IS-IS and OSPF Extensions for Inter-AS Multi-Topology Binding
draft-wang-lsr-inter-as-mt-binding-00
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Author | Aijun Wang | ||
| Last updated | 2026-10-14 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
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draft-wang-lsr-inter-as-mt-binding-00
Network Working Group A. Wang
Internet-Draft China Telecom
Intended status: Standards Track 14 October 2026
Expires: 17 April 2027
IS-IS and OSPF Extensions for Inter-AS Multi-Topology Binding
draft-wang-lsr-inter-as-mt-binding-00
Abstract
[RFC5120] and [RFC4915] define Multi-Topology (MT) routing for IS-IS
and OSPF, respectively. When a topology-specific path crosses an AS
boundary, a topology in one AS may need to be connected to a topology
identified by a different MT-ID in the adjacent AS.
Existing inter-AS Traffic Engineering extensions do not advertise
this relationship.
This document defines extensions to IS-IS and OSPF to advertise the
binding between local and remote MT-IDs associated with an inter-AS
link.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119] [RFC8174]
when, and only when, they appear in all capitals, as shown here.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 17 April 2027.
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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
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Inter-AS MT Binding . . . . . . . . . . . . . . . . . . . . . 4
4. IS-IS Extension . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Inter-AS MT Binding Sub-TLV . . . . . . . . . . . . . . . 5
4.2. Origination and Processing . . . . . . . . . . . . . . . 5
5. OSPF Extension . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Inter-AS MT Binding Sub-TLV . . . . . . . . . . . . . . . 6
5.2. Origination and Processing . . . . . . . . . . . . . . . 7
6. BGP-LS Extension . . . . . . . . . . . . . . . . . . . . . . 7
7. Operational Considations . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 9
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
10.1. IS-IS . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.2. OSPF . . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.3. BGP-LS . . . . . . . . . . . . . . . . . . . . . . . . . 10
11. Normative References . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
[RFC5120] defines Multi-Topology (MT) routing for IS-IS. RFC
[RFC4915] defines MT routing for OSPF. These mechanisms allow
multiple logical topologies to be maintained within an IGP. An
inter-AS path may traverse different MTs in different ASes. The MT-
ID used in one AS does not need to be the same as the MT-ID used for
the corresponding topology in an adjacent AS.
For example:
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AS 65001 AS 65002
MT-ID 10 MT-ID 20
| |
| |
ASBR1-----------------ASBR2
(Inter-AS Link)
Figure 1 Inter-AS Multi-Topology Binding
MT-ID 10 in AS 65001 may be connected to MT-ID 20 in AS 65002 over
the inter-AS link. The corresponding MT Binding is:
MT-ID 10-----> MT-ID 20
[RFC9346] defines IS-IS extensions for advertising inter-AS Traffic
Engineering (TE) links. [RFC5392] defines corresponding extensions
for OSPF. These mechanisms advertise inter-AS link information and
associated TE attributes, but do not identify the MTs connected by an
inter-AS link.
This document defines extensions to [RFC9346] and [RFC5392] to
advertise this information.
The MT Binding is configured on an ASBR and advertised into the local
IGP.
This document does not define a mechanism for exchanging MT Binding
information across an inter-AS link.
2. Terminology
This document uses the terminology defined in RFC 4915, RFC 5120, RFC
5392, and RFC 9346.
Local MT-ID: The MT-ID of a topology in the AS originating the inter-
AS link advertisement.
Remote MT-ID: The MT-ID of the topology in the adjacent AS to which
the local topology is connected over the inter-AS link.
Inter-AS MT Binding: An association between a Local MT-ID and a
Remote MT-ID on an inter-AS link..
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3. Inter-AS MT Binding
An Inter-AS MT Binding associates a local MT with an inter-AS link
and, when specified, identifies the corresponding MT in the adjacent
AS.
<Local MT-ID, Remote MT-ID>
For example:
<10, 20> indicates that local MT-ID 10 is connected to remote MT-ID
20 over the inter-AS link.
A single inter-AS link may support multiple MT Bindings.
For example:
AS 65001 AS 65002
MT10 MT11 MT20 MT21
| | | |
| | | |
ASBR1------------------ASBR2
(Inter-AS Link)
Figure 2 Inter-AS Multi-Topology Multiple Binding
The corresponding bindings are: <10, 20> <11, 21>
Each MT Binding is advertised independently.
The binding applies only to the inter-AS link with which it is
advertised.
The Local MT-ID and Remote MT-ID are configured on the advertising
ASBR. MT-ID values are independently assigned and managed within
each AS.
A receiver associates an advertised MT Binding with the inter-AS link
carrying the binding. A binding <X,Y> indicates that the inter-AS
link connects local MT X to remote MT Y.
When a configured MT Binding is added, changed, or removed, the ASBR
MUST update the corresponding IGP advertisement.
4. IS-IS Extension
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4.1. Inter-AS MT Binding Sub-TLV
[RFC9346] defines the Inter-AS Reachability Information TLV (TLV 141)
for advertising inter-AS TE links.
This document defines a new Inter-AS MT Binding sub-TLV of TLV 141.
Each instance of the sub-TLV advertises one MT Binding.
The sub-TLV 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 |R R R R| Local MT-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R R R U| Remote MT-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where:
Type: TBD1
Length: 4
Local MT-ID: A 12-bit field identifying the local MT.
Remote MT-ID: A 12-bit field identifying the corresponding MT in the
adjacent AS.
U: Remote MT-ID Unspecified. When set, the Remote MT-ID is
unspecified. The Remote MT-ID field MUST be set to zero by the
originator and MUST be ignored by the receiver. When clear, the
Remote MT-ID field identifies the corresponding MT in the adjacent
AS.
R: Reserved. These bits MUST be set to zero when originated and MUST
be ignored when received.
If an inter-AS link supports multiple MT Bindings, multiple instances
of the Inter-AS MT Binding sub-TLV MUST be included in the Inter-AS
Reachability Information TLV.
4.2. Origination and Processing
The Inter-AS MT Binding sub-TLV MUST be carried in the Inter-AS
Reachability Information TLV describing the inter-AS link to which
the binding applies.
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A receiver that does not support this extension ignores the unknown
sub-TLV according to normal IS-IS processing.
The sub-TLV has the same flooding scope as the enclosing Inter-AS
Reachability Information TLV.
5. OSPF Extension
5.1. Inter-AS MT Binding Sub-TLV
[RFC5392] defines the Inter-AS-TE-v2 LSA and Inter-AS-TE-v3 LSA for
advertising inter-AS TE links in OSPFv2 and OSPFv3.
Both LSAs contain a Link TLV describing an inter-AS link.
This document defines a new Inter-AS MT Binding sub-TLV of the Link
TLV.
Each instance of the sub-TLV advertises one MT Binding.
The sub-TLV 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R R R U| Local MT-ID | R R R R | Remote MT-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where:
Type: TBD2
Length: 4
Local MT-ID: A 12-bit field identifying the local MT.
Remote MT-ID: A 12-bit field identifying the corresponding MT in the
adjacent AS.
U: Remote MT-ID Unspecified. When set, the Remote MT-ID is
unspecified. The Remote MT-ID field MUST be set to zero by the
originator and MUST be ignored by the receiver. When clear, the
Remote MT-ID field identifies the corresponding MT in the adjacent
AS.
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R: Reserved. These bits MUST be set to zero when originated and MUST
be ignored when received.
If an inter-AS link supports multiple MT Bindings, multiple instances
of the Inter-AS MT Binding sub-TLV MUST be included in the Inter-AS
Reachability Information TLV.
The sub-TLV has the same semantics for OSPFv2 and OSPFv3.
5.2. Origination and Processing
The Inter-AS MT Binding sub-TLV MUST be carried in the Link TLV
describing the inter-AS link to which the binding applies.
A receiver that does not support this extension ignores the unknown
sub-TLV according to normal OSPF processing.
The sub-TLV has the same flooding scope as the enclosing Inter-AS-TE
LSA..
6. BGP-LS Extension
BGP-LS [RFC9552] provides a mechanism to distribute link-state and
Traffic Engineering information to external components, such as a
Path Computation Element (PCE) or a controller.
[I-D.ietf-idr-bgpls-inter-as-topology-ext] defines the Inter-AS Link
NLRI for advertising inter-AS link information via BGP-LS.
This document defines a new Inter-AS MT Binding TLV to advertise the
MT Binding information associated with an Inter-AS Link NLRI.
Each instance of the Inter-AS MT Binding TLV advertises one MT
Binding.
The TLV 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R R R U| Local MT-ID | R R R R | Remote MT-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: TBD3.
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Length: 4
U (Remote MT-ID Unspecified): When set, the Remote MT-ID is
unspecified. The Remote MT-ID field MUST be set to zero by the
originator and MUST be ignored by the receiver. When clear, the
Remote MT-ID field identifies the corresponding MT in the adjacent
AS.
Local MT-ID: A 12-bit field identifying the local MT.
Remote MT-ID: A 12-bit field identifying the corresponding MT in the
adjacent AS when the U bit is clear.
R: Reserved. These bits MUST be set to zero when originated and MUST
be ignored when received.
The Inter-AS MT Binding TLV is carried in the BGP-LS Attribute
associated with the Inter-AS Link NLRI defined in
[I-D.ietf-idr-bgpls-inter-as-topology-ext].
When an Inter-AS MT Binding sub-TLV defined in Section 4 or Section 5
is advertised for an inter-AS link, a BGP-LS speaker supporting this
extension MAY advertise the corresponding Inter-AS MT Binding TLV
with the Inter-AS Link NLRI representing that link.
The Local MT-ID, Remote MT-ID, and U bit MUST be copied from the
corresponding IGP Inter-AS MT Binding sub-TLV.
If multiple Inter-AS MT Bindings are associated with the same inter-
AS link, a separate Inter-AS MT Binding TLV MUST be advertised for
each binding.
7. Operational Considations
MT Bindings are configured independently at each end of an inter-AS
link.
No MT Binding information is exchanged between the ASBRs by the
mechanisms defined in this document.
The configuration mechanism is implementation specific.
An implementation SHOULD verify that the configured Local MT-ID is
valid in the local IGP.
The advertising ASBR is not required to verify the existence or
configuration of the Remote MT-ID in the adjacent AS.
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Operators are responsible for coordinating the MT Binding
configuration at the two ends of an inter-AS link.
MT-ID 0 represents the default topology. A binding involving MT-ID 0
MAY therefore be advertised.
8. Security Considerations
This document does not change the security mechanisms of IS-IS or
OSPF.
Incorrect MT Binding information may cause topology-specific path
computation to select an unintended topology in an adjacent AS,
resulting in incorrect forwarding or path selection.
The authentication mechanisms applicable to the underlying IGP SHOULD
be used to protect the advertisements.
Operators SHOULD ensure that the configured MT Bindings are
consistent with the intended inter-AS topology.
9. Acknowledgement
TBD
10. IANA Considerations
10.1. IS-IS
IANA is requested to allocate a new sub-TLV from the "Sub-TLVs for
22, 23, 25, 141, 222 and 223" registry:
+=========+=====================+=====+=====+==============+
| Type | Name | 141 | MP | reference |
+=========+=====================+=====+=====+==============+
| TBD1 | Inter-AS MT Binding | Y | Y | This document|
+---------+---------------------+-----+-----+--------------+
10.2. OSPF
IANA is requested to allocate a new sub-TLV from the applicable OSPF
Traffic Engineering registry for use in the Link TLV of the Inter-AS-
TE-v2 and Inter-AS-TE-v3 LSAs:
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+=========+=====================+==============+
| Type | Name | reference |
+=========+=====================+==============+
| TBD2 | Inter-AS MT Binding | This document|
+---------+---------------------+--------------+
10.3. BGP-LS
IANA is requested to allocate a new TLV from the "BGP-LS Node
Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs"
registry:
+=========+=====================+==============+
| Type | Name | reference |
+=========+=====================+==============+
| TBD3 | Inter-AS MT Binding | This document|
+---------+---------------------+--------------+
11. Normative References
[I-D.ietf-idr-bgpls-inter-as-topology-ext]
Wang, A., Chen, H., Talaulikar, K., Zhuang, S., and C.
Lin, "BGP-LS Extensions for Inter-AS Topology Retrieval",
Work in Progress, Internet-Draft, draft-ietf-idr-bgpls-
inter-as-topology-ext-46, 28 September 2026,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-
bgpls-inter-as-topology-ext-46>.
[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>.
[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>.
[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120,
DOI 10.17487/RFC5120, February 2008,
<https://www.rfc-editor.org/info/rfc5120>.
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[RFC5392] Chen, M., Zhang, R., and X. Duan, "OSPF Extensions in
Support of Inter-Autonomous System (AS) MPLS and GMPLS
Traffic Engineering", RFC 5392, DOI 10.17487/RFC5392,
January 2009, <https://www.rfc-editor.org/info/rfc5392>.
[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>.
[RFC9346] Chen, M., Ginsberg, L., Previdi, S., and D. Xiaodong, "IS-
IS Extensions in Support of Inter-Autonomous System (AS)
MPLS and GMPLS Traffic Engineering", RFC 9346,
DOI 10.17487/RFC9346, February 2023,
<https://www.rfc-editor.org/info/rfc9346>.
[RFC9552] Talaulikar, K., Ed., "Distribution of Link-State and
Traffic Engineering Information Using BGP", RFC 9552,
DOI 10.17487/RFC9552, December 2023,
<https://www.rfc-editor.org/info/rfc9552>.
Author's Address
Aijun Wang
China Telecom
Beiqijia Town, Changping District
Beijing
102209
China
Email: wangaj3@chinatelecom.cn
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