| RFC 8665 | OSPF Extensions for Segment Routing | December 2019 |
| Psenak, et al. | Standards Track | [Page] |
Segment Routing (SR) allows a flexible definition of end-to-end paths within IGP topologies by encoding paths as sequences of topological subpaths called "segments". These segments are advertised by the link-state routing protocols (IS-IS and OSPF).¶
This document describes the OSPFv2 extensions required for Segment Routing.¶
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 7841.¶
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained athttps://www.rfc-editor.org/info/rfc8665.¶
Copyright (c) 2019 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 (https://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.¶
Segment Routing (SR) allows a flexible definition of end-to-end paths within IGP topologies by encoding paths as sequences of topological subpaths called "segments". These segments are advertised by the link-state routing protocols (IS-IS and OSPF). Prefix segments represent an ECMP-aware shortest path to a prefix (or a node), as per the state of the IGP topology. Adjacency segments represent a hop over a specific adjacency between two nodes in the IGP. A prefix segment is typically a multi-hop path while an adjacency segment, in most cases, is a one-hop path. SR's control plane can be applied to both IPv6 and MPLS data planes, and it does not require any additional signaling (other than IGP extensions). The IPv6 data plane is out of the scope of this specification; it is not applicable to OSPFv2, which only supports the IPv4 address family. When used in MPLS networks, SR paths do not require any LDP or RSVP-TE signaling. However, SR can interoperate in the presence of LSPs established with RSVP or LDP.¶
There are additional segment types, e.g., Binding Segment Identifier (SID) defined in[RFC8402].¶
This document describes the OSPF extensions required for Segment Routing.¶
Segment Routing architecture is described in[RFC8402].¶
Segment Routing use cases are described in[RFC7855].¶
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[RFC2119][RFC8174] when, and only when, they appear in all capitals, as shown here.¶
Segment Routing defines various types of Segment Identifiers (SIDs): Prefix-SID, Adjacency SID, LAN Adjacency SID, and Binding SID.¶
Extended Prefix/Link Opaque Link State Advertisements (LSAs) defined in[RFC7684] are used for advertisements of the various SID types.¶
The SID/Label Sub-TLV appears in multiple TLVs or sub-TLVs defined later in this document. It is used to advertise the SID or label associated with a prefix or adjacency. The SID/Label 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 |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| SID/Label (variable) |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+¶
where:¶
Segment Routing requires some additional router capabilities to be advertised to other routers in the area.¶
These SR capabilities are advertised in the Router Information Opaque LSA (defined in[RFC7770]). The TLVs defined below are applicable to both OSPFv2 and OSPFv3; see also[RFC8666].¶
The SR-Algorithm TLV is a top-level TLV of the Router Information Opaque LSA (defined in[RFC7770]).¶
The SR-Algorithm TLV is optional. ItSHOULD only be advertised once in the Router Information Opaque LSA. If the SR-Algorithm TLV is not advertised by the node, such a node is considered as not being Segment Routing capable.¶
An SR Router can use various algorithms when calculating reachability to OSPF routers or prefixes in an OSPF area. Examples of these algorithms are metric-based Shortest Path First (SPF), various flavors of Constrained SPF, etc. The SR-Algorithm TLV allows a router to advertise the algorithms currently used by the router to other routers in an OSPF area. The SR-Algorithm 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 |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Algorithm 1 | Algorithm... | Algorithm n | |+- -+| |+ +¶
where:¶
Single octet identifying the algorithm. The following values are defined by this document:¶
When multiple SR-Algorithm TLVs are received from a given router, the receiverMUST use the first occurrence of the TLV in the Router Information Opaque LSA. If the SR-Algorithm TLV appears in multiple Router Information Opaque LSAs that have different flooding scopes, the SR-Algorithm TLV in the Router Information Opaque LSA with the area-scoped flooding scopeMUST be used. If the SR-Algorithm TLV appears in multiple Router Information Opaque LSAs that have the same flooding scope, the SR-Algorithm TLV in the Router Information (RI) Opaque LSA with the numerically smallest Instance IDMUST be used and subsequent instances of the SR-Algorithm TLVMUST be ignored.¶
The RI LSA can be advertised at any of the defined opaque flooding scopes (link, area, or Autonomous System (AS)). For the purpose of SR-Algorithm TLV advertisement, area-scoped flooding isREQUIRED.¶
Prefix-SIDsMAY be advertised in the form of an index as described inSection 5. Such an index defines the offset in the SID/Label space advertised by the router. The SID/Label Range TLV is used to advertise such SID/Label space.¶
The SID/Label Range TLV is a top-level TLV of the Router Information Opaque LSA (defined in[RFC7770]).¶
The SID/Label Range TLVMAY appear multiple times and 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 |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Range Size | Reserved |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Sub-TLVs (variable) |+- -+| |+ +¶
where:¶
Initially, the only supported sub-TLV is the SID/Label Sub-TLV as defined inSection 2.1. The SID/Label Sub-TLVMUST be included in the SID/Label Range TLV. The SID/Label advertised in the SID/Label Sub-TLV represents the first SID/Label in the advertised range.¶
Only a single SID/Label Sub-TLVMAY be advertised in the SID/Label Range TLV. If more than one SID/Label Sub-TLV is present, the SID/Label Range TLVMUST be ignored.¶
Multiple occurrences of the SID/Label Range TLVMAY be advertised in order to advertise multiple ranges. In such a case:¶
The following example illustrates the advertisement of multiple ranges.¶
The originating router advertises the following ranges:¶
Range 1: Range Size: 100 SID/Label Sub-TLV: 100 Range 1: Range Size: 100 SID/Label Sub-TLV: 1000 Range 1: Range Size: 100 SID/Label Sub-TLV: 500¶
The receiving routers concatenate the ranges and build the Segment Routing Global Block (SRGB) as follows:¶
SRGB = [100, 199] [1000, 1099] [500, 599]¶
The indexes span multiple ranges:¶
index 0 means label 100 ... index 99 means label 199 index 100 means label 1000 index 199 means label 1099 ... index 200 means label 500 ...¶
The RI LSA can be advertised at any of the defined flooding scopes (link, area, or autonomous system (AS)). For the purpose of SID/Label Range TLV advertisement, area-scoped flooding isREQUIRED.¶
The SR Local Block TLV (SRLB TLV) contains the range of labels the node has reserved for Local SIDs. SIDs from the SRLBMAY be used for Adjacency SIDs but also by components other than the OSPF protocol. As an example, an application or a controller can instruct the router to allocate a specific Local SID. Some controllers or applications can use the control plane to discover the available set of Local SIDs on a particular router. In such cases, the SRLB is advertised in the control plane. The requirement to advertise the SRLB is further described in[RFC8660]. The SRLB TLV is used to advertise the SRLB.¶
The SRLB TLV is a top-level TLV of the Router Information Opaque LSA (defined in[RFC7770]).¶
The SRLB TLVMAY appear multiple times in the Router Information Opaque LSA and 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 |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Range Size | Reserved |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Sub-TLVs (variable) |+- -+| |+ +¶
where:¶
Initially, the only supported sub-TLV is the SID/Label Sub-TLV as defined inSection 2.1. The SID/Label Sub-TLVMUST be included in the SRLB TLV. The SID/Label advertised in the SID/Label Sub-TLV represents the first SID/Label in the advertised range.¶
Only a single SID/Label Sub-TLVMAY be advertised in the SRLB TLV. If more than one SID/Label Sub-TLV is present, the SRLB TLVMUST be ignored.¶
The originating routerMUST NOT advertise overlapping ranges.¶
Each time a SID from the SRLB is allocated, itSHOULD also be reported to all components (e.g., controller or applications) in order for these components to have an up-to-date view of the current SRLB allocation. This is required to avoid collisions between allocation instructions.¶
Within the context of OSPF, the reporting of Local SIDs is done through OSPF sub-TLVs, such as the Adjacency SID (Section 6). However, the reporting of allocated Local SIDs can also be done through other means and protocols, which are outside the scope of this document.¶
A router advertising the SRLB TLVMAY also have other label ranges, outside of the SRLB, used for its local allocation purposes and not advertised in the SRLB TLV. For example, it is possible that an Adjacency SID is allocated using a local label that is not part of the SRLB.¶
The RI LSA can be advertised at any of the defined flooding scopes (link, area, or autonomous system (AS)). For the purpose of SRLB TLV advertisement, area-scoped flooding isREQUIRED.¶
The Segment Routing Mapping Server Preference TLV (SRMS Preference TLV) is used to advertise a preference associated with the node that acts as an SR Mapping Server. The role of an SRMS is described in[RFC8661]. SRMS preference is defined in[RFC8661].¶
The SRMS Preference TLV is a top-level TLV of the Router Information Opaque LSA (defined in[RFC7770]).¶
The SRMS Preference TLVMAY only be advertised once in the Router Information Opaque LSA and 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 |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Preference | Reserved |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+¶
where:¶
When multiple SRMS Preference TLVs are received from a given router, the receiverMUST use the first occurrence of the TLV in the Router Information Opaque LSA. If the SRMS Preference TLV appears in multiple Router Information Opaque LSAs that have different flooding scopes, the SRMS Preference TLV in the Router Information Opaque LSA with the narrowest flooding scopeMUST be used. If the SRMS Preference TLV appears in multiple Router Information Opaque LSAs that have the same flooding scope, the SRMS Preference TLV in the Router Information Opaque LSA with the numerically smallest Instance IDMUST be used and subsequent instances of the SRMS Preference TLVMUST be ignored.¶
The RI LSA can be advertised at any of the defined flooding scopes (link, area, or autonomous system (AS)). For the purpose of the SRMS Preference TLV advertisement, AS-scoped floodingSHOULD be used. This is because SRMS servers can be located in a different area than consumers of the SRMS advertisements. If the SRMS advertisements from the SRMS server are only used inside the SRMS server's area, area-scoped floodingMAY be used.¶
In some cases, it is useful to advertise attributes for a range of prefixes. The SR Mapping Server, which is described in[RFC8661], is an example where we need a single advertisement to advertise SIDs for multiple prefixes from a contiguous address range.¶
The OSPF Extended Prefix Range TLV, which is a top-level TLV of the Extended Prefix LSA described in[RFC7684] is defined for this purpose.¶
Multiple OSPF Extended Prefix Range TLVsMAY be advertised in each OSPF Extended Prefix Opaque LSA, but all prefix ranges included in a single OSPF Extended Prefix Opaque LSAMUST have the same flooding scope. The OSPF Extended Prefix Range 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 |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Prefix Length | AF | Range Size |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Flags | Reserved |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Address Prefix (variable) |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Sub-TLVs (variable) |+- -+| |¶
where:¶
Single-octet field. The following flags are defined:¶
0 1 2 3 4 5 6 7 +--+--+--+--+--+--+--+--+ |IA| | | | | | | | +--+--+--+--+--+--+--+--+¶
where:¶
Inter-Area Flag. If set, advertisement is of inter-area type. An Area Border Router (ABR) that is advertising the OSPF Extended Prefix Range TLV between areasMUST set this bit.¶
This bit is used to prevent redundant flooding of Prefix Range TLVs between areas as follows:¶
An ABR only propagates an inter-area Prefix Range advertisement from the backbone area to connected nonbackbone areas if the advertisement is considered to be the best one. The following rules are used to select the best range from the set of advertisements for the same Prefix Range:¶
The Prefix-SID Sub-TLV is a sub-TLV of the OSPF Extended Prefix TLV described in[RFC7684] and the OSPF Extended Prefix Range TLV described inSection 4. ItMAY appear more than once in the parent TLV and 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 |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Flags | Reserved | MT-ID | Algorithm | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| SID/Index/Label (variable) |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+¶
where:¶
Single-octet field. The following flags are defined:¶
0 1 2 3 4 5 6 7 +--+--+--+--+--+--+--+--+ | |NP|M |E |V |L | | | +--+--+--+--+--+--+--+--+¶
where:¶
Single octet identifying the algorithm the Prefix-SID is associated with as defined inSection 3.1¶
A router receiving a Prefix-SID from a remote node and with an algorithm value that the remote node has not advertised in the SR-Algorithm TLV (Section 3.1)MUST ignore the Prefix-SID Sub-TLV.¶
According to the V- and L-Flags, it contains:¶
If an OSPF router advertises multiple Prefix-SIDs for the same prefix, topology, and algorithm, all of themMUST be ignored.¶
When calculating the outgoing label for the prefix, the routerMUST take into account, as described below, the E-, NP-, and M-Flags advertised by the next-hop router if that router advertised the SID for the prefix. ThisMUST be done regardless of whether the next-hop router contributes to the best path to the prefix.¶
The NP-Flag (No-PHP)MUST be set and the E-FlagMUST be clear for Prefix-SIDs allocated to inter-area prefixes that are originated by the ABR based on intra-area or inter-area reachability between areas unless the advertised prefix is directly attached to the ABR.¶
The NP-Flag (No-PHP)MUST be set and the E-FlagMUST be clear for Prefix-SIDs allocated to redistributed prefixes, unless the redistributed prefix is directly attached to the Autonomous System Boundary Router (ASBR).¶
If the NP-Flag is not set, then:¶
If the NP-Flag is set and the E-Flag is not set, then:¶
If both the NP-Flag and E-Flag are set, then:¶
When the M-Flag is set, the NP-Flag and the E-FlagMUST be ignored on reception.¶
As the Mapping Server does not specify the originator of a prefix advertisement, it is not possible to determine PHP behavior solely based on the Mapping Server Advertisement. However, PHP behaviorSHOULD be done in the following cases:¶
When a Prefix-SID is advertised in an Extended Prefix Range TLV, then the value advertised in the Prefix-SID Sub-TLV is interpreted as a starting SID/Label value.¶
Example 1: If the following router addresses (loopback addresses) need to be mapped into the corresponding Prefix-SID indexes:¶
Router-A: 192.0.2.1/32, Prefix-SID: Index 1 Router-B: 192.0.2.2/32, Prefix-SID: Index 2 Router-C: 192.0.2.3/32, Prefix-SID: Index 3 Router-D: 192.0.2.4/32, Prefix-SID: Index 4¶
then the Prefix field in the Extended Prefix Range TLV would be set to 192.0.2.1, Prefix Length would be set to 32, Range Size would be set to 4, and the Index value in the Prefix-SID Sub-TLV would be set to 1.¶
Example 2: If the following prefixes need to be mapped into the corresponding Prefix-SID indexes:¶
192.0.2.0/30, Prefix-SID: Index 51 192.0.2.4/30, Prefix-SID: Index 52 192.0.2.8/30, Prefix-SID: Index 53 192.0.2.12/30, Prefix-SID: Index 54 192.0.2.16/30, Prefix-SID: Index 55 192.0.2.20/30, Prefix-SID: Index 56 192.0.2.24/30, Prefix-SID: Index 57¶
then the Prefix field in the Extended Prefix Range TLV would be set to 192.0.2.0, Prefix Length would be set to 30, Range Size would be 7, and the Index value in the Prefix-SID Sub-TLV would be set to 51.¶
An Adjacency Segment Identifier (Adj-SID) represents a router adjacency in Segment Routing.¶
Adj-SID is an optional sub-TLV of the Extended Link TLV defined in[RFC7684]. ItMAY appear multiple times in the Extended Link TLV. The Adj-SID 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 |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Flags | Reserved | MT-ID | Weight |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| SID/Label/Index (variable) |+---------------------------------------------------------------+¶
where:¶
Single-octet field containing the following flags:¶
0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |B|V|L|G|P| | +-+-+-+-+-+-+-+-+¶
where:¶
An SR-capable routerMAY allocate an Adj-SID for each of its adjacencies and set the B-Flag when the adjacency is eligible for protection by an FRR mechanism (IP or MPLS) as described inSection 3.5 of [RFC8402].¶
An SR-capable routerMAY allocate more than one Adj-SID to an adjacency.¶
An SR-capable routerMAY allocate the same Adj-SID to different adjacencies.¶
When the P-Flag is not set, the Adj-SIDMAY be persistent. When the P-Flag is set, the Adj-SIDMUST be persistent.¶
The LAN Adjacency SID is an optional sub-TLV of the Extended Link TLV defined in[RFC7684]. ItMAY appear multiple times in the Extended Link TLV. It is used to advertise a SID/Label for an adjacency to a non-DR (Designated Router) router on a broadcast, Non-Broadcast Multi-Access (NBMA), or hybrid[RFC6845] network.¶
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 |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Flags | Reserved | MT-ID | Weight |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Neighbor ID |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| SID/Label/Index (variable) |+---------------------------------------------------------------+¶
where:¶
When the P-Flag is not set, the LAN Adjacency SIDMAY be persistent. Whenthe P-Flag is set, the LAN Adjacency SIDMUST be persistent.¶
An OSPFv2 router that supports Segment RoutingMAY advertise Prefix- SIDs for any prefix to which it is advertising reachability (e.g., a loopback IP address as described inSection 5).¶
A Prefix-SID can also be advertised by the SR Mapping Servers (as described in[RFC8661]). A Mapping Server advertises Prefix-SIDs for remote prefixes that exist in the OSPFv2 routing domain. Multiple Mapping Servers can advertise Prefix-SIDs for the same prefix; in which case, the same Prefix-SIDMUST be advertised by all of them. The flooding scope of the OSPF Extended Prefix Opaque LSA that is generated by the SR Mapping Server could be either area scoped or AS scoped and is determined based on the configuration of the SR Mapping Server.¶
An SR Mapping ServerMUST use the OSPF Extended Prefix Range TLV when advertising SIDs for prefixes. Prefixes of different route types can be combined in a single OSPF Extended Prefix Range TLV advertised by an SR Mapping Server. Because the OSPF Extended Prefix Range TLV doesn't include a Route-Type field, as in the OSPF Extended Prefix TLV, it is possible to include adjacent prefixes from different route types in the OSPF Extended Prefix Range TLV.¶
Area-scoped OSPF Extended Prefix Range TLVs are propagated between areas. Similar to propagation of prefixes between areas, an ABR only propagates the OSPF Extended Prefix Range TLV that it considers to be the best from the set it received. The rules used to pick the best OSPF Extended Prefix Range TLV are described inSection 4.¶
When propagating an OSPF Extended Prefix Range TLV between areas, ABRsMUST set the IA-Flag. This is used to prevent redundant flooding of the OSPF Extended Prefix Range TLV between areas as described inSection 4.¶
In order to support SR in a multiarea environment, OSPFv2MUST propagate Prefix-SID information between areas. The following procedure is used to propagate Prefix-SIDs between areas.¶
When an OSPF ABR advertises a Type-3 Summary LSA from an intra-area prefix to all its connected areas, it will also originate an OSPF Extended Prefix Opaque LSA as described in[RFC7684]. The flooding scope of the OSPF Extended Prefix Opaque LSA type will be set to area-local scope. The route type in the OSPF Extended Prefix TLV is set to inter-area. The Prefix-SID Sub-TLV will be included in this LSA and the Prefix-SID value will be set as follows:¶
When an OSPF ABR advertises Type-3 Summary LSAs from an inter-area route to all its connected areas, it will also originate an OSPF Extended Prefix Opaque LSA as described in[RFC7684]. The flooding scope of the OSPF Extended Prefix Opaque LSA type will be set to area-local scope. The route type in the OSPF Extended Prefix TLV is set to inter-area. The Prefix-SID Sub-TLV will be included in this LSA and the Prefix-SID will be set as follows:¶
Type-5 LSAs are flooded domain wide. When an ASBR, which supports SR, generates Type-5 LSAs, itSHOULD also originate OSPF Extended Prefix Opaque LSAs as described in[RFC7684]. The flooding scope of the OSPF Extended Prefix Opaque LSA type is set to AS-wide scope. The route type in the OSPF Extended Prefix TLV is set to external. The Prefix-SID Sub-TLV is included in this LSA and the Prefix-SID value will be set to the SID that has been reserved for that prefix.¶
When a Not-So-Stubby Area (NSSA)[RFC3101] ABR translates Type-7 LSAs into Type-5 LSAs, itSHOULD also advertise the Prefix-SID for the prefix. The NSSA ABR determines its best path to the prefix advertised in the translated Type-7 LSA and finds the advertising router associated with that path. If the advertising router has advertised a Prefix-SID for the prefix, then the NSSA ABR uses it when advertising the Prefix-SID for the Type-5 prefix. Otherwise, the Prefix-SID advertised by any other router will be used.¶
The Adjacency Segment Routing Identifier (Adj-SID) is advertised using the Adj-SID Sub-TLV as described inSection 6.¶
An Adj-SIDMAY be advertised for any adjacency on a point-to-point (P2P) link that is in neighbor state 2-Way or higher. If the adjacency on a P2P link transitions from the FULL state, then the Adj-SID for that adjacencyMAY be removed from the area. If the adjacency transitions to a state lower than 2-Way, then the Adj-SID AdvertisementMUST be withdrawn from the area.¶
Broadcast, NBMA, or hybrid[RFC6845] networks in OSPF are represented by a star topology where the Designated Router (DR) is the central point to which all other routers on the broadcast, NBMA, or hybrid network connect. As a result, routers on the broadcast, NBMA, or hybrid network advertise only their adjacency to the DR. Routers that do not act as DR do not form or advertise adjacencies with each other. They do, however, maintain 2-Way adjacency state with each other and are directly reachable.¶
When Segment Routing is used, each router on the broadcast, NBMA, or hybrid networkMAY advertise the Adj-SID for its adjacency to the DR using the Adj-SID Sub-TLV as described inSection 6.1.¶
SR-capable routersMAY also advertise a LAN Adjacency SID for other neighbors (e.g., Backup Designated Router, DR-OTHER, etc.) on the broadcast, NBMA, or hybrid network using the LAN Adj-SID Sub-TLV as described inSection 6.2.¶
This specification updates several existing OSPF registries and creates a new IGP registry.¶
The following values have been allocated:¶
| Value | TLV Name | Reference |
|---|---|---|
| 8 | SR-Algorithm TLV | This document |
| 9 | SID/Label Range TLV | This document |
| 14 | SR Local Block TLV | This document |
| 15 | SRMS Preference TLV | This document |
The following values have been allocated:¶
| Value | Description | Reference |
|---|---|---|
| 2 | OSPF Extended Prefix Range TLV | This document |
The following values have been allocated:¶
| Value | Description | Reference |
|---|---|---|
| 1 | SID/Label Sub-TLV | This document |
| 2 | Prefix-SID Sub-TLV | This document |
The following initial values have been allocated:¶
| Value | Description | Reference |
|---|---|---|
| 1 | SID/Label Sub-TLV | This document |
| 2 | Adj-SID Sub-TLV | This document |
| 3 | LAN Adj-SID/Label Sub-TLV | This document |
IANA has set up a subregistry called "IGP Algorithm Type" under the "Interior Gateway Protocol (IGP) Parameters" registry. The registration policy for this registry is "Standards Action" ([RFC8126] and[RFC7120]).¶
Values in this registry come from the range 0-255.¶
The initial values in the IGP Algorithm Type registry are as follows:¶
| Value | Description | Reference |
|---|---|---|
| 0 | Shortest Path First (SPF) algorithm based on link metric. This is the standard shortest path algorithm as computed by the IGP protocol. Consistent with the deployed practice for link-state protocols, Algorithm 0 permits any node to overwrite the SPF path with a different path based on its local policy. | This document |
| 1 | Strict Shortest Path First (SPF) algorithm based on link metric. The algorithm is identical to Algorithm 0, but Algorithm 1 requires that all nodes along the path will honor the SPF routing decision. Local policy at the node claiming support for Algorithm 1MUST NOT alter the SPF paths computed by Algorithm 1. | This document |
For any new TLVs/sub-TLVs defined in this document, if the length isinvalid, the LSA in which it is advertised is considered malformed andMUST beignored. An errorSHOULD be logged subject to rate limiting.¶
With the OSPFv2 Segment Routing extensions defined herein, OSPFv2 will now program the MPLS data plane[RFC3031] in addition to the IP data plane. Previously, LDP[RFC5036] or another label distribution mechanism was required to advertise MPLS labels and program the MPLS data plane.¶
In general, the same types of attacks that can be carried out on the IP control plane can be carried out on the MPLS control plane resulting in traffic being misrouted in the respective data planes. However, the latter can be more difficult to detect and isolate.¶
Existing security extensions as described in[RFC2328] and[RFC7684] apply to these Segment Routing extensions. While OSPF is under a single administrative domain, there can be deployments where potential attackers have access to one or more networks in the OSPF routing domain. In these deployments, stronger authentication mechanisms such as those specified in[RFC7474]SHOULD be used.¶
ImplementationsMUST assure that malformed TLVs and sub-TLVs defined in this document are detected and do not provide a vulnerability for attackers to crash the OSPFv2 router or routing process. Reception of malformed TLVs or sub-TLVsSHOULD be counted and/or logged for further analysis. Logging of malformed TLVs and sub-TLVsSHOULD be rate limited to prevent a Denial of Service (DoS) attack (distributed or otherwise) from overloading the OSPF control plane.¶
We would like to thank Anton Smirnov for his contribution.¶
Thanks to Acee Lindem for the detailed review of the document, corrections, as well as discussion about details of the encoding.¶
The following people gave a substantial contribution to the content of this document: Acee Lindem, Ahmed Bashandy, Martin Horneffer, Bruno Decraene, Stephane Litkowski, Igor Milojevic, and Saku Ytti.¶