Abstract

There are deployments where the Layer 3 (L3) interface on which OSPF operates is a Layer 2 (L2) interface bundle. Existing OSPF advertisements only support advertising link attributes of the L3 interface. If entities external to OSPF wish to control traffic flows on the individual physical links that comprise the L2 interface bundle, link attribute information for the bundle members is required.¶

This document defines the protocol extensions for OSPF to advertise the link attributes of L2 bundle members. The document also specifies the advertisement of these OSPF extensions via the Border Gateway Protocol - Link State (BGP-LS) and thereby updates RFC 9085.¶

1. Introduction

There are deployments where the L3 interface on which an OSPF adjacency is established is a L2 interface bundle, for instance, a Link Aggregation Group (LAG) [IEEE802.1AX]. This reduces the number of adjacencies that need to be maintained by the OSPF protocol in cases where there are parallel links between the neighbors. Entities external to OSPF such as Path Computation Elements (PCEs) [RFC4655] may wish to control traffic flows on individual L2 member links of the underlying bundle interface (e.g., LAG). To do so, link attribute information for individual bundle members is required. The protocol extensions defined in this document provide the means to advertise this information.¶

This document defines sub-TLVs to advertise link attribute information for each of the L2 bundle members that comprise the L3 interface on which OSPF operates. Similar capabilities were introduced for IS-IS in [RFC8668].¶

[RFC8665] and [RFC8666] introduced the Adjacency Segment Identifier (Adj-SID) link attribute for OSPFv2 and OSPFv3, respectively, which can be used as an instruction to forward traffic over a specific link [RFC8402]. This document enables the advertisement of the Adj-SIDs using the same Adj-SID sub-TLV at the granularity level of each L2 bundle member link so that traffic may be steered over that specific member link.¶

Note that the advertisements at the L2 bundle member link level defined in this document are intended to be provided to entities external to OSPF and do not alter or change the OSPF route computation. The following items are intentionally not defined in and are outside the scope of this document:¶

• What link attributes will be advertised. This is determined by the needs of the external entities.¶
• A minimum or default set of link attributes.¶
• How these attributes are configured.¶
• How the advertisements are used.¶
• What impact the use of these advertisements may have on traffic flow in the network.¶
• How the advertisements are passed to external entities.¶

BGP Link State (BGP-LS) [RFC7752] was extended for the advertisement of L2 bundle members and their attributes in [RFC9085], which covered only IS-IS. This document updates [RFC9085] by specifying the advertisement from OSPF (refer to Section 3).¶

2. L2 Bundle Member Attributes

A new L2 Bundle Member Attributes sub-TLV is introduced to advertise L2 bundle member attributes in both OSPFv2 and OSPFv3. In the case of OSPFv2, this sub-TLV is an optional sub-TLV of the OSPFv2 Extended Link TLV that is used to describe link attributes via the OSPFv2 Extended Link Opaque LSA (Link State Advertisement) [RFC7684]. In the case of OSPFv3, this sub-TLV is an optional sub-TLV of the Router-Link TLV of the OSPFv3 E-Router-LSA [RFC8362].¶

When the OSPF adjacency is associated with an L2 bundle interface, this sub-TLV is used to advertise the underlying L2 bundle member links along with their respective link attributes. The inclusion of this information implies that the identified link is a member of the L2 bundle associated with an OSPF L3 link and that the member link is operationally up. Therefore, advertisements of member links MUST NOT be done when the member link becomes operationally down or is no longer a member of the identified L2 bundle.¶

The advertisement of the L2 Bundle Member Attributes sub-TLV may be asymmetric for an OSPF link, depending on the underlying L2 connectivity, i.e., advertised by the router on only one end.¶

The L2 Bundle Member Attributes 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               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   L2 Bundle Member Descriptor                 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           Member Link Attribute sub-TLVs (variable)          //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Where:¶

Type:

24 for OSPFv2 and 29 for OSPFv3¶

Length:

The total length (in octets) of the value portion of the TLV including nested sub-TLVs.¶

L2 Bundle Member Descriptor:

A 4-octet link-local identifier for the member link. This identifier is described as "link local identifier" in [RFC4202] and used as "Local Interface ID" in [RFC8510].¶

Link attributes for L2 bundle member links are advertised as sub-TLVs of the L2 Bundle Member Attributes sub-TLV.¶

In the case of OSPFv2, the L2 Bundle Member Attributes sub-TLV shares the sub-TLV space of the Extended Link TLV, and the sub-TLVs of the Extended Link TLV MAY be used to describe the attributes of the member link. Table 1 lists sub-TLVs and their applicability for L2 bundle member links. The sub-TLVs that are not applicable MUST NOT be used as sub-TLVs for the L2 Bundle Member Attributes sub-TLV. Specifications that introduce new sub-TLVs of the Extended Link TLV MUST indicate their applicability to the L2 Bundle Member Attributes sub-TLV. Typically, attributes that have L3 semantics would not be applicable, but L2 attributes would apply. An implementation MUST ignore any sub-TLVs received that are not applicable in the context of the L2 Bundle Member Attributes sub-TLV.¶

Applicability:¶

Y:

This sub-TLV MAY appear in the L2 Bundle Member Attributes sub-TLV.¶

N:

This sub-TLV MUST NOT appear in the L2 Bundle Member Attributes sub-TLV.¶

In the case of OSPFv3, the L2 Bundle Member Attributes sub-TLV shares the sub-TLV space of the Router-Link TLV, and the sub-TLVs of the Router-Link TLV MAY be used to describe the attributes of the member link. Table 2 lists sub-TLVs that are applicable to the Router-Link TLV and their applicability for L2 bundle member links. The sub-TLVs that are not applicable MUST NOT be used as sub-TLVs for the L2 Bundle Member Attributes sub-TLV. Specifications that introduce new sub-TLVs of the Router-Link TLV MUST indicate their applicability to the L2 Bundle Member Attributes sub-TLV. An implementation MUST ignore any sub-TLVs received that are not applicable in the context of the L2 Bundle Member Attributes sub-TLV.¶

Applicability:¶

Y:

This sub-TLV MAY appear in the L2 Bundle Member Attributes sub-TLV.¶

N:

This sub-TLV MUST NOT appear in the L2 Bundle Member Attributes sub-TLV.¶

X:

This is not a sub-TLV of the Router-Link TLV; it MUST NOT appear in the L2 Bundle Member Attributes sub-TLV.¶

7. References