Sub-interface VLAN YANG Data Models

Sub-interface VLAN YANG Data Models Cisco Systems

rwilton@cisco.com

Ericsson

scott.mansfield@ericsson.com

General Internet Engineering Task Force template This document defines YANG modules to add support for classifying traffic received on interfaces as Ethernet/VLAN framed packets to sub-interfaces based on the fields available in the Ethernet/VLAN frame headers. These modules allow configuration of Layer 3 and Layer 2 sub-interfaces (e.g. L2VPN attachment circuits) that can interoperate with IETF based forwarding protocols; such as IP and L3VPN services; or L2VPN services like VPWS, VPLS, and EVPN. The sub-interfaces also interoperate with VLAN tagged traffic originating from an IEEE 802.1Q compliant bridge. The model differs from an IEEE 802.1Q bridge model in that the configuration is interface/sub-interface based as opposed to being based on membership of an 802.1Q VLAN bridge. The YANG data models in this document conforms to the Network Management Datastore Architecture (NMDA) defined in RFC 8342.

Introduction This document defines two YANG 1.1 modules that augment the encapsulation choice YANG element defined in Interface Extensions YANG and the generic interfaces data model defined in . The two modules provide configuration nodes to support classification of Ethernet/VLAN traffic to sub-interfaces, that can have interface based feature and service configuration applied to them. The purpose of these models is to allow IETF defined forwarding protocols, for example, IPv6 , Ethernet Pseudo Wires and VPLS , when configured via appropriate YANG data models , to interoperate with VLAN tagged traffic received from an IEEE 802.1Q compliant bridge. In the case of layer 2 Ethernet services, the flexible encapsulation module also supports flexible rewriting of the VLAN tags contained in the frame header. For reference, a comparison between the sub-interface based YANG model documented in this draft and an IEEE 802.1Q bridge model is described in . In summary, the YANG modules defined in this internet draft are:

ietf-if-vlan-encapsulation.yang - Defines the model for basic classification of VLAN tagged traffic, normally to L3 packet forwarding services
ietf-if-flexible-encapsulation.yang - Defines the model for flexible classification of Ethernet/VLAN traffic, normally to L2 frame forwarding services

Terminology 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 when, and only when, they appear in all capitals, as shown here. The term 'sub-interface' is defined in section 2.6 of Interface Extensions YANG.

Note to RFC Editor This document uses two placeholder values throughout the document. Please replace them as follows and remove this note before publication. RFC AAAA, where AAAA is the number assigned to I-D.ietf-netmod-intf-ext-yang at the time of its publication. RFC BBBB, where BBBB is the number assigned to this document at the time of publication.

Tree Diagrams Tree diagrams used in this document follow the notation defined in .

Prefixes in Data Node Names In this document, names of data nodes and other data model objects are prefixed using the standard prefix associated with the corresponding YANG imported modules, as shown in . Prefixes for imported YANG modules

Prefix	YANG Module	Reference
if-vlan	ietf-if-vlan-encapsulation	This document
if-flex	ietf-if-flexible-encapsulation	This document
if-ext	ietf-if-extensions
if	ietf-interfaces
ianaift	iana-if-type
dot1q-types	ieee802-dot1q-types

Objectives The primary aim of the YANG modules contained in this draft is to provide the core model that is required to implement VLAN transport services on router based devices that is fully compatible with IEEE 802.1Q compliant bridges. A secondary aim is for the modules to be structured in such a way that they can be cleanly extended in the future.

Interoperability with IEEE 802.1Q compliant bridges The modules defined in this document are designed to fully interoperate with IEEE 802.1Q compliant bridges. In particular, the models are restricted to only matching, adding, or rewriting the 802.1Q VLAN tags in frames in ways that are compatible with IEEE 802.1Q compliant bridges.

Interface VLAN Encapsulation Model The Interface VLAN encapsulation model provides appropriate leaves for termination of an 802.1Q VLAN tagged segment to a sub-interface (or interface) based L3 service, such as IP. It allows for termination of traffic with one or two 802.1Q VLAN tags. The L3 service must be configured via a separate YANG data model, e.g., . A short example of configuring 802.1Q VLAN sub-interfaces with IP using YANG is provided in . The "ietf-if-vlan-encapsulation" YANG module has the following structure:

Interface Flexible Encapsulation Model The Interface Flexible Encapsulation model is designed to allow for the flexible provisioning of layer 2 services. It provides the capability to classify and demultiplex Ethernet/VLAN frames received on an Ethernet trunk interface to sub-interfaces based on the fields available in the layer 2 headers. Once classified to sub-interfaces, it provides the capability to selectively modify fields within the layer 2 frame header before the frame is handed off to the appropriate forwarding code for further handling. The forwarding instance, e.g., L2VPN, VPLS, etc., is configured using a separate YANG configuration model defined elsewhere. The model supports a common core set of layer 2 header matches based on the 802.1Q tag type and VLAN Ids contained within the header up to a tag stack depth of two tags. When there are two tags, there is an outer tag and a second-tag (inner). The model supports flexible rewrites of the layer 2 frame header for data frames as they are processed on the interface. It defines a set of standard tag manipulations that allow for the insertion, removal, or rewrite of one or two 802.1Q VLAN tags. The expectation is that manipulations are generally implemented in a symmetrical fashion, i.e. if a manipulation is performed on ingress traffic on an interface then the reverse manipulation is always performed on egress traffic out of the same interface. However, the model also allows for asymmetrical rewrites, which may be required to implement some forwarding models (such as E-Tree). The model also allows a flexible encapsulation and rewrite to be configured directly on an Ethernet or LAG interface without configuring separate child sub-interfaces. Ingress frames that do not match the encapsulation are dropped and counted against the "in-discard-unknown-encaps" counter specified in . Egress frames MUST conform to the encapsulation. The final aim for the model design is for it to be cleanly extensible to add in additional match and rewrite criteria of the layer 2 header, such as matching on the source or destination MAC address, PCP or DEI fields in the 802.1Q tags, or the EtherType of the frame payload. Rewrites can also be extended to allow for modification of other fields within the layer 2 frame header. In summary, the model provides a binding that defines how services are bound to an interface or sub-interface providing a mechanism to pass traffic to/from interface/sub-interface to a service. A short example of configuring 802.1Q VLAN sub-interfaces with L2VPN using YANG is provided in . The "ietf-if-flexible-encapsulation" YANG module has the following structure:

VLAN Encapsulation YANG Module This YANG module augments the 'encapsulation' container defined in ietf-if-extensions.yang. It also contains references to , , and . WG List: Editors: Robert Wilton Scott Mansfield "; description "This YANG module models configuration to classify IEEE 802.1Q VLAN tagged Ethernet traffic by exactly matching the tag type and VLAN identifier of one or two 802.1Q VLAN tags in the frame. Copyright (c) 2025 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). This version of this YANG module is part of RFC BBBB (https://www.rfc-editor.org/info/rfcBBBB); see the RFC itself for full legal notices. 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 (RFC 2119) (RFC 8174) when, and only when, they appear in all capitals, as shown here."; revision 2025-09-03 { description "Initial version of the module"; reference "RFC BBBB: Sub-interface VLAN YANG Data Models"; } augment "/if:interfaces/if:interface/if-ext:encapsulation/" + "if-ext:encaps-type" { when "derived-from-or-self(../if:type, 'ianaift:ethernetCsmacd') or derived-from-or-self(../if:type, 'ianaift:ieee8023adLag') or derived-from-or-self(../if:type, 'ianaift:l2vlan')" { description "Applies only to Ethernet-like interfaces and sub-interfaces."; } description "Augment the generic interface encapsulation with basic 802.1Q VLAN tag classifications"; case dot1q-vlan { container dot1q-vlan { description "Classifies 802.1Q VLAN tagged Ethernet frames to a sub-interface (or interface) by exactly matching the number of tags, tag type(s) and VLAN identifier(s). Only frames matching the classification configured on a sub-interface/interface are processed on that sub-interface/interface. Frames that do not match any sub-interface are processed directly on the parent interface, if it is associated with a forwarding instance, otherwise they are dropped."; container outer-tag { must 'tag-type = "dot1q-types:s-vlan" or ' + 'tag-type = "dot1q-types:c-vlan"' { error-message "Only C-VLAN and S-VLAN tags can be matched."; description "For IEEE 802.1Q interoperability, only C-VLAN and S-VLAN tags are matched."; } description "Specifies the VLAN tag values to match against the outermost (first) 802.1Q VLAN tag in the frame."; uses dot1q-types:dot1q-tag-classifier-grouping; } container second-tag { must '../outer-tag/tag-type = "dot1q-types:s-vlan" and ' + 'tag-type = "dot1q-types:c-vlan"' { error-message "When matching two 802.1Q VLAN tags, the outermost (first) tag in the frame MUST be specified and be of S-VLAN type and the second tag in the frame must be of C-VLAN tag type."; description "For IEEE 802.1Q interoperability, when matching two 802.1Q VLAN tags, it is REQUIRED that the outermost tag exists and is an S-VLAN, and the second tag is a C-VLAN."; } presence "Classify frames that have two 802.1Q VLAN tags."; description "Specifies the VLAN tag values to match against the second outermost 802.1Q VLAN tag in the frame."; uses dot1q-types:dot1q-tag-classifier-grouping; } } } } } ]]>

Flexible Encapsulation YANG Module This YANG module augments the 'encapsulation' container defined in ietf-if-extensions.yang. This YANG module also augments the 'interface' list entry defined in . It also contains references to , and . WG List: Editors: Robert Wilton Scott Mansfield "; description "This YANG module describes interface configuration for flexible classification and rewrites of IEEE 802.1Q VLAN tagged Ethernet traffic. Copyright (c) 2025 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). This version of this YANG module is part of RFC BBBB (https://www.rfc-editor.org/info/rfcBBBB); see the RFC itself for full legal notices. 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 (RFC 2119) (RFC 8174) when, and only when, they appear in all capitals, as shown here."; revision 2025-09-03 { description "Initial version of the module"; reference "RFC BBBB: Sub-interface VLAN YANG Data Models"; } feature flexible-rewrites { description "This feature indicates that the network element supports specifying flexible rewrite operations."; } feature asymmetric-rewrites { description "This feature indicates that the network element supports specifying different rewrite operations for the ingress rewrite operation and egress rewrite operation."; } feature dot1q-tag-rewrites { description "This feature indicates that the network element supports the flexible rewrite functionality specifying 802.1Q tag rewrites."; } grouping flexible-match { description "Represents a flexible frame classification: The rules for a flexible match are: 1. Match-type: default, untagged, priority tag, or tag stack. 2. Each tag in the stack of tags matches: a. tag type (802.1Q or 802.1ad) + b. tag value: i. single tag ii. set of tag ranges/values. iii. 'any' keyword"; choice match-type { mandatory true; description "Provides a choice of how the frames may be matched"; case default { description "Default match"; leaf default { type empty; description "Default match. Matches all traffic not matched to any other peer sub-interface by a more specific encapsulation."; } } case untagged { description "Match untagged Ethernet frames only"; leaf untagged { type empty; description "Untagged match. Matches all untagged traffic."; } } case dot1q-priority-tagged { description "Match 802.1Q priority tagged Ethernet frames only"; container dot1q-priority-tagged { description "802.1Q priority tag match"; leaf tag-type { type dot1q-types:dot1q-tag-type; mandatory true; description "The 802.1Q tag type of matched priority tagged packets"; } } } case dot1q-vlan-tagged { container dot1q-vlan-tagged { description "Matches VLAN tagged frames"; container outer-tag { must 'tag-type = "dot1q-types:s-vlan" or ' + 'tag-type = "dot1q-types:c-vlan"' { error-message "Only C-VLAN and S-VLAN tags can be matched."; description "For IEEE 802.1Q interoperability, only C-VLAN and S-VLAN tags can be matched."; } description "Classifies traffic using the outermost (first) VLAN tag on the frame."; uses "dot1q-types:" + "dot1q-tag-ranges-or-any-classifier-grouping"; } container second-tag { must '../outer-tag/tag-type = "dot1q-types:s-vlan" and ' + 'tag-type = "dot1q-types:c-vlan"' { error-message "When matching two tags, the outermost (first) tag must be specified and of S-VLAN type and the second outermost tag must be of C-VLAN tag type."; description "For IEEE 802.1Q interoperability, when matching two tags, it is required that the outermost (first) tag exists and is an S-VLAN, and the second outermost tag is a C-VLAN."; } presence "Also classify on the second VLAN tag."; description "Classifies traffic using the second outermost VLAN tag on the frame."; uses "dot1q-types:" + "dot1q-tag-ranges-or-any-classifier-grouping"; } leaf match-exact-tags { type empty; description "If set, indicates that all 802.1Q VLAN tags in the Ethernet frame header must be explicitly matched, i.e. the EtherType following the matched tags must not be a 802.1Q tag EtherType. If unset then extra 802.1Q VLAN tags are allowed."; } } } } } grouping dot1q-tag-rewrite { description "Flexible rewrite grouping. Can be either be expressed symmetrically, or independently in the ingress and/or egress directions."; leaf pop-tags { type uint8 { range "1..2"; } description "The number of 802.1Q VLAN tags to pop, or translate if used in conjunction with push-tags. Popped tags are the outermost tags on the frame."; } container push-tags { presence "802.1Q tags are pushed or translated"; description "The 802.1Q tags to push on the front of the frame, or translate if configured in conjunction with pop-tags."; container outer-tag { must 'tag-type = "dot1q-types:s-vlan" or ' + 'tag-type = "dot1q-types:c-vlan"' { error-message "Only C-VLAN and S-VLAN tags can be pushed."; description "For IEEE 802.1Q interoperability, only C-VLAN and S-VLAN tags can be pushed."; } description "The outermost (first) VLAN tag to push onto the frame."; uses dot1q-types:dot1q-tag-classifier-grouping; } container second-tag { must '../outer-tag/tag-type = "dot1q-types:s-vlan" and ' + 'tag-type = "dot1q-types:c-vlan"' { error-message "When pushing/rewriting two tags, the outermost tag must be specified and of S-VLAN type and the second outermost tag must be of C-VLAN tag type."; description "For IEEE 802.1Q interoperability, when pushing two tags, it is required that the outermost tag exists and is an S-VLAN, and the second outermost tag is a C-VLAN."; } presence "In addition to the first tag, also push/rewrite a second VLAN tag."; description "The second outermost VLAN tag to push onto the frame."; uses dot1q-types:dot1q-tag-classifier-grouping; } } } grouping flexible-rewrite { description "Grouping for flexible rewrites of fields in the L2 header. Restricted to flexible 802.1Q VLAN tag rewrites, but could be extended to cover rewrites of other fields in the L2 header in future."; container dot1q-tag-rewrite { if-feature "dot1q-tag-rewrites"; description "802.1Q VLAN tag rewrite. Translate operations are expressed as a combination of tag push and pop operations. E.g., translating the outer tag is expressed as popping a single tag, and pushing a single tag. 802.1Q tags that are translated SHOULD preserve the PCP and DEI fields unless if a different QoS behavior has been specified."; uses dot1q-tag-rewrite; } } augment "/if:interfaces/if:interface/if-ext:encapsulation/" + "if-ext:encaps-type" { when "derived-from-or-self(../if:type, 'ianaift:ethernetCsmacd') or derived-from-or-self(../if:type, 'ianaift:ieee8023adLag') or derived-from-or-self(../if:type, 'ianaift:l2vlan')" { description "Applies only to Ethernet-like interfaces and sub-interfaces."; } description "Augment the generic interface encapsulation with flexible match and rewrite for VLAN sub-interfaces."; case flexible { description "Flexible encapsulation and rewrite"; container flexible { description "Flexible encapsulation allows for the matching of ranges and sets of 802.1Q VLAN Tags and performing rewrite operations on the VLAN tags. The structure is also designed to be extended to allow for matching/rewriting other fields within the L2 frame header if required."; container match { description "Flexibly classifies Ethernet frames to a sub-interface (or interface) based on the L2 header fields. Only frames matching the classification configured on a sub-interface/interface are processed on that sub-interface/interface. Frames that do not match any sub-interface are processed directly on the parent interface, if it is associated with a forwarding instance, otherwise they are dropped. If a frame could be classified to multiple sub-interfaces then they get classified to the sub-interface with the most specific match. E.g., matching two VLAN tags in the frame is more specific than matching the outermost VLAN tag, which is more specific than the catch-all 'default' match."; uses flexible-match; } container rewrite { if-feature "flexible-rewrites"; description "L2 frame rewrite operations. Rewrites allows for modifications to the L2 frame header as it transits the interface/sub-interface. Examples include adding a VLAN tag, removing a VLAN tag, or rewriting the VLAN Id carried in a VLAN tag."; choice direction { description "Whether the rewrite policy is symmetrical or asymmetrical."; case symmetrical { container symmetrical { uses flexible-rewrite; description "Symmetrical rewrite. Expressed in the ingress direction, but the reverse operation is applied to egress traffic. E.g., if a tag is pushed on ingress traffic, then the reverse operation is a 'pop 1', that is performed on traffic egressing the interface, so a peer device sees a consistent L2 encapsulation for both ingress and egress traffic."; } } case asymmetrical { if-feature "asymmetric-rewrites"; description "Asymmetrical rewrite. Rewrite operations may be specified in only a single direction, or different rewrite operations may be specified in each direction."; container ingress { uses flexible-rewrite; description "A rewrite operation that only applies to ingress traffic. Ingress rewrite operations are performed before the frame is subsequently processed by the forwarding operation."; } container egress { uses flexible-rewrite; description "A rewrite operation that only applies to egress traffic."; } } } } container local-traffic-default-encaps { presence "A local traffic default encapsulation has been specified."; description "Specifies the 802.1Q VLAN tags to use by default for locally sourced traffic from the interface. Used for encapsulations that match a range of VLANs (or 'any'), where the source VLAN Ids are otherwise ambiguous."; container outer-tag { must 'tag-type = "dot1q-types:s-vlan" or ' + 'tag-type = "dot1q-types:c-vlan"' { error-message "Only C-VLAN and S-VLAN tags can be matched."; description "For IEEE 802.1Q interoperability, only C-VLAN and S-VLAN tags can be matched."; } description "The outermost (first) VLAN tag for locally sourced traffic."; uses dot1q-types:dot1q-tag-classifier-grouping; } container second-tag { must '../outer-tag/tag-type = "dot1q-types:s-vlan" and ' + 'tag-type = "dot1q-types:c-vlan"' { error-message "When specifying two tags, the outermost (first) tag must be specified and of S-VLAN type and the second outermost tag must be of C-VLAN tag type."; description "For IEEE 802.1Q interoperability, when specifying two tags, it is required that the outermost (first) tag exists and is an S-VLAN, and the second outermost tag is a C-VLAN."; } presence "Indicates existence of a second outermost VLAN tag."; description "The second outermost VLAN tag for locally sourced traffic."; uses dot1q-types:dot1q-tag-classifier-grouping; } } } } } } ]]>

Examples The following sections give examples of configuring a sub-interface supporting L3 forwarding, and a sub-interface being used in conjunction with an L2VPN.

Layer 3 sub-interfaces with IPv6 This example illustrates two layer sub-interfaces, 'eth0.1' and 'eth0.2', both are child interfaces of the Ethernet interface 'eth0'. 'eth0.1' is configured to match traffic with two VLAN tags: an outer S-VLAN of 10 and an inner C-VLAN of 20. 'eth0.2' is configured to match traffic with a single S-VLAN tag, with VLAN Id 11.

Layer 2 sub-interfaces with L2VPN This example illustrates a layer 2 sub-interface 'eth0.3' configured to match traffic with an S-VLAN tag of 10, and a C-VLAN tag of 21; and remove the outer tag (S-VLAN 10) before the traffic is passed off to the L2VPN service. It also illustrates another sub-interface 'eth1.0' under a separate physical interface configured to match traffic with a C-VLAN of 50, with the tag removed before traffic is given to any service. Sub-interface 'eth1.0' is not currently bound to any service and hence traffic classified to that sub-interface is dropped.

Acknowledgements The authors would particularly like to thank Benoit Claise, John Messenger, Glenn Parsons, and Dan Romascanu for their help progressing this draft. The authors would also like to thank Martin Bjorklund, Alex Campbell, Don Fedyk, Eric Gray, Giles Heron, Marc Holness, Iftekhar Hussain, Neil Ketley, William Lupton, John Messenger, Glenn Parsons, Ludwig Pauwels, Joseph White, Vladimir Vassilev, David Ball and members of the IEEE 802.1 WG for their helpful reviews and feedback on this draft.

IANA Considerations

YANG Module Registrations The following YANG modules are requested to be registered in the IANA "YANG Module Names" registry: The ietf-if-vlan-encapsulation module:

Name: ietf-if-vlan-encapsulation
XML Namespace: urn:ietf:params:xml:ns:yang:ietf-if-vlan-encapsulation
Prefix: if-vlan
Reference: RFCXXXX

The ietf-if-flexible-encapsulation module:

Name: ietf-if-flexible-encapsulation
XML Namespace: urn:ietf:params:xml:ns:yang:ietf-if-flexible-encapsulation
Prefix: if-flex
Reference: RFCXXXX

This document registers two URIs in the "IETF XML Registry" . Following the format in RFC 3688, the following registrations have been made.

URI: urn:ietf:params:xml:ns:yang:ietf-if-vlan-encapsulation
Registrant Contact: The IESG.
XML: N/A, the requested URI is an XML namespace.

URI: urn:ietf:params:xml:ns:yang:ietf-if-flexible-encapsulation
Registrant Contact: The IESG.
XML: N/A, the requested URI is an XML namespace.

Security Considerations The "ietf-if-flexible-encapsulation" and "ietf-if-vlan-encapsulation" YANG modules define data models that are designed to be accessed via YANG-based management protocols, such as NETCONF and RESTCONF . These protocols have to use a secure transport layer (e.g., SSH , TLS , and QUIC ) and have to use mutual authentication. The Network Configuration Access Control Model (NACM) 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 these YANG modules that are writable/creatable/deletable (i.e. config true, which is the default). All writable data nodes are likely to be reasonably 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 following subtrees and data nodes have particular sensitivities/vulnerabilities: There are no particularly sensitive writable data nodes. Some of the readable data nodes in this YANG module may be considered sensitive or vulnerable in some network environments. It is thus important to control read access (e.g., via get, get-config, or notification) to these data nodes. Specifically, the following subtrees and data nodes have particular sensitivities/vulnerabilities: There are no particularly sensitive readable data nodes.

References Normative References Informative References

Comparison with the IEEE 802.1Q Configuration Model In addition to the sub-interface based YANG model proposed here, the IEEE 802.1Q working group has developed a YANG model for the configuration of 802.1Q VLANs. This raises the valid question as to whether the models overlap and whether it is necessary or beneficial to have two different models for superficially similar constructs. This section aims to answer that question by summarizing and comparing the two models.

Sub-interface based configuration model overview The key features of the sub-interface based configuration model can be summarized as:

The model is primarily designed to enable layer 2 and layer 3 services on Ethernet interfaces that can be defined in a very flexible way to meet the varied requirements of service providers.
Traffic is classified from an Ethernet-like interface to sub-interfaces based on fields in the layer 2 header. This is often based on VLAN Ids contained in the frame, but the model is extensible to other arbitrary fields in the frame header.
Sub-interfaces are just a type of if:interface and hence support any feature configuration YANG models that can be applied generally to interfaces. For example, QoS or ACL models that reference if:interface can be applied to the sub-interfaces, or the sub-interface can be used as an Access Circuit in L2VPN or L3VPN models that reference if:interface.
In the sub-interface based configuration model, the classification of traffic arriving on an interface to a given sub-interface, based on fields in the layer 2 header, is completely independent of how the traffic is forwarded. The sub-interface can be referenced (via references to if:interface) by other models that specify how traffic is forwarded; thus sub-interfaces can support multiple different forwarding paradigms, including but not limited to: layer 3 (IPv4/IPv6), layer 2 pseudowires (over MPLS or IP), VPLS instances, EVPN instance.
The model is flexible in the scope of the VLAN Identifier space. I.e. by default VLAN Ids can be scoped locally to a single Ethernet-like trunk interface, but the scope is determined by the forwarding paradigm that is used.

IEEE 802.1Q Bridge Configuration Model Overview The key features of the IEEE 802.1Q bridge configuration model can be summarized as:

Each VLAN bridge component has a set of Ethernet interfaces that are members of that bridge. Sub-interfaces are not used, nor required in the 802.1Q bridge model.
Within a VLAN bridge component, the VLAN tag in the packet is used, along with the destination MAC address, to determine how to forward the packet. Other forwarding paradigms are not supported by the 802.1Q model.
Classification of traffic to a VLAN bridge component is based only on the Ethernet interface that it arrived on.
VLAN Identifiers are scoped to a VLAN bridge component. Often devices only support a single bridge component and hence VLANs are scoped globally within the device.
Feature configuration is specified in the context of the bridge, or particular VLANs on a bridge.

Possible Overlap Between the Two Models Both models can be used for configuring similar basic layer 2 forwarding topologies. The 802.1Q bridge configuration model is optimized for configuring Virtual LANs that span across enterprises and data centers. The sub-interface model can also be used for configuring equivalent Virtual LAN networks that span across enterprises and data centers, but often requires more configuration to be able to configure the equivalent constructs to the 802.1Q bridge model. The sub-interface model really excels when implementing flexible L2 and L3 services, where those services may be handled on the same physical interface, and where the VLAN Identifier is being solely used to identify the customer or service that is being provided rather than a Virtual LAN. The sub-interface model provides more flexibility as to how traffic can be classified, how features can be applied to traffic streams, and how the traffic is to be forwarded. Conversely, the 802.1Q bridge model can also be used to implement L2 services in some scenarios, but only if the forwarding paradigm being used to implement the service is the native Ethernet forwarding specified in 802.1Q - other forwarding paradigms such as pseudowires or VPLS are not supported. The 802.1Q bridge model does not implement L3 services at all, although this can be partly mitigated by using a virtual L3 interface construct that is a separate logical Ethernet-like interface which is a member of the bridge. In conclusion, it is valid for both of these models to exist since they have different deployment scenarios for which they are optimized. Devices may choose which of the models (or both) to implement depending on what functionality the device is being designed for.