Advertising Layer 2 Bundle Member Link Attributes in IS-IS

There are deployments where the Layer 3 interface on which an IS-IS 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. The protocol extensions defined in this document provide the means to advertise this information. This document introduces a new TLV to advertise link attribute information for each of the L2 Bundle Members that comprise the Layer 3 interface on which IS-IS operates. introduces a new link attribute, adjacency segment identifier (Adj-SID), which can be used as an instruction to forwarding to send traffic over a specific link. This document introduces additional sub-TLVs to advertise Adj-SIDs for L2 Bundle Members. Note that the new advertisements defined in this document are intended to be provided to external (to IS-IS) entities. The following items are intentionally not defined and/or 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.

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.

A new TLV is introduced to advertise L2 Bundle Member attributes. Although much of the information is identical to and uses the same sub-TLVs included in Extended IS-Neighbor advertisements (TLVs 22 and 222), a new TLV is used so that changes to the advertisement of the L2 Bundle Member link attributes do not trigger unnecessary action by the Decision Process. Advertisement of this information implies that the identified link is a member of the L2 Bundle associated with the identified Parent L3 Neighbor and that the member link is operationally up. Therefore, advertisements MUST be withdrawn if the link becomes operationally down or it is no longer a member of the identified L2 Bundle. This new TLV utilizes the sub-TLV space defined for TLVs 22, 23, 141, 222, and 223. The following new TLV is introduced:

L2 Bundle Member Attributes
- Type:
  25
  
  Length:
  Number of octets to follow

Parent L3 Neighbor Descriptor
- L3 Neighbor System ID + pseudonode ID (7 octets)
  
  Flags: 1-octet field of the following flags:
  - where:
  - P-flag:
    When set to 1, one of the sub-TLVs described in Section 3.1 immediately follows the flags field. If the P-flag is set to 0, then none of the sub-TLVs described in Section 3.1 are present.
    
    Other bits:
    MUST be zero when originated and ignored when received.
One or more L2 Bundle Attribute Descriptors (as defined below).
- Length of L2 Bundle Attribute Descriptor (1 octet)
  - NOTE: This includes all fields described below.
- Number of L2 Bundle Member Descriptors (1 octet)
- L2 Bundle Member Link Local Identifiers (4 * Number of L2 Bundle Member Descriptors octets)
  - NOTE: An L2 Bundle Member Descriptor is a Link Local Identifier as defined in .
- Sub-TLV(s)
  A sub-TLV may define an attribute common to all of the bundle members listed, or it may define an attribute unique to each bundle member. Use of these two classes of sub-TLVs is described in the following sections.

NOTE: Only one Parent L3 Neighbor Descriptor is present in a given TLV. Multiple L2 Bundle Attribute Descriptors may be present in a single TLV.

When there exist multiple L3 adjacencies to the same neighbor, additional information is required to uniquely identify the L3 Neighbor. One and only one of the following three sub-TLVs is used to uniquely identify the L3 adjacency: IPv4 Interface Address (sub-TLV 6 defined in ) IPv6 Interface Address (sub-TLV 12 defined in ) Link Local/Remote Identifiers (sub-TLV 4 defined in ) When the P-flag is set in the flags field in the Parent L3 Neighbor Descriptor, one and only one of the above sub-TLVs MUST be present. The chosen sub-TLV MUST immediately follow the flags field described in . These sub-TLVs MAY be omitted if no parallel adjacencies to the neighbor exist.

These sub-TLVs advertise a single copy of an attribute (e.g., link bandwidth). The attribute applies to all of the L2 Bundle Members in the set advertised under the preceding L2 Bundle Member Attribute Descriptor. No more than one copy of a given sub-TLV in this category may appear in the set of sub-TLVs under the preceding L2 Bundle Member Attribute Descriptor. If multiple copies of a given sub-TLV are present, all copies MUST be ignored. The set of L2 Bundle Member Descriptors that may be advertised under a single L2 Bundle Member Attribute Descriptor is therefore limited to bundle members that share the set of attributes advertised in the shared attribute sub-TLVs. All existing sub-TLVs defined in the IANA registry for sub-TLVs for TLVs 22, 23, 141, 222, and 223 are in the category of shared attribute sub-TLVs unless otherwise specified in this document.

defines sub-TLVs to advertise Adj-SIDs for L3 adjacencies. However, these sub-TLVs only support the advertisement of a single Adj-SID. As it is expected that each L2 Bundle Member will have unique Adj-SIDs in many deployments, it is desirable to define a new sub-TLV that allows more efficient encoding of a set of Adj-SIDs in a single sub-TLV. Two new sub-TLVs are therefore introduced to support advertising Adj-SIDs for L2 Bundle Members. The format of the new sub-TLVs is similar to that used for L3 adjacencies, but it is optimized to allow advertisement of a set of Adj-SIDs (one per L2 Bundle Member) in a single sub-TLV. The two new sub-TLVs defined in the following sections do not fall into the category of shared attribute sub-TLVs.

This sub-TLV is used to advertise Adj-SIDs for L2 Bundle Members associated with a parent L3 adjacency that is point-to-point. The following format is defined for this sub-TLV:

. NOTE: Flags and weight are shared by all L2 Bundle Members listed in the L2 Bundle Attribute Descriptor. L2 Bundle Member Adj-SID Descriptors: There MUST be one descriptor for each of the L2 Bundle Members advertised under the preceding L2 Bundle Member Attribute Descriptor. Each descriptor consists of one of the following fields: SID/Index/Label: according to the V and L flags, it contains either: * A 3-octet local label where the 20 rightmost bits are used for encoding the label value. In this case, the V and L flags MUST be set. * A 4-octet index defining the offset in the SID/Label space advertised by this router. See [RFC8667]. In this case, V and L flags MUST be unset. ]]>

This sub-TLV is used to advertise Adj-SIDs for L2 Bundle Members associated with a parent L3 adjacency that is a LAN adjacency. In LAN subnetworks, the Designated Intermediate System (DIS) is elected and originates the Pseudonode-LSP (PN-LSP) including all neighbors of the DIS. When Segment Routing is used, each router in the LAN MAY advertise the Adj-SID of each of its neighbors on the LAN. Similarly, for each L2 Bundle Member, a router MAY advertise an Adj-SID to each neighbor on the LAN. The following format is defined for this sub-TLV:

. NOTE: Flags and weight are shared by all L2 Bundle Members listed in the L2 Bundle Attribute Descriptor. L2 Bundle Member LAN Adj-SID Descriptors. There MUST be one descriptor for each of the L2 Bundle Members advertised under the preceding L2 Bundle Member Attribute Descriptor. Each descriptor consists of one of the following fields: SID/Index/Label: According to the V and L flags, it contains either: * A 3-octet local label where the 20 rightmost bits are used for encoding the label value. In this case, the V and L flags MUST be set. * A 4-octet index defining the offset in the SID/Label space advertised by this router. See [RFC8667]. In this case, V and L flags MUST be unset. ]]>

This document adds the following new TLV to the IS-IS "TLV Codepoints Registry". Value: 25 Name: L2 Bundle Member Attributes The name of the IANA registry for sub-TLVs for TLVs 22, 23, 141, 222, and 223 has been changed to include sub-TLV 25. An additional column has been added to the registry to indicate which sub-TLVs may appear in the new L2 Bundle Member Attributes TLV. The column for TLV 25 has one of the following three values: sub-TLV may appear in TLV 25 but MUST NOT be shared by multiple L2 Bundle Members sub-TLV may appear in TLV 25 and MAY be shared by multiple L2 Bundle Members sub-TLV MUST NOT appear in TLV 25 The following table indicates the appropriate settings for all currently defined sub-TLVs with regard to their use in the new L2 Bundle Member Attributes TLV.

This document adds the following new sub-TLVs to the above registry. Value: 41 Name: L2 Bundle Member Adj-SID This sub-TLV is allowed in the following TLVs:

Value: 42 Name: L2 Bundle Member LAN Adj-SID This sub-TLV is allowed in the following TLVs:

The IS-IS protocol has supported the advertisement of link attribute information, including link identifiers, for many years. The advertisements defined in this document are identical to existing advertisements defined in , , , and , but are associated with L2 links that are part of a bundle interface on which the IS-IS protocol operates. There are therefore no new security issues introduced by the extensions in this document. As always, if the protocol is used in an environment where unauthorized access to the physical links on which IS-IS Protocol Data Units (PDUs) are sent occurs, then attacks are possible. The use of authentication as defined in and is recommended to prevent such attacks.

Information technology -- Telecommunications and information exchange between systems -- Intermediate System to Intermediate System intra-domain routeing information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode network service (ISO 8473) International Organization for Standardization IEEE Standard for Local and metropolitan area networks -- Link Aggregation IEEE IS-IS Extensions for Segment Routing

Below is an example encoding of L2 Bundle advertisements in a case where we have two parallel adjacencies to the same neighbor whose system-id is 1234.1234.1234.00. The two L2 bundles have the following sets of attributes:

This requires two TLVs, one for each L3 adjacency. TLV for Adjacency #1:

TLV for Adjacency #2

The authors would like to thank Jon Mitchell for his careful review.

The following people gave a substantial contribution to the content of this document and should be considered coauthors: