| Internet-Draft | inter-AS-MT-Binding | October 2026 |
| Wang | Expires 17 April 2027 | [Page] |
- Workgroup:
- Network Working Group
- Internet-Draft:
- draft-wang-lsr-inter-as-mt-binding-00
- Published:
- Intended Status:
- Standards Track
- Expires:
IS-IS and OSPF Extensions for Inter-AS Multi-Topology Binding
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.¶
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.¶
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:¶
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..¶
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
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.¶
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.¶
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.¶
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.¶
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:¶
+=========+=====================+==============+ | 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, , <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, , <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, , <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, , <https://www.rfc-editor.org/info/rfc5120>.
- [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, , <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, , <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, , <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, , <https://www.rfc-editor.org/info/rfc9552>.