RFC 7752
North-Bound Distribution of Link-State and Traffic Engineering (TE) Information Using BGP
Internet Engineering Task Force (IETF) H. Gredler, Ed. Request for Comments: 7752 Individual Contributor Category: Standards Track J. Medved ISSN: 2070-1721 S. Previdi Cisco Systems, Inc. A. Farrel Juniper Networks, Inc. S. Ray March 2016 North-Bound Distribution of Link-State and Traffic Engineering (TE) Information Using BGPAbstract
In a number of environments, a component external to a network is called upon to perform computations based on the network topology and current state of the connections within the network, including Traffic Engineering (TE) information. This is information typically distributed by IGP routing protocols within the network. This document describes a mechanism by which link-state and TE information can be collected from networks and shared with external components using the BGP routing protocol. This is achieved using a new BGP Network Layer Reachability Information (NLRI) encoding format. The mechanism is applicable to physical and virtual IGP links. The mechanism described is subject to policy control. Applications of this technique include Application-Layer Traffic Optimization (ALTO) servers and Path Computation Elements (PCEs). Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7752.
Copyright Notice Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.Table of Contents
1. Introduction ....................................................3 1.1. Requirements Language ......................................5 2. Motivation and Applicability ....................................5 2.1. MPLS-TE with PCE ...........................................5 2.2. ALTO Server Network API ....................................6 3. Carrying Link-State Information in BGP ..........................7 3.1. TLV Format .................................................8 3.2. The Link-State NLRI ........................................8 3.2.1. Node Descriptors ...................................12 3.2.2. Link Descriptors ...................................16 3.2.3. Prefix Descriptors .................................18 3.3. The BGP-LS Attribute ......................................19 3.3.1. Node Attribute TLVs ................................20 3.3.2. Link Attribute TLVs ................................23 3.3.3. Prefix Attribute TLVs ..............................28 3.4. BGP Next-Hop Information ..................................31 3.5. Inter-AS Links ............................................32 3.6. Router-ID Anchoring Example: ISO Pseudonode ...............32 3.7. Router-ID Anchoring Example: OSPF Pseudonode ..............33 3.8. Router-ID Anchoring Example: OSPFv2 to IS-IS Migration ....34 4. Link to Path Aggregation .......................................34 4.1. Example: No Link Aggregation ..............................35 4.2. Example: ASBR to ASBR Path Aggregation ....................35 4.3. Example: Multi-AS Path Aggregation ........................36 5. IANA Considerations ............................................36 5.1. Guidance for Designated Experts ...........................37 6. Manageability Considerations ...................................38 6.1. Operational Considerations ................................38 6.1.1. Operations .........................................38 6.1.2. Installation and Initial Setup .....................38 6.1.3. Migration Path .....................................38
6.1.4. Requirements on Other Protocols and
Functional Components ..............................38
6.1.5. Impact on Network Operation ........................38
6.1.6. Verifying Correct Operation ........................39
6.2. Management Considerations .................................39
6.2.1. Management Information .............................39
6.2.2. Fault Management ...................................39
6.2.3. Configuration Management ...........................40
6.2.4. Accounting Management ..............................40
6.2.5. Performance Management .............................40
6.2.6. Security Management ................................41
7. TLV/Sub-TLV Code Points Summary ................................41
8. Security Considerations ........................................42
9. References .....................................................43
9.1. Normative References ......................................43
9.2. Informative References ....................................45
Acknowledgements ..................................................47
Contributors ......................................................47
Authors' Addresses ................................................48
1. Introduction
The contents of a Link-State Database (LSDB) or of an IGP's Traffic
Engineering Database (TED) describe only the links and nodes within
an IGP area. Some applications, such as end-to-end Traffic
Engineering (TE), would benefit from visibility outside one area or
Autonomous System (AS) in order to make better decisions.
The IETF has defined the Path Computation Element (PCE) [RFC4655] as
a mechanism for achieving the computation of end-to-end TE paths that
cross the visibility of more than one TED or that require CPU-
intensive or coordinated computations. The IETF has also defined the
ALTO server [RFC5693] as an entity that generates an abstracted
network topology and provides it to network-aware applications.
Both a PCE and an ALTO server need to gather information about the
topologies and capabilities of the network in order to be able to
fulfill their function.
This document describes a mechanism by which link-state and TE
information can be collected from networks and shared with external
components using the BGP routing protocol [RFC4271]. This is
achieved using a new BGP Network Layer Reachability Information
(NLRI) encoding format. The mechanism is applicable to physical and
virtual links. The mechanism described is subject to policy control.
A router maintains one or more databases for storing link-state information about nodes and links in any given area. Link attributes stored in these databases include: local/remote IP addresses, local/ remote interface identifiers, link metric and TE metric, link bandwidth, reservable bandwidth, per Class-of-Service (CoS) class reservation state, preemption, and Shared Risk Link Groups (SRLGs). The router's BGP process can retrieve topology from these LSDBs and distribute it to a consumer, either directly or via a peer BGP speaker (typically a dedicated Route Reflector), using the encoding specified in this document. The collection of link-state and TE information and its distribution to consumers is shown in the following figure. +-----------+ | Consumer | +-----------+ ^ | +-----------+ | BGP | +-----------+ | Speaker | | Consumer | +-----------+ +-----------+ ^ ^ ^ ^ | | | | +---------------+ | +-------------------+ | | | | | +-----------+ +-----------+ +-----------+ | BGP | | BGP | | BGP | | Speaker | | Speaker | . . . | Speaker | +-----------+ +-----------+ +-----------+ ^ ^ ^ | | | IGP IGP IGP Figure 1: Collection of Link-State and TE Information A BGP speaker may apply configurable policy to the information that it distributes. Thus, it may distribute the real physical topology from the LSDB or the TED. Alternatively, it may create an abstracted topology, where virtual, aggregated nodes are connected by virtual paths. Aggregated nodes can be created, for example, out of multiple routers in a Point of Presence (POP). Abstracted topology can also be a mix of physical and virtual nodes and physical and virtual links. Furthermore, the BGP speaker can apply policy to determine when information is updated to the consumer so that there is a
reduction of information flow from the network to the consumers. Mechanisms through which topologies can be aggregated or virtualized are outside the scope of this document1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].2. Motivation and Applicability
This section describes use cases from which the requirements can be derived.2.1. MPLS-TE with PCE
As described in [RFC4655], a PCE can be used to compute MPLS-TE paths within a "domain" (such as an IGP area) or across multiple domains (such as a multi-area AS or multiple ASes). o Within a single area, the PCE offers enhanced computational power that may not be available on individual routers, sophisticated policy control and algorithms, and coordination of computation across the whole area. o If a router wants to compute a MPLS-TE path across IGP areas, then its own TED lacks visibility of the complete topology. That means that the router cannot determine the end-to-end path and cannot even select the right exit router (Area Border Router (ABR)) for an optimal path. This is an issue for large-scale networks that need to segment their core networks into distinct areas but still want to take advantage of MPLS-TE. Previous solutions used per-domain path computation [RFC5152]. The source router could only compute the path for the first area because the router only has full topological visibility for the first area along the path, but not for subsequent areas. Per-domain path computation uses a technique called "loose-hop-expansion" [RFC3209] and selects the exit ABR and other ABRs or AS Border Routers (ASBRs) using the IGP-computed shortest path topology for the remainder of the path. This may lead to sub-optimal paths, makes alternate/back- up path computation hard, and might result in no TE path being found when one really does exist. The PCE presents a computation server that may have visibility into more than one IGP area or AS, or may cooperate with other PCEs to perform distributed path computation. The PCE obviously needs access
to the TED for the area(s) it serves, but [RFC4655] does not describe how this is achieved. Many implementations make the PCE a passive participant in the IGP so that it can learn the latest state of the network, but this may be sub-optimal when the network is subject to a high degree of churn or when the PCE is responsible for multiple areas. The following figure shows how a PCE can get its TED information using the mechanism described in this document. +----------+ +---------+ | ----- | | BGP | | | TED |<-+-------------------------->| Speaker | | ----- | TED synchronization | | | | | mechanism: +---------+ | | | BGP with Link-State NLRI | v | | ----- | | | PCE | | | ----- | +----------+ ^ | Request/ | Response v Service +----------+ Signaling +----------+ Request | Head-End | Protocol | Adjacent | -------->| Node |<------------>| Node | +----------+ +----------+ Figure 2: External PCE Node Using a TED Synchronization Mechanism The mechanism in this document allows the necessary TED information to be collected from the IGP within the network, filtered according to configurable policy, and distributed to the PCE as necessary.2.2. ALTO Server Network API
An ALTO server [RFC5693] is an entity that generates an abstracted network topology and provides it to network-aware applications over a web-service-based API. Example applications are peer-to-peer (P2P) clients or trackers, or Content Distribution Networks (CDNs). The abstracted network topology comes in the form of two maps: a Network Map that specifies allocation of prefixes to Partition Identifiers (PIDs), and a Cost Map that specifies the cost between PIDs listed in the Network Map. For more details, see [RFC7285].
ALTO abstract network topologies can be auto-generated from the physical topology of the underlying network. The generation would typically be based on policies and rules set by the operator. Both prefix and TE data are required: prefix data is required to generate ALTO Network Maps, and TE (topology) data is required to generate ALTO Cost Maps. Prefix data is carried and originated in BGP, and TE data is originated and carried in an IGP. The mechanism defined in this document provides a single interface through which an ALTO server can retrieve all the necessary prefix and network topology data from the underlying network. Note that an ALTO server can use other mechanisms to get network data, for example, peering with multiple IGP and BGP speakers. The following figure shows how an ALTO server can get network topology information from the underlying network using the mechanism described in this document. +--------+ | Client |<--+ +--------+ | | ALTO +--------+ BGP with +---------+ +--------+ | Protocol | ALTO | Link-State NLRI | BGP | | Client |<--+------------| Server |<----------------| Speaker | +--------+ | | | | | | +--------+ +---------+ +--------+ | | Client |<--+ +--------+ Figure 3: ALTO Server Using Network Topology Information3. Carrying Link-State Information in BGP
This specification contains two parts: definition of a new BGP NLRI that describes links, nodes, and prefixes comprising IGP link-state information and definition of a new BGP path attribute (BGP-LS attribute) that carries link, node, and prefix properties and attributes, such as the link and prefix metric or auxiliary Router- IDs of nodes, etc. It is desirable to keep the dependencies on the protocol source of this attribute to a minimum and represent any content in an IGP- neutral way, such that applications that want to learn about a link- state topology do not need to know about any OSPF or IS-IS protocol specifics.
3.1. TLV Format
Information in the new Link-State NLRIs and attributes is encoded in Type/Length/Value triplets. The TLV format is shown in Figure 4. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // Value (variable) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: TLV Format The Length field defines the length of the value portion in octets (thus, a TLV with no value portion would have a length of zero). The TLV is not padded to 4-octet alignment. Unrecognized types MUST be preserved and propagated. In order to compare NLRIs with unknown TLVs, all TLVs MUST be ordered in ascending order by TLV Type. If there are more TLVs of the same type, then the TLVs MUST be ordered in ascending order of the TLV value within the TLVs with the same type by treating the entire Value field as an opaque hexadecimal string and comparing leftmost octets first, regardless of the length of the string. All TLVs that are not specified as mandatory are considered optional.3.2. The Link-State NLRI
The MP_REACH_NLRI and MP_UNREACH_NLRI attributes are BGP's containers for carrying opaque information. Each Link-State NLRI describes either a node, a link, or a prefix. All non-VPN link, node, and prefix information SHALL be encoded using AFI 16388 / SAFI 71. VPN link, node, and prefix information SHALL be encoded using AFI 16388 / SAFI 72. In order for two BGP speakers to exchange Link-State NLRI, they MUST use BGP Capabilities Advertisement to ensure that they are both capable of properly processing such NLRI. This is done as specified in [RFC4760], by using capability code 1 (multi-protocol BGP), with AFI 16388 / SAFI 71 for BGP-LS, and AFI 16388 / SAFI 72 for BGP-LS-VPN.
The format of the Link-State NLRI is shown in the following figures. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NLRI Type | Total NLRI Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | // Link-State NLRI (variable) // | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5: Link-State AFI 16388 / SAFI 71 NLRI 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NLRI Type | Total NLRI Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + Route Distinguisher + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | // Link-State NLRI (variable) // | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: Link-State VPN AFI 16388 / SAFI 72 NLRI Format The Total NLRI Length field contains the cumulative length, in octets, of the rest of the NLRI, not including the NLRI Type field or itself. For VPN applications, it also includes the length of the Route Distinguisher. +------+---------------------------+ | Type | NLRI Type | +------+---------------------------+ | 1 | Node NLRI | | 2 | Link NLRI | | 3 | IPv4 Topology Prefix NLRI | | 4 | IPv6 Topology Prefix NLRI | +------+---------------------------+ Table 1: NLRI Types
Route Distinguishers are defined and discussed in [RFC4364]. The Node NLRI (NLRI Type = 1) is shown in the following figure. 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 +-+-+-+-+-+-+-+-+ | Protocol-ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identifier | | (64 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // Local Node Descriptors (variable) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7: The Node NLRI Format The Link NLRI (NLRI Type = 2) is shown in the following figure. 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 +-+-+-+-+-+-+-+-+ | Protocol-ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identifier | | (64 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // Local Node Descriptors (variable) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // Remote Node Descriptors (variable) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // Link Descriptors (variable) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 8: The Link NLRI Format
The IPv4 and IPv6 Prefix NLRIs (NLRI Type = 3 and Type = 4) use the same format, as shown in the following figure. 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 +-+-+-+-+-+-+-+-+ | Protocol-ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identifier | | (64 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // Local Node Descriptors (variable) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // Prefix Descriptors (variable) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9: The IPv4/IPv6 Topology Prefix NLRI Format The Protocol-ID field can contain one of the following values: +-------------+----------------------------------+ | Protocol-ID | NLRI information source protocol | +-------------+----------------------------------+ | 1 | IS-IS Level 1 | | 2 | IS-IS Level 2 | | 3 | OSPFv2 | | 4 | Direct | | 5 | Static configuration | | 6 | OSPFv3 | +-------------+----------------------------------+ Table 2: Protocol Identifiers The 'Direct' and 'Static configuration' protocol types SHOULD be used when BGP-LS is sourcing local information. For all information derived from other protocols, the corresponding Protocol-ID MUST be used. If BGP-LS has direct access to interface information and wants to advertise a local link, then the Protocol-ID 'Direct' SHOULD be used. For modeling virtual links, such as described in Section 4, the Protocol-ID 'Static configuration' SHOULD be used. Both OSPF and IS-IS MAY run multiple routing protocol instances over the same link. See [RFC6822] and [RFC6549]. These instances define independent "routing universes". The 64-bit Identifier field is used to identify the routing universe where the NLRI belongs. The NLRIs representing link-state objects (nodes, links, or prefixes) from the same routing universe MUST have the same 'Identifier' value. NLRIs
with different 'Identifier' values MUST be considered to be from different routing universes. Table 3 lists the 'Identifier' values that are defined as well-known in this document. +------------+----------------------------------+ | Identifier | Routing Universe | +------------+----------------------------------+ | 0 | Default Layer 3 Routing topology | +------------+----------------------------------+ Table 3: Well-Known Instance Identifiers If a given protocol does not support multiple routing universes, then it SHOULD set the Identifier field according to Table 3. However, an implementation MAY make the 'Identifier' configurable for a given protocol. Each Node Descriptor and Link Descriptor consists of one or more TLVs, as described in the following sections.3.2.1. Node Descriptors
Each link is anchored by a pair of Router-IDs that are used by the underlying IGP, namely, a 48-bit ISO System-ID for IS-IS and a 32-bit Router-ID for OSPFv2 and OSPFv3. An IGP may use one or more additional auxiliary Router-IDs, mainly for Traffic Engineering purposes. For example, IS-IS may have one or more IPv4 and IPv6 TE Router-IDs [RFC5305] [RFC6119]. These auxiliary Router-IDs MUST be included in the link attribute described in Section 3.3.2. It is desirable that the Router-ID assignments inside the Node Descriptor are globally unique. However, there may be Router-ID spaces (e.g., ISO) where no global registry exists, or worse, Router- IDs have been allocated following the private-IP allocation described in RFC 1918 [RFC1918]. BGP-LS uses the Autonomous System (AS) Number and BGP-LS Identifier (see Section 3.2.1.4) to disambiguate the Router-IDs, as described in Section 3.2.1.1.3.2.1.1. Globally Unique Node/Link/Prefix Identifiers
One problem that needs to be addressed is the ability to identify an IGP node globally (by "globally", we mean within the BGP-LS database collected by all BGP-LS speakers that talk to each other). This can be expressed through the following two requirements: (A) The same node MUST NOT be represented by two keys (otherwise, one node will look like two nodes).
(B) Two different nodes MUST NOT be represented by the same key
(otherwise, two nodes will look like one node).
We define an "IGP domain" to be the set of nodes (hence, by extension
links and prefixes) within which each node has a unique IGP
representation by using the combination of Area-ID, Router-ID,
Protocol-ID, Multi-Topology ID, and Instance-ID. The problem is that
BGP may receive node/link/prefix information from multiple
independent "IGP domains", and we need to distinguish between them.
Moreover, we can't assume there is always one and only one IGP domain
per AS. During IGP transitions, it may happen that two redundant
IGPs are in place.
In Section 3.2.1.4, a set of sub-TLVs is described, which allows
specification of a flexible key for any given node/link information
such that global uniqueness of the NLRI is ensured.
3.2.1.2. Local Node Descriptors
The Local Node Descriptors TLV contains Node Descriptors for the node
anchoring the local end of the link. This is a mandatory TLV in all
three types of NLRIs (node, link, and prefix). The length of this
TLV is variable. The value contains one or more Node Descriptor
Sub-TLVs defined in Section 3.2.1.4.
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 Descriptor Sub-TLVs (variable) //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: Local Node Descriptors TLV Format
3.2.1.3. Remote Node Descriptors
The Remote Node Descriptors TLV contains Node Descriptors for the
node anchoring the remote end of the link. This is a mandatory TLV
for Link NLRIs. The length of this TLV is variable. The value
contains one or more Node Descriptor Sub-TLVs defined in
Section 3.2.1.4.
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 Descriptor Sub-TLVs (variable) // | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 11: Remote Node Descriptors TLV Format3.2.1.4. Node Descriptor Sub-TLVs
The Node Descriptor Sub-TLV type code points and lengths are listed in the following table: +--------------------+-------------------+----------+ | Sub-TLV Code Point | Description | Length | +--------------------+-------------------+----------+ | 512 | Autonomous System | 4 | | 513 | BGP-LS Identifier | 4 | | 514 | OSPF Area-ID | 4 | | 515 | IGP Router-ID | Variable | +--------------------+-------------------+----------+ Table 4: Node Descriptor Sub-TLVs The sub-TLV values in Node Descriptor TLVs are defined as follows: Autonomous System: Opaque value (32-bit AS Number) BGP-LS Identifier: Opaque value (32-bit ID). In conjunction with Autonomous System Number (ASN), uniquely identifies the BGP-LS domain. The combination of ASN and BGP-LS ID MUST be globally unique. All BGP-LS speakers within an IGP flooding-set (set of IGP nodes within which an LSP/LSA is flooded) MUST use the same ASN, BGP-LS ID tuple. If an IGP domain consists of multiple flooding-sets, then all BGP-LS speakers within the IGP domain SHOULD use the same ASN, BGP-LS ID tuple. Area-ID: Used to identify the 32-bit area to which the NLRI belongs. The Area Identifier allows different NLRIs of the same router to be discriminated. IGP Router-ID: Opaque value. This is a mandatory TLV. For an IS-IS non-pseudonode, this contains a 6-octet ISO Node-ID (ISO system- ID). For an IS-IS pseudonode corresponding to a LAN, this
contains the 6-octet ISO Node-ID of the Designated Intermediate
System (DIS) followed by a 1-octet, nonzero PSN identifier (7
octets in total). For an OSPFv2 or OSPFv3 non-pseudonode, this
contains the 4-octet Router-ID. For an OSPFv2 pseudonode
representing a LAN, this contains the 4-octet Router-ID of the
Designated Router (DR) followed by the 4-octet IPv4 address of the
DR's interface to the LAN (8 octets in total). Similarly, for an
OSPFv3 pseudonode, this contains the 4-octet Router-ID of the DR
followed by the 4-octet interface identifier of the DR's interface
to the LAN (8 octets in total). The TLV size in combination with
the protocol identifier enables the decoder to determine the type
of the node.
There can be at most one instance of each sub-TLV type present in
any Node Descriptor. The sub-TLVs within a Node Descriptor MUST
be arranged in ascending order by sub-TLV type. This needs to be
done in order to compare NLRIs, even when an implementation
encounters an unknown sub-TLV. Using stable sorting, an
implementation can do binary comparison of NLRIs and hence allow
incremental deployment of new key sub-TLVs.
3.2.1.5. Multi-Topology ID
The Multi-Topology ID (MT-ID) TLV carries one or more IS-IS or OSPF
Multi-Topology IDs for a link, node, or prefix.
Semantics of the IS-IS MT-ID are defined in Section 7.2 of RFC 5120
[RFC5120]. Semantics of the OSPF MT-ID are defined in Section 3.7 of
RFC 4915 [RFC4915]. If the value in the MT-ID TLV is derived from
OSPF, then the upper 9 bits MUST be set to 0. Bits R are reserved
and SHOULD be set to 0 when originated and ignored on receipt.
The format of the MT-ID TLV is shown in the following figure.
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=2*n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R R R R| Multi-Topology ID 1 | .... //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// .... |R R R R| Multi-Topology ID n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: Multi-Topology ID TLV Format
where Type is 263, Length is 2*n, and n is the number of MT-IDs
carried in the TLV.
The MT-ID TLV MAY be present in a Link Descriptor, a Prefix Descriptor, or the BGP-LS attribute of a Node NLRI. In a Link or Prefix Descriptor, only a single MT-ID TLV containing the MT-ID of the topology where the link or the prefix is reachable is allowed. In case one wants to advertise multiple topologies for a given Link Descriptor or Prefix Descriptor, multiple NLRIs need to be generated where each NLRI contains an unique MT-ID. In the BGP-LS attribute of a Node NLRI, one MT-ID TLV containing the array of MT-IDs of all topologies where the node is reachable is allowed.3.2.2. Link Descriptors
The Link Descriptor field is a set of Type/Length/Value (TLV) triplets. The format of each TLV is shown in Section 3.1. The Link Descriptor TLVs uniquely identify a link among multiple parallel links between a pair of anchor routers. A link described by the Link Descriptor TLVs actually is a "half-link", a unidirectional representation of a logical link. In order to fully describe a single logical link, two originating routers advertise a half-link each, i.e., two Link NLRIs are advertised for a given point-to-point link. The format and semantics of the Value fields in most Link Descriptor TLVs correspond to the format and semantics of Value fields in IS-IS Extended IS Reachability sub-TLVs, defined in [RFC5305], [RFC5307], and [RFC6119]. Although the encodings for Link Descriptor TLVs were originally defined for IS-IS, the TLVs can carry data sourced by either IS-IS or OSPF.
The following TLVs are valid as Link Descriptors in the Link NLRI: +-----------+---------------------+--------------+------------------+ | TLV Code | Description | IS-IS TLV | Reference | | Point | | /Sub-TLV | (RFC/Section) | +-----------+---------------------+--------------+------------------+ | 258 | Link Local/Remote | 22/4 | [RFC5307]/1.1 | | | Identifiers | | | | 259 | IPv4 interface | 22/6 | [RFC5305]/3.2 | | | address | | | | 260 | IPv4 neighbor | 22/8 | [RFC5305]/3.3 | | | address | | | | 261 | IPv6 interface | 22/12 | [RFC6119]/4.2 | | | address | | | | 262 | IPv6 neighbor | 22/13 | [RFC6119]/4.3 | | | address | | | | 263 | Multi-Topology | --- | Section 3.2.1.5 | | | Identifier | | | +-----------+---------------------+--------------+------------------+ Table 5: Link Descriptor TLVs The information about a link present in the LSA/LSP originated by the local node of the link determines the set of TLVs in the Link Descriptor of the link. If interface and neighbor addresses, either IPv4 or IPv6, are present, then the IP address TLVs are included in the Link Descriptor but not the link local/remote Identifier TLV. The link local/remote identifiers MAY be included in the link attribute. If interface and neighbor addresses are not present and the link local/remote identifiers are present, then the link local/remote Identifier TLV is included in the Link Descriptor. The Multi-Topology Identifier TLV is included in Link Descriptor if that information is present.
3.2.3. Prefix Descriptors
The Prefix Descriptor field is a set of Type/Length/Value (TLV) triplets. Prefix Descriptor TLVs uniquely identify an IPv4 or IPv6 prefix originated by a node. The following TLVs are valid as Prefix Descriptors in the IPv4/IPv6 Prefix NLRI: +-------------+---------------------+----------+--------------------+ | TLV Code | Description | Length | Reference | | Point | | | (RFC/Section) | +-------------+---------------------+----------+--------------------+ | 263 | Multi-Topology | variable | Section 3.2.1.5 | | | Identifier | | | | 264 | OSPF Route Type | 1 | Section 3.2.3.1 | | 265 | IP Reachability | variable | Section 3.2.3.2 | | | Information | | | +-------------+---------------------+----------+--------------------+ Table 6: Prefix Descriptor TLVs3.2.3.1. OSPF Route Type
The OSPF Route Type TLV is an optional TLV that MAY be present in Prefix NLRIs. It is used to identify the OSPF route type of the prefix. It is used when an OSPF prefix is advertised in the OSPF domain with multiple route types. The Route Type TLV allows the discrimination of these advertisements. The format of the OSPF Route Type TLV is shown in the following figure. 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 Type | +-+-+-+-+-+-+-+-+ Figure 13: OSPF Route Type TLV Format where the Type and Length fields of the TLV are defined in Table 6. The OSPF Route Type field values are defined in the OSPF protocol and can be one of the following: o Intra-Area (0x1) o Inter-Area (0x2) o External 1 (0x3)
o External 2 (0x4) o NSSA 1 (0x5) o NSSA 2 (0x6)3.2.3.2. IP Reachability Information
The IP Reachability Information TLV is a mandatory TLV that contains one IP address prefix (IPv4 or IPv6) originally advertised in the IGP topology. Its purpose is to glue a particular BGP service NLRI by virtue of its BGP next hop to a given node in the LSDB. A router SHOULD advertise an IP Prefix NLRI for each of its BGP next hops. The format of the IP Reachability Information TLV is shown in the following figure: 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Prefix Length | IP Prefix (variable) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 14: IP Reachability Information TLV Format The Type and Length fields of the TLV are defined in Table 6. The following two fields determine the reachability information of the address family. The Prefix Length field contains the length of the prefix in bits. The IP Prefix field contains the most significant octets of the prefix, i.e., 1 octet for prefix length 1 up to 8, 2 octets for prefix length 9 to 16, 3 octets for prefix length 17 up to 24, 4 octets for prefix length 25 up to 32, etc.
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