RFC 8665: OSPF Extensions for Segment Routing

RFC 8665	OSPF Extensions for Segment Routing	December 2019
Psenak, et al.	Standards Track	[Page]

Abstract

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.¶

3. Segment Routing Capabilities

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].¶

3.1. SR-Algorithm TLV

The SR-Algorithm TLV is a top-level TLV of the Router Information Opaque LSA (defined in [RFC7770]).¶

The SR-Algorithm TLV is optional. It SHOULD 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:¶

Type:

8¶

Length:

Variable, in octets, depending on the number of algorithms advertised¶

Algorithm:

Single octet identifying the algorithm. The following values are defined by this document:¶

0:

Shortest Path First (SPF) algorithm based on link metric. This is the standard shortest path algorithm as computed by the OSPF 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. If the SR-Algorithm TLV is advertised, Algorithm 0 MUST be included.¶

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 1 MUST NOT alter the SPF paths computed by Algorithm 1.¶

When multiple SR-Algorithm TLVs are received from a given router, the receiver MUST 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 scope MUST 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 ID MUST be used and subsequent instances of the SR-Algorithm TLV MUST 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 is REQUIRED.¶

3.2. SID/Label Range TLV

Prefix-SIDs MAY be advertised in the form of an index as described in Section 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 TLV MAY 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:¶

Type:

9¶

Length:

Variable, in octets, depending on the sub-TLVs¶

Range Size:

3-octet SID/label range size (i.e., the number of SIDs or labels in the range including the first SID/label). It MUST be greater than 0.¶

Reserved:

SHOULD be set to 0 on transmission and MUST be ignored on reception¶

Initially, the only supported sub-TLV is the SID/Label Sub-TLV as defined in Section 2.1. The SID/Label Sub-TLV MUST 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-TLV MAY be advertised in the SID/Label Range TLV. If more than one SID/Label Sub-TLV is present, the SID/Label Range TLV MUST be ignored.¶

Multiple occurrences of the SID/Label Range TLV MAY be advertised in order to advertise multiple ranges. In such a case:¶

The originating router MUST encode each range into a different SID/Label Range TLV.¶
The originating router decides the order in which the set of SID/Label Range TLVs are advertised inside the Router Information Opaque LSA. The originating router MUST ensure the order is the same after a graceful restart (using checkpointing, nonvolatile storage, or any other mechanism) in order to ensure the SID/Label range and SID index correspondence is preserved across graceful restarts.¶
The receiving router MUST adhere to the order in which the ranges are advertised when calculating a SID/Label from a SID index.¶
The originating router MUST NOT advertise overlapping ranges.¶
When a router receives multiple overlapping ranges, it MUST conform to the procedures defined in [RFC8660].¶

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 is REQUIRED.¶

3.3. SR Local Block TLV

The SR Local Block TLV (SRLB TLV) contains the range of labels the node has reserved for Local SIDs. SIDs from the SRLB MAY 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 TLV MAY 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:¶

Type:

14¶

Length:

Variable, in octets, depending on the sub-TLVs¶

Range Size:

3-octet SID/Label range size (i.e., the number of SIDs or labels in the range including the first SID/Label). It MUST be greater than 0.¶

Reserved:

SHOULD be set to 0 on transmission and MUST be ignored on reception¶

Initially, the only supported sub-TLV is the SID/Label Sub-TLV as defined in Section 2.1. The SID/Label Sub-TLV MUST 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-TLV MAY be advertised in the SRLB TLV. If more than one SID/Label Sub-TLV is present, the SRLB TLV MUST be ignored.¶

The originating router MUST NOT advertise overlapping ranges.¶

Each time a SID from the SRLB is allocated, it SHOULD 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 TLV MAY 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 is REQUIRED.¶

3.4. SRMS Preference TLV

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 TLV MAY 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:¶

Type:

15¶

Length:

4 octets¶

Preference:

1 octet, with an SRMS preference value from 0 to 255¶

Reserved:

SHOULD be set to 0 on transmission and MUST be ignored on reception¶

When multiple SRMS Preference TLVs are received from a given router, the receiver MUST 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 scope MUST 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 ID MUST be used and subsequent instances of the SRMS Preference TLV MUST 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 flooding SHOULD 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 flooding MAY be used.¶

4. OSPF Extended Prefix Range TLV

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 TLVs MAY be advertised in each OSPF Extended Prefix Opaque LSA, but all prefix ranges included in a single OSPF Extended Prefix Opaque LSA MUST 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:¶

Type:

2¶

Length:

Variable, in octets, depending on the sub-TLVs¶

Prefix Length:

Length of prefix in bits¶

AF:

Address family for the prefix. Currently, the only supported value is 0 for IPv4 unicast. The inclusion of address family in this TLV allows for future extension.¶

Range Size:

Represents the number of prefixes that are covered by the advertisement. The Range Size MUST NOT exceed the number of prefixes that could be satisfied by the Prefix Length without including the IPv4 multicast address range (224.0.0.0/3).¶

Flags:
Single-octet field. The following flags are defined:¶
```
     0  1  2  3  4  5  6  7 
   +--+--+--+--+--+--+--+--+
   |IA|  |  |  |  |  |  |  |
   +--+--+--+--+--+--+--+--+
```
¶
where:¶
- IA-Flag:
  
  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 areas MUST 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:¶
  
  An ABR always prefers intra-area Prefix Range advertisements over inter-area advertisements.¶
  
  An ABR does not consider inter-area Prefix Range advertisements coming from nonbackbone areas.¶
Reserved:

SHOULD be set to 0 on transmission and MUST be ignored on reception¶

Address Prefix:

For the address family IPv4 unicast, the prefix itself is encoded as a 32-bit value. The default route is represented by a prefix of length 0. Prefix encoding for other address families is beyond the scope of this specification.¶

5. Prefix-SID Sub-TLV

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 in Section 4. It MAY 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:¶

Type:

2¶

Length:

7 or 8 octets, depending on the V-Flag¶

Flags:
Single-octet field. The following flags are defined:¶
```
                      
     0  1  2  3  4  5  6  7 
   +--+--+--+--+--+--+--+--+
   |  |NP|M |E |V |L |  |  |
   +--+--+--+--+--+--+--+--+
```
¶
where:¶
- NP-Flag:
  
  No-PHP (Penultimate Hop Popping) Flag. If set, then the penultimate hop MUST NOT pop the Prefix-SID before delivering packets to the node that advertised the Prefix-SID.¶
  
  M-Flag:
  
  Mapping Server Flag. If set, the SID was advertised by an SR Mapping Server as described in [RFC8661].¶
  
  E-Flag:
  
  Explicit Null Flag. If set, any upstream neighbor of the Prefix-SID originator MUST replace the Prefix-SID with the Explicit NULL label (0 for IPv4) before forwarding the packet.¶
  
  V-Flag:
  
  Value/Index Flag. If set, then the Prefix-SID carries an absolute value. If not set, then the Prefix-SID carries an index.¶
  
  L-Flag:
  
  Local/Global Flag. If set, then the value/index carried by the Prefix-SID has local significance. If not set, then the value/index carried by this sub-TLV has global significance.¶
  
  Other bits:
  
  Reserved. These MUST be zero when sent and are ignored when received.¶
Reserved:

SHOULD be set to 0 on transmission and MUST be ignored on reception¶

MT-ID:

Multi-Topology ID (as defined in [RFC4915])¶

Algorithm:

Single octet identifying the algorithm the Prefix-SID is associated with as defined in Section 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.¶

SID/Index/Label:
According to the V- and L-Flags, it contains:¶
- V-Flag is set to 0 and L-Flag is set to 0: The SID/Index/Label field is a 4-octet index defining the offset in the SID/Label space advertised by this router.¶
- V-Flag is set to 1 and L-Flag is set to 1: The SID/Index/Label field is a 3-octet local label where the 20 rightmost bits are used for encoding the label value.¶
- All other combinations of V-Flag and L-Flag are invalid and any SID Advertisement received with an invalid setting for V- and L-Flags MUST be ignored.¶

If an OSPF router advertises multiple Prefix-SIDs for the same prefix, topology, and algorithm, all of them MUST be ignored.¶

When calculating the outgoing label for the prefix, the router MUST 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. This MUST 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-Flag MUST 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-Flag MUST 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:¶

Any upstream neighbor of the Prefix-SID originator MUST pop the Prefix-SID. This is equivalent to the penultimate hop-popping mechanism used in the MPLS data plane.¶
The received E-Flag is ignored.¶

If the NP-Flag is set and the E-Flag is not set, then:¶

Any upstream neighbor of the Prefix-SID originator MUST keep the Prefix-SID on top of the stack. This is useful when the originator of the Prefix-SID needs to stitch the incoming packet into a continuing MPLS LSP to the final destination. This could occur at an ABR (prefix propagation from one area to another) or at an ASBR (prefix propagation from one domain to another).¶

If both the NP-Flag and E-Flag are set, then:¶

Any upstream neighbor of the Prefix-SID originator MUST replace the Prefix-SID with an Explicit NULL label. This is useful, e.g., when the originator of the Prefix-SID is the final destination for the related prefix and the originator wishes to receive the packet with the original EXP bits.¶

When the M-Flag is set, the NP-Flag and the E-Flag MUST 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 behavior SHOULD be done in the following cases:¶

The Prefix is intra-area type and the downstream neighbor is the originator of the prefix.¶
The Prefix is inter-area type and the downstream neighbor is an ABR, which is advertising prefix reachability and is also generating the Extended Prefix TLV with the A-Flag set for this prefix as described in Section 2.1 of [RFC7684].¶
The Prefix is external type and the downstream neighbor is an ASBR, which is advertising prefix reachability and is also generating the Extended Prefix TLV with the A-Flag set for this prefix as described in Section 2.1 of [RFC7684].¶

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.¶

6. Adjacency Segment Identifier (Adj-SID)

An Adjacency Segment Identifier (Adj-SID) represents a router adjacency in Segment Routing.¶

6.1. Adj-SID Sub-TLV

Adj-SID is an optional sub-TLV of the Extended Link TLV defined in [RFC7684]. It MAY 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:¶

Type:

2¶

Length:

7 or 8 octets, depending on the V-Flag¶

Flags:
Single-octet field containing the following flags:¶
```
    
    0 1 2 3 4 5 6 7 
   +-+-+-+-+-+-+-+-+
   |B|V|L|G|P|     |
   +-+-+-+-+-+-+-+-+
```
¶
where:¶
- B-Flag:
  
  Backup Flag. If set, the Adj-SID refers to an adjacency that is eligible for protection (e.g., using IP Fast Reroute or MPLS-FRR (MPLS-Fast Reroute) as described in Section 2.1 of [RFC8402].¶
  
  V-Flag:
  
  Value/Index Flag. If set, then the Adj-SID carries an absolute value. If not set, then the Adj-SID carries an index.¶
  
  L-Flag:
  
  Local/Global Flag. If set, then the value/index carried by the Adj-SID has local significance. If not set, then the value/index carried by this sub-TLV has global significance.¶
  
  G-Flag:
  
  Group Flag. When set, the G-Flag indicates that the Adj-SID refers to a group of adjacencies (and therefore MAY be assigned to other adjacencies as well).¶
  
  P-Flag:
  
  Persistent Flag. When set, the P-Flag indicates that the Adj-SID is persistently allocated, i.e., the Adj-SID value remains consistent across router restart and/or interface flap.¶
  
  Other bits:
  
  Reserved. These MUST be zero when sent and are ignored when received.¶
Reserved:

SHOULD be set to 0 on transmission and MUST be ignored on reception¶

MT-ID:

Multi-Topology ID (as defined in [RFC4915]¶

Weight:

Weight used for load-balancing purposes. The use of the weight is defined in [RFC8402].¶

SID/Index/Label:

As described in Section 5 ¶

An SR-capable router MAY 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 in Section 3.5 of [RFC8402].¶

An SR-capable router MAY allocate more than one Adj-SID to an adjacency.¶

An SR-capable router MAY allocate the same Adj-SID to different adjacencies.¶

When the P-Flag is not set, the Adj-SID MAY be persistent. When the P-Flag is set, the Adj-SID MUST be persistent.¶

6.2. LAN Adj-SID Sub-TLV

The LAN Adjacency SID is an optional sub-TLV of the Extended Link TLV defined in [RFC7684]. It MAY 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:¶

Type:

3¶

Length:

11 or 12 octets, depending on the V-Flag¶

Flags:

Same as in Section 6.1 ¶

Reserved:

SHOULD be set to 0 on transmission and MUST be ignored on reception¶

MT-ID:

Multi-Topology ID (as defined in [RFC4915])¶

Weight:

Weight used for load-balancing purposes. The use of the weight is defined in [RFC8402].¶

Neighbor ID:

The Router ID of the neighbor for which the LAN Adjacency SID is advertised¶

SID/Index/Label:

As described in Section 5 ¶

When the P-Flag is not set, the LAN Adjacency SID MAY be persistent. When the P-Flag is set, the LAN Adjacency SID MUST be persistent.¶

7. Elements of Procedure

7.1. Intra-area Segment Routing in OSPFv2

An OSPFv2 router that supports Segment Routing MAY advertise Prefix- SIDs for any prefix to which it is advertising reachability (e.g., a loopback IP address as described in Section 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-SID MUST 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 Server MUST 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 in Section 4.¶

When propagating an OSPF Extended Prefix Range TLV between areas, ABRs MUST set the IA-Flag. This is used to prevent redundant flooding of the OSPF Extended Prefix Range TLV between areas as described in Section 4.¶

7.2. Inter-area Segment Routing in OSPFv2

In order to support SR in a multiarea environment, OSPFv2 MUST 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:¶

The ABR will look at its best path to the prefix in the source area and find the advertising router associated with the best path to that prefix.¶
The ABR will then determine if this router advertised a Prefix-SID for the prefix and use it when advertising the Prefix-SID to other connected areas.¶
If no Prefix-SID was advertised for the prefix in the source area by the router that contributes to the best path to the prefix, the originating ABR will use the Prefix-SID advertised by any other router when propagating the Prefix-SID for the prefix to other areas.¶

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:¶

The ABR will look at its best path to the prefix in the backbone area and find the advertising router associated with the best path to that prefix.¶
The ABR will then determine if such a router advertised a Prefix-SID for the prefix and use it when advertising the Prefix-SID to other connected areas.¶
If no Prefix-SID was advertised for the prefix in the backbone area by the ABR that contributes to the best path to the prefix, the originating ABR will use the Prefix-SID advertised by any other router when propagating the Prefix-SID for the prefix to other areas.¶

7.3. Segment Routing for External Prefixes

Type-5 LSAs are flooded domain wide. When an ASBR, which supports SR, generates Type-5 LSAs, it SHOULD 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, it SHOULD 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.¶

7.4. Advertisement of Adj-SID

The Adjacency Segment Routing Identifier (Adj-SID) is advertised using the Adj-SID Sub-TLV as described in Section 6.¶

7.4.1. Advertisement of Adj-SID on Point-to-Point Links

An Adj-SID MAY 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 adjacency MAY be removed from the area. If the adjacency transitions to a state lower than 2-Way, then the Adj-SID Advertisement MUST be withdrawn from the area.¶

7.4.2. Adjacency SID on Broadcast or NBMA Interfaces

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 network MAY advertise the Adj-SID for its adjacency to the DR using the Adj-SID Sub-TLV as described in Section 6.1.¶

SR-capable routers MAY 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 in Section 6.2.¶

Table 1: OSPF Router Information (RI) TLVs
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

Table 2: OSPFv2 Extended Prefix Opaque LSA TLVs
Value	Description	Reference
2	OSPF Extended Prefix Range TLV	This document

Table 3: OSPFv2 Extended Prefix TLV Sub-TLVs
Value	Description	Reference
1	SID/Label Sub-TLV	This document
2	Prefix-SID Sub-TLV	This document

Table 4: OSPFv2 Extended Link TLV Sub-TLVs
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

Table 5: IGP Algorithm Types
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 1 MUST NOT alter the SPF paths computed by Algorithm 1.	This document

11. References

11.1. Normative References

[RFC2119]: Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.
[RFC2328]: Moy, J., "OSPF Version 2", STD 54, RFC 2328, DOI 10.17487/RFC2328, April 1998, <https://www.rfc-editor.org/info/rfc2328>.
[RFC3101]: Murphy, P., "The OSPF Not-So-Stubby Area (NSSA) Option", RFC 3101, DOI 10.17487/RFC3101, January 2003, <https://www.rfc-editor.org/info/rfc3101>.
[RFC4915]: Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P. Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF", RFC 4915, DOI 10.17487/RFC4915, June 2007, <https://www.rfc-editor.org/info/rfc4915>.
[RFC6845]: Sheth, N., Wang, L., and J. Zhang, "OSPF Hybrid Broadcast and Point-to-Multipoint Interface Type", RFC 6845, DOI 10.17487/RFC6845, January 2013, <https://www.rfc-editor.org/info/rfc6845>.
[RFC7120]: Cotton, M., "Early IANA Allocation of Standards Track Code Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January 2014, <https://www.rfc-editor.org/info/rfc7120>.
[RFC7684]: Psenak, P., Gredler, H., Shakir, R., Henderickx, W., Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute Advertisement", RFC 7684, DOI 10.17487/RFC7684, November 2015, <https://www.rfc-editor.org/info/rfc7684>.
[RFC7770]: Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and S. Shaffer, "Extensions to OSPF for Advertising Optional Router Capabilities", RFC 7770, DOI 10.17487/RFC7770, February 2016, <https://www.rfc-editor.org/info/rfc7770>.
[RFC8126]: Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, <https://www.rfc-editor.org/info/rfc8126>.
[RFC8174]: Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8402]: Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., Decraene, B., Litkowski, S., and R. Shakir, "Segment Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8660]: Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S., Decraene, B., Litkowski, S., and R. Shakir, "Segment Routing with MPLS Data Plane", RFC 8660, DOI 10.17487/RFC8660, December 2019, <https://www.rfc-editor.org/info/rfc8660>.
[RFC8661]: Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S., Decraene, B., and S. Litkowski, "Segment Routing Interworking with LDP", RFC 8661, DOI 10.17487/RFC8661, December 2019, <https://www.rfc-editor.org/info/rfc8661>.

11.2. Informative References

[RFC3031]: Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol Label Switching Architecture", RFC 3031, DOI 10.17487/RFC3031, January 2001, <https://www.rfc-editor.org/info/rfc3031>.
[RFC5036]: Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed., "LDP Specification", RFC 5036, DOI 10.17487/RFC5036, October 2007, <https://www.rfc-editor.org/info/rfc5036>.
[RFC7474]: Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed., "Security Extension for OSPFv2 When Using Manual Key Management", RFC 7474, DOI 10.17487/RFC7474, April 2015, <https://www.rfc-editor.org/info/rfc7474>.
[RFC7855]: Previdi, S., Ed., Filsfils, C., Ed., Decraene, B., Litkowski, S., Horneffer, M., and R. Shakir, "Source Packet Routing in Networking (SPRING) Problem Statement and Requirements", RFC 7855, DOI 10.17487/RFC7855, May 2016, <https://www.rfc-editor.org/info/rfc7855>.
[RFC8666]: Psenak, P., Ed. and S. Previdi, Ed., "OSPFv3 Extensions for Segment Routing", RFC 8666, DOI 10.17487/RFC8666, December 2019, <https://www.rfc-editor.org/info/rfc8666>.