Advertising SRv6 SIDs for Layer 2 Bundle Member Links in IGP

There are deployments where the Layer-3 interface on which an IGP adjacency is established is a Layer-2 interface bundle, for instance, a Link Aggregation Group (LAG) . This reduces the number of adjacencies that need to be maintained by the routing protocol in cases where there are parallel links between the neighbors. Entities external to IS-IS such as Path Computation Elements (PCEs) may wish to control traffic flows on individual members of the underlying Layer-2 bundle. In order to do so, link attribute information about individual bundle members is required. and specify the IGP extensions to advertise link attribute information for each of the L2 Bundle members which comprise the Layer-3 interface, in which the encoding and advertisement of SR-MPLS adjacency SIDs of each bundle member link is defined. When SRv6 is used with Layer-2 interface bundle, to control traffic flows on the individual member links, the SRv6 SIDs which represent the Layer-2 member links of the L2 bundle needs to be advertised. This document proposes the IGP extensions to advertise the SRv6 SIDs of the L2 bundle member links.

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 BCP14 RFC 2119 RFC 8174 when, and only when, they appear in all capitals, as shown here.

As defined in , the SRv6 End.X behavior means "Endpoint with cross-connect to an array of layer-3 adjacencies". An SRv6 SID associated with a Layer 2 bundle member link of an L3 adjacency can be considered as an instance of the End.X SID, which identifies a subset of the L3 adjacency. Thus separate End.X SID can be allocated for each L2 bundle member link.

This section defines the IS-IS sub-TLVs to advertise SRv6 SIDs for L2 Bundle Members. Following the encoding mechanism used in for the advertisement of SR-MPLS adj-SIDs for L2 Bundle Members, this allows the advertisement of a set of SRv6 End.X SIDs (one per L2 Bundle Member) in a single sub-TLV.

defines the SRv6 End.X SID sub-TLV to advertise an SRv6 SID associated with a point-to-point adjacency. This document defines a new sub-TLV called "L2 Bundle Member SRv6 End.X SIDs" to advertise the SRv6 End.X SIDs for the L2 Bundle Members. This sub-TLV is allowed to be carried in TLV 25 "L2 Bundle Member Attributes" only. The format of the sub-TLV is as below:

Where: Type: TBA. Length: Variable. Flags: 1 octet. The format is the same as the Flags field in the SRv6 End.X SID sub-TLV as defined in . Algorithm: 1 octet. The algorithm values are defined in the IGP Algorithm Type registry. Weight: 1 octet. The value represents the weight of the SID for the purpose of load balancing. Endpoint Behavior: 2 octets. It SHOULD be set to the value of End.X behavior. SIDs. One or multiple 16-octet SRv6 SIDs. The number of the SIDs is determined by the "Number of L2 Bundle Member Descriptors" field as defined in . Sub-sub-tlv-length: 1 octet. Number of octets used by sub-sub-TLVs. Optional Sub-sub-TLVs: Supported sub-sub-TLVs are specified in Section 11.6 of . Multiple L2 Bundle Member SRv6 End.X SID Sub-TLV may be required in order to advertise all the SRv6 End.X SIDs associated with the member links of an L2 bundle.

defines the SRv6 LAN End.X SID sub-TLV to advertise an SRv6 SID associated with a LAN adjacency. This document defines a new sub-TLV called "L2 Bundle Member SRv6 LAN End.X SIDs" to advertise the SRv6 LAN End.X SIDs for the L2 Bundle Members. The format of the sub-TLV is as below:

Where: Type: TBA. Length: Variable. Neighbor System-ID: IS-IS System-ID of length "ID Length" as defined in . Flags: 1 octet, the format is the same as the Flags field in the SRv6 End.X SID sub-TLV as defined in . Algorithm: 1 octet. The algorithm values are defined in the IGP Algorithm Type registry. Weight: 1 octet. The value represents the weight of the End.X SID for the purpose of load balancing. Endpoint Behavior: 2 octets. It SHOULD be set to the value of End.X behavior. SIDs. One or multiple 16-octet SRv6 SIDs. The number of the SIDs is determined by the "Number of L2 Bundle Member Descriptors" field as defined in . Sub-sub-tlv-length: 1 octet. Number of octets used by sub-sub-TLVs. Optional Sub-sub-TLVs: Supported sub-sub-TLVs are specified in Section 11.6 of .

This section describes the mechanism to advertise SRv6 SIDs for L2 Bundle Members in OSPF. For SR-MPLS data plane, as described in , the SR-MPLS Adjacency SID sub-TLVs for the L3 adjacency is used for the advertisement of Adjacency SIDs of L2 Bundle Members. And for OSPFv3, the L2 Bundle Member Attributes sub-TLV is an optional sub-TLV of the Router-Link TLV of the OSPFv3 E-Router-LSA . For SRv6 data plane, the SRv6 related sub-TLVs as defined in can be used under the L2 Bundle Member Attributes sub-TLV for the advertisement of SRv6 SIDs of the L2 bundle members in OSPFv3. Table 1 lists the SRv6 related sub-TLVs that are applicable to the Router-Link TLV and their applicability for L2 bundle member links.

IANA is requested to assign two new code points from the IS-IS "Sub-TLVs for TLVs 22, 23, 25, 141, 222, and 223" registry.

In the "OSPFv3 Extended- LSA Sub-TLVs" registry, IANA has already set the L2BM flag for the code points 30, 31 and 32 to "Y", per the request of .

The security considerations in and apply. This document introduces no additional security vulnerabilities to IS-IS and OSPF.

The authors would like to thank Zhenbin Li for the review and discussion of this document.