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RFC 7752

North-Bound Distribution of Link-State and Traffic Engineering (TE) Information Using BGP

Pages: 48
Proposed Standard
Errata
Updated by:  9029
Part 2 of 3 – Pages 19 to 37
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Top   ToC   RFC7752 - Page 19   prevText

3.3. The BGP-LS Attribute

The BGP-LS attribute is an optional, non-transitive BGP attribute that is used to carry link, node, and prefix parameters and attributes. It is defined as a set of Type/Length/Value (TLV) triplets, described in the following section. This attribute SHOULD only be included with Link-State NLRIs. This attribute MUST be ignored for all other address families.
Top   ToC   RFC7752 - Page 20

3.3.1. Node Attribute TLVs

Node attribute TLVs are the TLVs that may be encoded in the BGP-LS attribute with a Node NLRI. The following Node Attribute TLVs are defined: +-------------+----------------------+----------+-------------------+ | TLV Code | Description | Length | Reference | | Point | | | (RFC/Section) | +-------------+----------------------+----------+-------------------+ | 263 | Multi-Topology | variable | Section 3.2.1.5 | | | Identifier | | | | 1024 | Node Flag Bits | 1 | Section 3.3.1.1 | | 1025 | Opaque Node | variable | Section 3.3.1.5 | | | Attribute | | | | 1026 | Node Name | variable | Section 3.3.1.3 | | 1027 | IS-IS Area | variable | Section 3.3.1.2 | | | Identifier | | | | 1028 | IPv4 Router-ID of | 4 | [RFC5305]/4.3 | | | Local Node | | | | 1029 | IPv6 Router-ID of | 16 | [RFC6119]/4.1 | | | Local Node | | | +-------------+----------------------+----------+-------------------+ Table 7: Node Attribute TLVs
3.3.1.1. Node Flag Bits TLV
The Node Flag Bits TLV carries a bit mask describing node attributes. The value is a variable-length bit array of flags, where each bit represents a node capability. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |O|T|E|B|R|V| Rsvd| +-+-+-+-+-+-+-+-+-+ Figure 15: Node Flag Bits TLV Format
Top   ToC   RFC7752 - Page 21
   The bits are defined as follows:

        +-----------------+-------------------------+------------+
        |       Bit       | Description             | Reference  |
        +-----------------+-------------------------+------------+
        |       'O'       | Overload Bit            | [ISO10589] |
        |       'T'       | Attached Bit            | [ISO10589] |
        |       'E'       | External Bit            | [RFC2328]  |
        |       'B'       | ABR Bit                 | [RFC2328]  |
        |       'R'       | Router Bit              | [RFC5340]  |
        |       'V'       | V6 Bit                  | [RFC5340]  |
        | Reserved (Rsvd) | Reserved for future use |            |
        +-----------------+-------------------------+------------+

                    Table 8: Node Flag Bits Definitions

3.3.1.2. IS-IS Area Identifier TLV
An IS-IS node can be part of one or more IS-IS areas. Each of these area addresses is carried in the IS-IS Area Identifier TLV. If multiple area addresses are present, multiple TLVs are used to encode them. The IS-IS Area Identifier TLV may be present in the BGP-LS attribute only when advertised in the Link-State Node NLRI. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // Area Identifier (variable) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 16: IS-IS Area Identifier TLV Format
3.3.1.3. Node Name TLV
The Node Name TLV is optional. Its structure and encoding has been borrowed from [RFC5301]. The Value field identifies the symbolic name of the router node. This symbolic name can be the Fully Qualified Domain Name (FQDN) for the router, it can be a subset of the FQDN (e.g., a hostname), or it can be any string operators want to use for the router. The use of FQDN or a subset of it is strongly RECOMMENDED. The maximum length of the Node Name TLV is 255 octets.
Top   ToC   RFC7752 - Page 22
   The Value field is encoded in 7-bit ASCII.  If a user interface for
   configuring or displaying this field permits Unicode characters, that
   user interface is responsible for applying the ToASCII and/or
   ToUnicode algorithm as described in [RFC5890] to achieve the correct
   format for transmission or display.

   Although [RFC5301] describes an IS-IS-specific extension, usage of
   the Node Name TLV is possible for all protocols.  How a router
   derives and injects node names, e.g., OSPF nodes, is outside of the
   scope of this document.

      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                     Node Name (variable)                    //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 17: Node Name Format

3.3.1.4. Local IPv4/IPv6 Router-ID TLVs
The local IPv4/IPv6 Router-ID TLVs are used to describe auxiliary Router-IDs that the IGP might be using, e.g., for TE and migration purposes such as correlating a Node-ID between different protocols. If there is more than one auxiliary Router-ID of a given type, then each one is encoded in its own TLV.
3.3.1.5. Opaque Node Attribute TLV
The Opaque Node Attribute TLV is an envelope that transparently carries optional Node Attribute TLVs advertised by a router. An originating router shall use this TLV for encoding information specific to the protocol advertised in the NLRI header Protocol-ID field or new protocol extensions to the protocol as advertised in the NLRI header Protocol-ID field for which there is no protocol-neutral representation in the BGP Link-State NLRI. The primary use of the Opaque Node Attribute TLV is to bridge the document lag between, e.g., a new IGP link-state attribute being defined and the protocol- neutral BGP-LS extensions being published. A router, for example, could use this extension in order to advertise the native protocol's Node Attribute TLVs, such as the OSPF Router Informational Capabilities TLV defined in [RFC7770] or the IGP TE Node Capability Descriptor TLV described in [RFC5073].
Top   ToC   RFC7752 - Page 23
      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //               Opaque node attributes (variable)             //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 18: Opaque Node Attribute Format

3.3.2. Link Attribute TLVs

Link Attribute TLVs are TLVs that may be encoded in the BGP-LS attribute with a Link NLRI. Each 'Link Attribute' is a Type/Length/ Value (TLV) triplet formatted as defined in Section 3.1. The format and semantics of the Value fields in some Link Attribute TLVs correspond to the format and semantics of the Value fields in IS-IS Extended IS Reachability sub-TLVs, defined in [RFC5305] and [RFC5307]. Other Link Attribute TLVs are defined in this document. Although the encodings for Link Attribute TLVs were originally defined for IS-IS, the TLVs can carry data sourced by either IS-IS or OSPF.
Top   ToC   RFC7752 - Page 24
   The following Link Attribute TLVs are valid in the BGP-LS attribute
   with a Link NLRI:

   +-----------+---------------------+--------------+------------------+
   |  TLV Code | Description         |  IS-IS TLV   | Reference        |
   |   Point   |                     |   /Sub-TLV   | (RFC/Section)    |
   +-----------+---------------------+--------------+------------------+
   |    1028   | IPv4 Router-ID of   |   134/---    | [RFC5305]/4.3    |
   |           | Local Node          |              |                  |
   |    1029   | IPv6 Router-ID of   |   140/---    | [RFC6119]/4.1    |
   |           | Local Node          |              |                  |
   |    1030   | IPv4 Router-ID of   |   134/---    | [RFC5305]/4.3    |
   |           | Remote Node         |              |                  |
   |    1031   | IPv6 Router-ID of   |   140/---    | [RFC6119]/4.1    |
   |           | Remote Node         |              |                  |
   |    1088   | Administrative      |     22/3     | [RFC5305]/3.1    |
   |           | group (color)       |              |                  |
   |    1089   | Maximum link        |     22/9     | [RFC5305]/3.4    |
   |           | bandwidth           |              |                  |
   |    1090   | Max. reservable     |    22/10     | [RFC5305]/3.5    |
   |           | link bandwidth      |              |                  |
   |    1091   | Unreserved          |    22/11     | [RFC5305]/3.6    |
   |           | bandwidth           |              |                  |
   |    1092   | TE Default Metric   |    22/18     | Section 3.3.2.3  |
   |    1093   | Link Protection     |    22/20     | [RFC5307]/1.2    |
   |           | Type                |              |                  |
   |    1094   | MPLS Protocol Mask  |     ---      | Section 3.3.2.2  |
   |    1095   | IGP Metric          |     ---      | Section 3.3.2.4  |
   |    1096   | Shared Risk Link    |     ---      | Section 3.3.2.5  |
   |           | Group               |              |                  |
   |    1097   | Opaque Link         |     ---      | Section 3.3.2.6  |
   |           | Attribute           |              |                  |
   |    1098   | Link Name           |     ---      | Section 3.3.2.7  |
   +-----------+---------------------+--------------+------------------+

                       Table 9: Link Attribute TLVs

3.3.2.1. IPv4/IPv6 Router-ID TLVs
The local/remote IPv4/IPv6 Router-ID TLVs are used to describe auxiliary Router-IDs that the IGP might be using, e.g., for TE purposes. All auxiliary Router-IDs of both the local and the remote node MUST be included in the link attribute of each Link NLRI. If there is more than one auxiliary Router-ID of a given type, then multiple TLVs are used to encode them.
Top   ToC   RFC7752 - Page 25
3.3.2.2. MPLS Protocol Mask TLV
The MPLS Protocol Mask TLV carries a bit mask describing which MPLS signaling protocols are enabled. The length of this TLV is 1. The value is a bit array of 8 flags, where each bit represents an MPLS Protocol capability. Generation of the MPLS Protocol Mask TLV is only valid for and SHOULD only be used with originators that have local link insight, for example, the Protocol-IDs 'Static configuration' or 'Direct' as per Table 2. The MPLS Protocol Mask TLV MUST NOT be included in NLRIs with the other Protocol-IDs listed in Table 2. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |L|R| Reserved | +-+-+-+-+-+-+-+-+ Figure 19: MPLS Protocol Mask TLV The following bits are defined: +------------+------------------------------------------+-----------+ | Bit | Description | Reference | +------------+------------------------------------------+-----------+ | 'L' | Label Distribution Protocol (LDP) | [RFC5036] | | 'R' | Extension to RSVP for LSP Tunnels | [RFC3209] | | | (RSVP-TE) | | | 'Reserved' | Reserved for future use | | +------------+------------------------------------------+-----------+ Table 10: MPLS Protocol Mask TLV Codes
Top   ToC   RFC7752 - Page 26
3.3.2.3. TE Default Metric TLV
The TE Default Metric TLV carries the Traffic Engineering metric for this link. The length of this TLV is fixed at 4 octets. If a source protocol uses a metric width of less than 32 bits, then the high- order bits of this field MUST be padded with zero. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TE Default Link Metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 20: TE Default Metric TLV Format
3.3.2.4. IGP Metric TLV
The IGP Metric TLV carries the metric for this link. The length of this TLV is variable, depending on the metric width of the underlying protocol. IS-IS small metrics have a length of 1 octet (the two most significant bits are ignored). OSPF link metrics have a length of 2 octets. IS-IS wide metrics have a length of 3 octets. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // IGP Link Metric (variable length) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 21: IGP Metric TLV Format
3.3.2.5. Shared Risk Link Group TLV
The Shared Risk Link Group (SRLG) TLV carries the Shared Risk Link Group information (see Section 2.3 ("Shared Risk Link Group Information") of [RFC4202]). It contains a data structure consisting of a (variable) list of SRLG values, where each element in the list has 4 octets, as shown in Figure 22. The length of this TLV is 4 * (number of SRLG values).
Top   ToC   RFC7752 - Page 27
      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  Shared Risk Link Group Value                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                         ............                        //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  Shared Risk Link Group Value                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 22: Shared Risk Link Group TLV Format

   The SRLG TLV for OSPF-TE is defined in [RFC4203].  In IS-IS, the SRLG
   information is carried in two different TLVs: the IPv4 (SRLG) TLV
   (Type 138) defined in [RFC5307] and the IPv6 SRLG TLV (Type 139)
   defined in [RFC6119].  In Link-State NLRI, both IPv4 and IPv6 SRLG
   information are carried in a single TLV.

3.3.2.6. Opaque Link Attribute TLV
The Opaque Link Attribute TLV is an envelope that transparently carries optional Link Attribute TLVs advertised by a router. An originating router shall use this TLV for encoding information specific to the protocol advertised in the NLRI header Protocol-ID field or new protocol extensions to the protocol as advertised in the NLRI header Protocol-ID field for which there is no protocol-neutral representation in the BGP Link-State NLRI. The primary use of the Opaque Link Attribute TLV is to bridge the document lag between, e.g., a new IGP link-state attribute being defined and the 'protocol- neutral' BGP-LS extensions being published. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // Opaque link attributes (variable) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 23: Opaque Link Attribute TLV Format
3.3.2.7. Link Name TLV
The Link Name TLV is optional. The Value field identifies the symbolic name of the router link. This symbolic name can be the FQDN for the link, it can be a subset of the FQDN, or it can be any string
Top   ToC   RFC7752 - Page 28
   operators want to use for the link.  The use of FQDN or a subset of
   it is strongly RECOMMENDED.  The maximum length of the Link Name TLV
   is 255 octets.

   The Value field is encoded in 7-bit ASCII.  If a user interface for
   configuring or displaying this field permits Unicode characters, that
   user interface is responsible for applying the ToASCII and/or
   ToUnicode algorithm as described in [RFC5890] to achieve the correct
   format for transmission or display.

   How a router derives and injects link names is outside of the scope
   of this document.

      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                     Link Name (variable)                    //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 24: Link Name TLV Format

3.3.3. Prefix Attribute TLVs

Prefixes are learned from the IGP topology (IS-IS or OSPF) with a set of IGP attributes (such as metric, route tags, etc.) that MUST be reflected into the BGP-LS attribute with a prefix NLRI. This section describes the different attributes related to the IPv4/IPv6 prefixes. Prefix Attribute TLVs SHOULD be used when advertising NLRI types 3 and 4 only. The following Prefix Attribute TLVs are defined: +---------------+----------------------+----------+-----------------+ | TLV Code | Description | Length | Reference | | Point | | | | +---------------+----------------------+----------+-----------------+ | 1152 | IGP Flags | 1 | Section 3.3.3.1 | | 1153 | IGP Route Tag | 4*n | [RFC5130] | | 1154 | IGP Extended Route | 8*n | [RFC5130] | | | Tag | | | | 1155 | Prefix Metric | 4 | [RFC5305] | | 1156 | OSPF Forwarding | 4 | [RFC2328] | | | Address | | | | 1157 | Opaque Prefix | variable | Section 3.3.3.6 | | | Attribute | | | +---------------+----------------------+----------+-----------------+ Table 11: Prefix Attribute TLVs
Top   ToC   RFC7752 - Page 29
3.3.3.1. IGP Flags TLV
The IGP Flags TLV contains IS-IS and OSPF flags and bits originally assigned to the prefix. The IGP Flags TLV is encoded as follows: 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |D|N|L|P| Resvd.| +-+-+-+-+-+-+-+-+ Figure 25: IGP Flag TLV Format The Value field contains bits defined according to the table below: +----------+---------------------------+-----------+ | Bit | Description | Reference | +----------+---------------------------+-----------+ | 'D' | IS-IS Up/Down Bit | [RFC5305] | | 'N' | OSPF "no unicast" Bit | [RFC5340] | | 'L' | OSPF "local address" Bit | [RFC5340] | | 'P' | OSPF "propagate NSSA" Bit | [RFC5340] | | Reserved | Reserved for future use. | | +----------+---------------------------+-----------+ Table 12: IGP Flag Bits Definitions
3.3.3.2. IGP Route Tag TLV
The IGP Route Tag TLV carries original IGP Tags (IS-IS [RFC5130] or OSPF) of the prefix and is encoded as follows: 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // Route Tags (one or more) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 26: IGP Route Tag TLV Format Length is a multiple of 4. The Value field contains one or more Route Tags as learned in the IGP topology.
Top   ToC   RFC7752 - Page 30
3.3.3.3. Extended IGP Route Tag TLV
The Extended IGP Route Tag TLV carries IS-IS Extended Route Tags of the prefix [RFC5130] and is encoded as follows: 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // Extended Route Tag (one or more) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 27: Extended IGP Route Tag TLV Format Length is a multiple of 8. The Extended Route Tag field contains one or more Extended Route Tags as learned in the IGP topology.
3.3.3.4. Prefix Metric TLV
The Prefix Metric TLV is an optional attribute and may only appear once. If present, it carries the metric of the prefix as known in the IGP topology as described in Section 4 of [RFC5305] (and therefore represents the reachability cost to the prefix). If not present, it means that the prefix is advertised without any reachability. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 28: Prefix Metric TLV Format Length is 4.
3.3.3.5. OSPF Forwarding Address TLV
The OSPF Forwarding Address TLV [RFC2328] [RFC5340] carries the OSPF forwarding address as known in the original OSPF advertisement. Forwarding address can be either IPv4 or IPv6.
Top   ToC   RFC7752 - Page 31
      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                Forwarding Address (variable)                //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 29: OSPF Forwarding Address TLV Format

   Length is 4 for an IPv4 forwarding address, and 16 for an IPv6
   forwarding address.

3.3.3.6. Opaque Prefix Attribute TLV
The Opaque Prefix Attribute TLV is an envelope that transparently carries optional Prefix Attribute TLVs advertised by a router. An originating router shall use this TLV for encoding information specific to the protocol advertised in the NLRI header Protocol-ID field or new protocol extensions to the protocol as advertised in the NLRI header Protocol-ID field for which there is no protocol-neutral representation in the BGP Link-State NLRI. The primary use of the Opaque Prefix Attribute TLV is to bridge the document lag between, e.g., a new IGP link-state attribute being defined and the protocol- neutral BGP-LS extensions being published. The format of the TLV is as follows: 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // Opaque Prefix Attributes (variable) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 30: Opaque Prefix Attribute TLV Format Type is as specified in Table 11. Length is variable.

3.4. BGP Next-Hop Information

BGP link-state information for both IPv4 and IPv6 networks can be carried over either an IPv4 BGP session or an IPv6 BGP session. If an IPv4 BGP session is used, then the next hop in the MP_REACH_NLRI SHOULD be an IPv4 address. Similarly, if an IPv6 BGP session is used, then the next hop in the MP_REACH_NLRI SHOULD be an IPv6 address. Usually, the next hop will be set to the local endpoint
Top   ToC   RFC7752 - Page 32
   address of the BGP session.  The next-hop address MUST be encoded as
   described in [RFC4760].  The Length field of the next-hop address
   will specify the next-hop address family.  If the next-hop length is
   4, then the next hop is an IPv4 address; if the next-hop length is
   16, then it is a global IPv6 address; and if the next-hop length is
   32, then there is one global IPv6 address followed by a link-local
   IPv6 address.  The link-local IPv6 address should be used as
   described in [RFC2545].  For VPN Subsequent Address Family Identifier
   (SAFI), as per custom, an 8-byte Route Distinguisher set to all zero
   is prepended to the next hop.

   The BGP Next Hop attribute is used by each BGP-LS speaker to validate
   the NLRI it receives.  In case identical NLRIs are sourced by
   multiple originators, the BGP Next Hop attribute is used to tiebreak
   as per the standard BGP path decision process.  This specification
   doesn't mandate any rule regarding the rewrite of the BGP Next Hop
   attribute.

3.5. Inter-AS Links

The main source of TE information is the IGP, which is not active on inter-AS links. In some cases, the IGP may have information of inter-AS links [RFC5392] [RFC5316]. In other cases, an implementation SHOULD provide a means to inject inter-AS links into BGP-LS. The exact mechanism used to provision the inter-AS links is outside the scope of this document

3.6. Router-ID Anchoring Example: ISO Pseudonode

Encoding of a broadcast LAN in IS-IS provides a good example of how Router-IDs are encoded. Consider Figure 31. This represents a Broadcast LAN between a pair of routers. The "real" (non-pseudonode) routers have both an IPv4 Router-ID and IS-IS Node-ID. The pseudonode does not have an IPv4 Router-ID. Node1 is the DIS for the LAN. Two unidirectional links (Node1, Pseudonode1) and (Pseudonode1, Node2) are being generated. The Link NLRI of (Node1, Pseudonode1) is encoded as follows. The IGP Router-ID TLV of the local Node Descriptor is 6 octets long and contains the ISO-ID of Node1, 1920.0000.2001. The IGP Router-ID TLV of the remote Node Descriptor is 7 octets long and contains the ISO- ID of Pseudonode1, 1920.0000.2001.02. The BGP-LS attribute of this link contains one local IPv4 Router-ID TLV (TLV type 1028) containing 192.0.2.1, the IPv4 Router-ID of Node1. The Link NLRI of (Pseudonode1, Node2) is encoded as follows. The IGP Router-ID TLV of the local Node Descriptor is 7 octets long and contains the ISO-ID of Pseudonode1, 1920.0000.2001.02. The IGP
Top   ToC   RFC7752 - Page 33
   Router-ID TLV of the remote Node Descriptor is 6 octets long and
   contains the ISO-ID of Node2, 1920.0000.2002.  The BGP-LS attribute
   of this link contains one remote IPv4 Router-ID TLV (TLV type 1030)
   containing 192.0.2.2, the IPv4 Router-ID of Node2.

     +-----------------+    +-----------------+    +-----------------+
     |      Node1      |    |   Pseudonode1   |    |      Node2      |
     |1920.0000.2001.00|--->|1920.0000.2001.02|--->|1920.0000.2002.00|
     |     192.0.2.1   |    |                 |    |     192.0.2.2   |
     +-----------------+    +-----------------+    +-----------------+

                       Figure 31: IS-IS Pseudonodes

3.7. Router-ID Anchoring Example: OSPF Pseudonode

Encoding of a broadcast LAN in OSPF provides a good example of how Router-IDs and local Interface IPs are encoded. Consider Figure 32. This represents a Broadcast LAN between a pair of routers. The "real" (non-pseudonode) routers have both an IPv4 Router-ID and an Area Identifier. The pseudonode does have an IPv4 Router-ID, an IPv4 Interface Address (for disambiguation), and an OSPF Area. Node1 is the DR for the LAN; hence, its local IP address 10.1.1.1 is used as both the Router-ID and Interface IP for the pseudonode keys. Two unidirectional links, (Node1, Pseudonode1) and (Pseudonode1, Node2), are being generated. The Link NLRI of (Node1, Pseudonode1) is encoded as follows: o Local Node Descriptor TLV #515: IGP Router-ID: 11.11.11.11 TLV #514: OSPF Area-ID: ID:0.0.0.0 o Remote Node Descriptor TLV #515: IGP Router-ID: 11.11.11.11:10.1.1.1 TLV #514: OSPF Area-ID: ID:0.0.0.0 The Link NLRI of (Pseudonode1, Node2) is encoded as follows: o Local Node Descriptor TLV #515: IGP Router-ID: 11.11.11.11:10.1.1.1 TLV #514: OSPF Area-ID: ID:0.0.0.0
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   o  Remote Node Descriptor

         TLV #515: IGP Router-ID: 33.33.33.34

         TLV #514: OSPF Area-ID: ID:0.0.0.0

     +-----------------+    +-----------------+    +-----------------+
     |      Node1      |    |   Pseudonode1   |    |      Node2      |
     |   11.11.11.11   |--->|   11.11.11.11   |--->|  33.33.33.34    |
     |                 |    |     10.1.1.1    |    |                 |
     |      Area 0     |    |      Area 0     |    |      Area 0     |
     +-----------------+    +-----------------+    +-----------------+

                        Figure 32: OSPF Pseudonodes

3.8. Router-ID Anchoring Example: OSPFv2 to IS-IS Migration

Graceful migration from one IGP to another requires coordinated operation of both protocols during the migration period. Such a coordination requires identifying a given physical link in both IGPs. The IPv4 Router-ID provides that "glue", which is present in the Node Descriptors of the OSPF Link NLRI and in the link attribute of the IS-IS Link NLRI. Consider a point-to-point link between two routers, A and B, that initially were OSPFv2-only routers and then IS-IS is enabled on them. Node A has IPv4 Router-ID and ISO-ID; node B has IPv4 Router-ID, IPv6 Router-ID, and ISO-ID. Each protocol generates one Link NLRI for the link (A, B), both of which are carried by BGP-LS. The OSPFv2 Link NLRI for the link is encoded with the IPv4 Router-ID of nodes A and B in the local and remote Node Descriptors, respectively. The IS-IS Link NLRI for the link is encoded with the ISO-ID of nodes A and B in the local and remote Node Descriptors, respectively. In addition, the BGP-LS attribute of the IS-IS Link NLRI contains the TLV type 1028 containing the IPv4 Router-ID of node A, TLV type 1030 containing the IPv4 Router-ID of node B, and TLV type 1031 containing the IPv6 Router-ID of node B. In this case, by using IPv4 Router-ID, the link (A, B) can be identified in both the IS-IS and OSPF protocol.

4. Link to Path Aggregation

Distribution of all links available in the global Internet is certainly possible; however, it not desirable from a scaling and privacy point of view. Therefore, an implementation may support a link to path aggregation. Rather than advertising all specific links of a domain, an ASBR may advertise an "aggregate link" between a non- adjacent pair of nodes. The "aggregate link" represents the
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   aggregated set of link properties between a pair of non-adjacent
   nodes.  The actual methods to compute the path properties (of
   bandwidth, metric, etc.) are outside the scope of this document.  The
   decision whether to advertise all specific links or aggregated links
   is an operator's policy choice.  To highlight the varying levels of
   exposure, the following deployment examples are discussed.

4.1. Example: No Link Aggregation

Consider Figure 33. Both AS1 and AS2 operators want to protect their inter-AS {R1, R3}, {R2, R4} links using RSVP-FRR LSPs. If R1 wants to compute its link-protection LSP to R3, it needs to "see" an alternate path to R3. Therefore, the AS2 operator exposes its topology. All BGP-TE-enabled routers in AS1 "see" the full topology of AS2 and therefore can compute a backup path. Note that the computing router decides if the direct link between {R3, R4} or the {R4, R5, R3} path is used. AS1 : AS2 : R1-------R3 | : | \ | : | R5 | : | / R2-------R4 : : Figure 33: No Link Aggregation

4.2. Example: ASBR to ASBR Path Aggregation

The brief difference between the "no-link aggregation" example and this example is that no specific link gets exposed. Consider Figure 34. The only link that gets advertised by AS2 is an "aggregate" link between R3 and R4. This is enough to tell AS1 that there is a backup path. However, the actual links being used are hidden from the topology.
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          AS1   :   AS2
                :
           R1-------R3
            |   :   |
            |   :   |
            |   :   |
           R2-------R4
                :
                :

         Figure 34: ASBR Link Aggregation

4.3. Example: Multi-AS Path Aggregation

Service providers in control of multiple ASes may even decide to not expose their internal inter-AS links. Consider Figure 35. AS3 is modeled as a single node that connects to the border routers of the aggregated domain. AS1 : AS2 : AS3 : : R1-------R3----- | : : \ | : : vR0 | : : / R2-------R4----- : : : : Figure 35: Multi-AS Aggregation

5. IANA Considerations

IANA has assigned address family number 16388 (BGP-LS) in the "Address Family Numbers" registry with this document as a reference. IANA has assigned SAFI values 71 (BGP-LS) and 72 (BGP-LS-VPN) in the "SAFI Values" sub-registry under the "Subsequent Address Family Identifiers (SAFI) Parameters" registry. IANA has assigned value 29 (BGP-LS Attribute) in the "BGP Path Attributes" sub-registry under the "Border Gateway Protocol (BGP) Parameters" registry. IANA has created a new "Border Gateway Protocol - Link State (BGP-LS) Parameters" registry at <http://www.iana.org/assignments/bgp-ls- parameters>. All of the following registries are BGP-LS specific and are accessible under this registry:
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   o  "BGP-LS NLRI-Types" registry

      Value 0 is reserved.  The maximum value is 65535.  The registry
      has been populated with the values shown in Table 1.  Allocations
      within the registry require documentation of the proposed use of
      the allocated value (Specification Required) and approval by the
      Designated Expert assigned by the IESG (see [RFC5226]).

   o  "BGP-LS Protocol-IDs" registry

      Value 0 is reserved.  The maximum value is 255.  The registry has
      been populated with the values shown in Table 2.  Allocations
      within the registry require documentation of the proposed use of
      the allocated value (Specification Required) and approval by the
      Designated Expert assigned by the IESG (see [RFC5226]).

   o  "BGP-LS Well-Known Instance-IDs" registry

      The registry has been populated with the values shown in Table 3.
      New allocations from the range 1-31 use the IANA allocation policy
      "Specification Required" and require approval by the Designated
      Expert assigned by the IESG (see [RFC5226]).  Values in the range
      32 to 2^64-1 are for "Private Use" and are not recorded by IANA.

   o  "BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor, and
      Attribute TLVs" registry

      Values 0-255 are reserved.  Values 256-65535 will be used for code
      points.  The registry has been populated with the values shown in
      Table 13.  Allocations within the registry require documentation
      of the proposed use of the allocated value (Specification
      Required) and approval by the Designated Expert assigned by the
      IESG (see [RFC5226]).

5.1. Guidance for Designated Experts

In all cases of review by the Designated Expert (DE) described here, the DE is expected to ascertain the existence of suitable documentation (a specification) as described in [RFC5226] and to verify that the document is permanently and publicly available. The DE is also expected to check the clarity of purpose and use of the requested code points. Last, the DE must verify that any specification produced in the IETF that requests one of these code points has been made available for review by the IDR working group and that any specification produced outside the IETF does not conflict with work that is active or already published within the IETF.


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