Bit Mask Length in octets as defined in
Reserved: 2-octet field that MUST
be set to zero on transmission and MUST
be ignored on reception.
Standard Application Identifier Bit Mask: An optional set of bits
(of size 0, 4, or 8 octets as indicated by the SABM Length), where
each bit represents a single standard application as defined in
User-Defined Application Identifier Bit Mask: An optional set of
bits (of size 0, 4, or 8 octets as indicated by the UDABM Length),
where each bit represents a single user-defined application as
defined in [RFC8919
] and [RFC8920
Link Attribute sub-TLVs: BGP-LS Attribute TLVs corresponding to the
Link NLRI that are application-specific (including existing ones
as specified in Section 3
) are included as sub-TLVs of the ASLA
The semantics associated with the standard and user-defined bit masks
as well as the encoding scheme for application-specific attributes
are as specified in [RFC8920
The ASLA TLV and its sub-TLVs can only be added to the BGP-LS
Attribute associated with the Link NLRI of the node that originates
the underlying IGP link attribute TLVs and sub-TLVs. The procedures
for originating link attributes in the ASLA TLV from underlying IGPs
are specified in Section 4
3. Application-Specific Link Attributes
Several BGP-LS Attribute TLVs corresponding to the Link NLRI are
defined in BGP-LS [RFC7752
], and more may be added in the future.
Those attributes that have been determined to be, and advertised as,
application-specific in the underlying IGPs are also encoded
similarly in BGP-LS.
The following table lists the currently defined BGP-LS Attribute TLVs
corresponding to Link NLRI that can have application-specific
semantics based on the underlying IGP specifications [RFC8919
]. These were originally defined with semantics for RSVP-TE
and GMPLS applications in BGP-LS by the respective reference
| TLV Code Point | Description | Reference Document |
| 1088 | Administrative group | [RFC7752
| | (color) | |
| 1092 | TE Default Metric | [RFC7752
| 1096 | Shared Risk Link Group | [RFC7752
| 1114 | Unidirectional Link | [RFC8571
| | Delay | |
| 1115 | Min/Max Unidirectional | [RFC8571
| | Link Delay | |
| 1116 | Unidirectional Delay | [RFC8571
| | Variation | |
| 1117 | Unidirectional Link | [RFC8571
| | Loss | |
| 1118 | Unidirectional | [RFC8571
| | Residual Bandwidth | |
| 1119 | Unidirectional | [RFC8571
| | Available Bandwidth | |
| 1120 | Unidirectional | [RFC8571
| | Utilized Bandwidth | |
| 1173 | Extended | [RFC9104
| | Administrative Group | |
Table 1: Existing BGP-LS TLVs Identified as Application-Specific
All the BGP-LS Attribute TLVs listed in the table above are REQUIRED
to be advertised as a top-level TLV in the BGP-LS Attribute when used
to carry link attributes specific to RSVP-TE.
BGP-LS Attribute TLVs corresponding to Link NLRI that are advertised
in the underlying IGP as application-specific are REQUIRED
encoded within an ASLA TLV.
Link attributes that do not have application-specific semantics MUST
be advertised within the ASLA TLV.
When the same application-specific link attributes are advertised
both within the ASLA TLV and as top-level TLVs in the BGP-LS
Attribute, the attributes advertised within the ASLA TLV take
precedence for the applications indicated in the ASLA TLV encoding.
The BGP-LS originator learns of the association of an application-
specific attribute to one or more applications from the underlying
IGP protocol Link State Advertisements (LSAs) or Link State Packets
(LSPs) from which it is advertising the topology information.
] and [RFC8919
] specify the mechanisms for advertising
application-specific link attributes in OSPF and IS-IS, respectively.
Application-specific link attributes received from an IGP node
without the use of ASLA encodings continue to be encoded using the
respective BGP-LS top-level TLVs listed in Table 1 as specified in
their respective reference documents.
While the ASLA encoding in OSPF is similar to that of BGP-LS, the
encoding in IS-IS differs and requires additional procedures when
conveying information into BGP-LS. One of these differences arises
from the presence of the L-flag in the IS-IS encoding. Another
difference arises due to the requirement to collate information from
two types of IS-IS encodings for application-specific link
information (i.e., the IS-IS ASLA sub-TLV and the IS-IS Application-
Specific Shared Risk Link Group (SRLG) TLV) [RFC8919
] and to carry
them together in the BGP-LS ASLA TLV.
A BGP-LS originator node that is advertising link-state information
from the underlying IGP using ASLA encodings determines their BGP-LS
encoding based on the following rules: 1.
Application-specific link attributes received from an OSPF node
using an ASLA sub-TLV or from an IS-IS node using either an ASLA
sub-TLV or an Application-Specific SRLG TLV MUST
be encoded in
the BGP-LS ASLA TLV as sub-TLVs. Exceptions to this rule are
specified in (2)(F) and (2)(G) below. 2.
In the case of IS-IS, the specific procedures below are to be
A. When application-specific link attributes are received from a
node with the L-flag set in the IS-IS ASLA sub-TLV and when
application bits (other than RSVP-TE) are set in the
application bit masks, then the application-specific link
attributes advertised in the corresponding legacy IS-IS TLVs
and sub-TLVs MUST
be encoded within the BGP-LS ASLA TLV as
sub-TLVs with the application bits (other than the RSVP-TE
bit) copied from the IS-IS ASLA sub-TLV. The link attributes
advertised in the legacy IS-IS TLVs and sub-TLVs are also
advertised in BGP-LS top-level TLVs as per [RFC7752
], and [RFC9104
]. The same procedure also applies
for the advertisement of the SRLG values from the IS-IS
Application-Specific SRLG TLV.
B. When the IS-IS ASLA sub-TLV has the RSVP-TE application bit
set, then the link attributes for the corresponding IS-IS
ASLA sub-TLVs MUST
be encoded using the respective BGP-LS
top-level TLVs as per [RFC7752
], and [RFC9104
Similarly, when the IS-IS Application-Specific SRLG TLV has
the RSVP-TE application bit set, then the SRLG values within
be encoded using the top-level BGP-LS SRLG TLV (1096)
as per [RFC7752
C. The SRLGs advertised in one or more IS-IS Application-
Specific SRLG TLVs and the other link attributes advertised
in one or more IS-IS ASLA sub-TLVs are REQUIRED
collated, on a per-application basis, only for those
applications that meet all the following criteria:
* their bit is set in the SABM or UDABM in one of the two
types of IS-IS encodings (e.g., IS-IS ASLA sub-TLV)
* the other encoding type (e.g., IS-IS Application Specific
SRLG TLV) has an advertisement with zero-length
application bit masks
* there is no corresponding advertisement of that other
encoding type (following the example, IS-IS Application
Specific SRLG TLV) with that specific application bit set
For each such application, its collated information MUST
carried in a BGP-LS ASLA TLV with that application's bit set
in the SABM or UDABM. See the illustration in Section 4.1
D. If the resulting set of collated link attributes and SRLG
values is common across multiple applications, they MAY
advertised in a common BGP-LS ASLA TLV instance where the
bits for all such applications would be set in the
application bit mask.
E. Both the SRLG values from IS-IS Application-Specific SRLG
TLVs and the link attributes from IS-IS ASLA sub-TLVs, with
the zero-length application bit mask, MUST
be advertised into
a BGP-LS ASLA TLV with a zero-length application bit mask,
independent of the collation described above.
] allows the advertisement of the Maximum Link
Bandwidth within an IS-IS ASLA sub-TLV even though it is not
an application-specific attribute. However, when originating
the Maximum Link Bandwidth into BGP-LS, the attribute MUST
encoded only in the top-level BGP-LS Maximum Link Bandwidth
TLV (1089) and MUST NOT
be advertised within the BGP-LS ASLA
] also allows the advertisement of the Maximum
Reservable Link Bandwidth and the Unreserved Bandwidth within
an IS-IS ASLA sub-TLV even though these attributes are
specific to RSVP-TE application. However, when originating
the Maximum Reservable Link Bandwidth and Unreserved
Bandwidth into BGP-LS, these attributes MUST
be encoded only
in the BGP-LS top-level Maximum Reservable Link Bandwidth TLV
(1090) and Unreserved Bandwidth TLV (1091), respectively, and
not within the BGP-LS ASLA TLV.
These rules ensure that a BGP-LS originator performs the
advertisement for all application-specific link attributes from the
IGP nodes that support the ASLA extension. Furthermore, it also
ensures that the top-level BGP-LS TLVs defined for RSVP-TE and GMPLS
applications continue to be used for advertisement of their
A BGP-LS speaker would normally advertise all the application-
specific link attributes corresponding to RSVP-TE and GMPLS
applications as existing top-level BGP-LS TLVs while for other
applications they are encoded in the ASLA TLV(s) with appropriate
applicable bit mask setting. An application-specific attribute value
received via a sub-TLV within the ASLA TLV has precedence over the
value received via a top-level TLV.
4.1. Illustration for IS-IS
This section illustrates the procedure for the advertisement of
application-specific link attributes from IS-IS into BGP-LS.
Consider the following advertisements for a link in IS-IS. We start
with this set:
a. IS-IS ASLA sub-TLV with the S, F, and X bits set on it that
carries certain application-specific link attributes
b. IS-IS Application-Specific SRLG TLV with zero-length bit masks
with a set of application-specific SRLGs
c. IS-IS Application-Specific SRLG TLV with the X bit set on it with
a set of application-specific SRLGs
The corresponding BGP-LS advertisements for that link are determined
First, based on rule (1), the advertisements are conveyed to BGP-LS
to get the following "updated set": 1.
ASLA with the S, F, and X bits set on it that carries link
attributes from IS-IS advertisement (a) 2.
ASLA SRLG with zero-length bit masks with a set of SRLGs from IS-
IS advertisement (b) 3.
ASLA SRLG with the X bit set on it with a set of SRLGs from IS-IS
Next, we apply the rules from (2) to this "updated set", because all
of them were sourced from IS-IS, to derive a new set.
The next rule that applies is (2)(c), and it is determined that
collation is required for applications S and F; therefore, we get the
following "final set": 1.
ASLA with the S bit set on it that carries link attributes from
IS-IS advertisement (a) and SRLGs from IS-IS advertisement (b)
(this is collation for application S based on (2)(c)) 2.
ASLA with the F bit set on it that carries link attributes from
IS-IS advertisement (a) and SRLGs from IS-IS advertisement (b)
(this is collation for application F based on (2)(c)) 3.
ASLA with the X bit set on it that carries link attributes from
IS-IS advertisement (a) (remaining application not affected by
collation based on (2)(c)) 4.
ASLA with zero-length bit masks with SRLGs from IS-IS
advertisement (b) (not affected by (2)(c) and therefore carried
forward unchanged from the "updated set") 5.
ASLA with the X bit set on it with SRLGs from IS-IS advertisement
(c) (not affected by (2)(c) and therefore carried forward
unchanged from the "updated set")
Implementations may optionally perform further consolidation by
processing the "final set" above based on (2)(d) to determine the
following "consolidated final set": 1.
ASLA with the S and F bits set on it that carries application-
specific link attributes from IS-IS advertisement (a) and SRLGs
from IS-IS advertisement (b) (this is the consolidation of items 1
and 2 of the "final set" based on (2)(d)) 2.
ASLA with the X bit set on it that carries certain application-
specific link attributes from IS-IS advertisement (a) (it is
unaffected by this consolidation) 3.
ASLA with zero-length bit masks with a set of application-
specific SRLGs from IS-IS advertisement (b) (this is retained
based on (2)(e) and is unaffected by any further consolidation) 4.
ASLA with the X bit set on it with a set of application-specific
SRLGs from IS-IS advertisement (c) (it is unaffected by this
Further optimization (e.g., combining (2) and (4) from the
"consolidated final set" above into a single BGP-LS ASLA TLV) may be
possible while ensuring that the semantics are preserved between the
IS-IS and BGP-LS advertisements. Such optimizations are outside the
scope of this document.
5. Deployment Considerations
BGP-LS sources the link-state topology information (including the
extensions introduced by this document) from the underlying link-
state IGP protocols. The various deployment aspects related to the
advertisement and use of application-specific link attributes are
discussed in the Deployment Considerations sections of [RFC8920
]. The IGP backward-compatibility aspects described in those
documents associated with application-specific link attributes along
with the BGP-LS procedures specified in this document enable backward
compatibility in deployments of existing implementations of
], and [RFC9104
] for applications such as RSVP-TE,
SR Policy, and LFA.
It is recommended that only nodes supporting this specification are
selected as originators of BGP-LS information when advertising the
link-state information from the IGPs in deployments supporting
application-specific link attributes.
BGP-LS consumers that do not support this specification can continue
to use the existing top-level TLVs for link attributes for existing
applications as discussed above. However, they would be able to
support neither the application-specific link attributes nor newer
applications that may be encoded only using the ASLA TLV.
6. IANA Considerations
IANA has assigned a code point from the "BGP-LS Node Descriptor, Link
Descriptor, Prefix Descriptor, and Attribute TLVs" registry as
described in the following table. There is no "IS-IS TLV/Sub-TLV"
value for this entry.
| TLV Code Point | Description | Reference |
| 1122 | Application-Specific | RFC 9294
| | Link Attributes | |
Table 2: ASLA TLV Code Point Allocation
7. Manageability Considerations
The protocol extensions introduced in this document augment the
existing IGP topology information defined in [RFC7752
and protocol extensions defined in this document do not affect the
BGP protocol operations and management other than as discussed in the
Manageability Considerations section of [RFC7752
]. Specifically, the
malformed NLRI attribute tests in the Fault Management section of
] now encompass the BGP-LS TLVs defined in this document.
The extensions specified in this document do not specify any new
configuration or monitoring aspects in BGP or BGP-LS. The
specification of BGP models is an ongoing work based on
8. Security Considerations
Security considerations for acquiring and distributing BGP-LS
information are discussed in [RFC7752
]. Specifically, the
considerations related to topology information, which are related to
traffic engineering, apply.
The TLVs introduced in this document are used to propagate the
application-specific link attributes IGP extensions defined in
] and [RFC8920
]. It is assumed that the IGP instances
originating these TLVs will support all the required security (as
described in [RFC8919
] and [RFC8920
This document defines a new way to advertise link attributes.
Tampering with the information defined in this document may affect
applications using it, including impacting traffic engineering, which
uses various link attributes for its path computation. As the
advertisements defined in this document limit the scope to specific
applications, the impact of tampering is similarly limited in scope.
The advertisement of the link attribute information defined in this
document presents no significant additional risk beyond that
associated with the existing link attribute information already
supported in [RFC7752
9.1. Normative References
] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119
, March 1997,
] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752
, March 2016,
] 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
] Ginsberg, L., Ed., Previdi, S., Wu, Q., Tantsura, J., and
C. Filsfils, "BGP - Link State (BGP-LS) Advertisement of
IGP Traffic Engineering Performance Metric Extensions", RFC 8571
, DOI 10.17487/RFC8571
, March 2019,
] Ginsberg, L., Psenak, P., Previdi, S., Henderickx, W., and
J. Drake, "IS-IS Application-Specific Link Attributes", RFC 8919
, DOI 10.17487/RFC8919
, October 2020,
] Psenak, P., Ed., Ginsberg, L., Henderickx, W., Tantsura,
J., and J. Drake, "OSPF Application-Specific Link
Attributes", RFC 8920
, DOI 10.17487/RFC8920
, October 2020,
] Tantsura, J., Wang, Z., Wu, Q., and K. Talaulikar,
"Distribution of Traffic Engineering Extended
Administrative Groups Using the Border Gateway Protocol -
Link State (BGP-LS)", RFC 9104
, DOI 10.17487/RFC9104
August 2021, <https://www.rfc-editor.org/info/rfc9104
9.2. Informative References
Jethanandani, M., Patel, K., Hares, S., and J. Haas, "BGP
YANG Model for Service Provider Networks", Work in
Progress, Internet-Draft, draft-ietf-idr-bgp-model-14, 3
July 2022, <https://datatracker.ietf.org/doc/html/draft-
] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195
, DOI 10.17487/RFC1195
December 1990, <https://www.rfc-editor.org/info/rfc1195
] Moy, J., "OSPF Version 2", STD 54, RFC 2328
, April 1998,
] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209
, DOI 10.17487/RFC3209
, December 2001,
] Kompella, K., Ed. and Y. Rekhter, Ed., "Routing Extensions
in Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 4202
, DOI 10.17487/RFC4202
, October 2005,
] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286
, September 2008,
] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340
, DOI 10.17487/RFC5340
, July 2008,
] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402
, DOI 10.17487/RFC8402
July 2018, <https://www.rfc-editor.org/info/rfc8402
The authors would like to thank Les Ginsberg, Baalajee S., Amalesh
Maity, Acee Lindem, Keyur Patel, Paul Wouters, Rudy Selderslaghs,
Kristy Paine, and Shraddha Hegde for their review and feedback on
this document. The authors would like to thank Alvaro Retana for his
very detailed AD review and comments that improved this document.
The authors would also like to thank John Scudder for his detailed
review and feedback on clarifying the procedures along with the
example in Section 4
Ketan Talaulikar (editor)