IS-IS for SRv6

Segment routing (SR) uses an IGP to advertise topology, prefix, locator, and SID information. To complete the preceding functions, the IGP extends some TLVs of protocol packets. Table 1 shows IS-IS extensions for SRv6.

**Table 1** Extended IS-IS TLVs for SRv6
Name	Function	Carried In
SRv6 Locator TLV	Advertises an SRv6 locator and the End SIDs associated with it.	IS-IS LSP
SRv6 Capabilities sub-TLV	Advertises SRv6 capabilities.	IS-IS Router Capability TLV-242
SRv6 End SID sub-TLV	Advertises SRv6 SIDs.	SRv6 Locator TLV
SRv6 End.X SID sub-TLV	Advertises SRv6 SIDs on a point-to-point (P2P) network.	IS-IS Extended IS reachability TLV-22 IS-IS IS Neighbor Attribute TLV-23 IS-IS inter-AS reachability information TLV-141 IS-IS Multitopology IS TLV-222 IS-IS Multitopology IS Neighbor Attribute TLV-223
SRv6 LAN End.X SID sub-TLV	Advertises SRv6 SIDs on a local area network (LAN).	IS-IS Extended IS reachability TLV-22 IS-IS IS Neighbor Attribute TLV-23 IS-IS inter-AS reachability information TLV-141 IS-IS Multitopology IS TLV-222 IS-IS Multitopology IS Neighbor Attribute TLV-223
Node MSD sub-TLV	Advertises the maximum SID depth (MSD) supported by a device.	IS-IS Router Capability TLV-242
IS-IS FAD sub-TLV	Advertises the Flex-Algo definition (FAD) of IS-IS. For details, see SRv6 Flex-Algo.	IS-IS Router Capability TLV-242
SR-Algorithm sub-TLV	Advertises the algorithm that is used. For details, see SRv6 Flex-Algo.	IS-IS Router Capability TLV-242

SRv6 Locator TLV

The SRv6 Locator TLV is used to advertise an SRv6 locator and the End SIDs associated with it. Figure 1 shows its format.

Figure 1 SRv6 Locator TLV

**Table 2** Fields in the SRv6 Locator TLV
Field	Length	Description
Type	8 bits	Type.
Length	8 bits	Length.
MTID	12 bits	Multi-topology identifier.
Metric	32 bits	Metric.
Flags	8 bits	Flags bit, expressed using the format shown in Figure 2. Currently, only the D bit is available. When a SID is leaked from Level-2 to Level-1, the D bit must be set. SIDs with the D bit set must not be leaked from Level-1 to Level-2 to prevent route loops. Figure 2 Flags field format
Algorithm	8 bits	Associated algorithm: 0: shortest path first 1: strict shortest path first
Loc Size	8 bits	Number of bits in the Locator field.
Locator (variable)	Variable length	Advertised SRv6 locator.
sub-tlv-len	8 bits	Sub-TLV length.
Sub-TLVs (variable)	Variable length	Included sub-TLVs, for example, SRv6 End SID sub-TLV.

SRv6 locators can be advertised using the SRv6 Locator TLV. After receiving the TLV, SRv6-capable IS-IS devices install corresponding locator route entries to their local forwarding tables, but SRv6-incapable IS-IS devices do not.

SRv6 locators can also be advertised using the Prefix Reachability TLV 236 or 237. In this advertisement mode, SRv6-incapable devices can install corresponding locator route entries to their local forwarding tables, thereby achieving interworking with SRv6-capable devices. In cases where a locator advertisement is received in both a Prefix Reachability TLV and an SRv6 Locator TLV, the Prefix Reachability TLV is preferentially used.

SRv6 Capabilities Sub-TLV

In SRv6, segment list information is stored in the Segment Routing header (SRH). SRv6-capable nodes must be able to process SRH information under certain restrictions. The SRv6 Capabilities sub-TLV is used to advertise the SRv6 capabilities supported by the local node. Figure 3 shows its format.

Figure 3 SRv6 Capabilities sub-TLV

**Table 3** Fields in the SRv6 Capabilities sub-TLV
Field	Length	Description
Type	8 bits	Type.
Length	8 bits	Length.
Flags	16 bits	Flag bit.
Optional sub-sub-TLVs	Variable length	Optional sub-sub-TLVs.

SRv6 End SID Sub-TLV

The SRv6 End SID sub-TLV is used to advertise SRv6 End SIDs with endpoint functions. Figure 4 shows its format.

Figure 4 SRv6 End SID sub-TLV

**Table 4** Fields in the SRv6 End SID sub-TLV
Field	Length	Description
Type	8 bits	Type.
Length	8 bits	Length.
Flags	8 bits	Flag bit.
SRv6 Endpoint Function	16 bits	SRv6 endpoint function. For details about supported values, see SRv6 Endpoint Function.
SID	16 octets	Advertised SRv6 SID.
Sub-sub-tlv-len	8 bits	Sub-sub-TLV length.
Sub-sub-TLVs (variable)	Variable length	Included sub-sub-TLVs.

SRv6 End.X SID Sub-TLV

The SRv6 End.X SID sub-TLV is used to advertise an SRv6 End.X SID associated with a P2P adjacency. Figure 5 shows its format.

Figure 5 SRv6 End.X SID sub-TLV

**Table 5** Fields in the SRv6 End.X SID sub-TLV
Field	Length	Description
Type	8 bits	Type.
Length	8 bits	Length.
Flags	8 bits	Flag bit. Figure 6 shows the format of this field. Figure 6 Flags field format B-Flag: backup flag. S-Flag: set flag. When set, it indicates that the End.X SID represents a set of adjacencies. P-Flag: persistent flag. When set, it indicates that the End.X SID is persistently allocated.
Algorithm	8 bits	Associated algorithm.
Weight	8 bits	Weight of the End.X SID for the purpose of load balancing.
SRv6 Endpoint Function	16 bits	SRv6 endpoint function. For details about supported values, see SRv6 Endpoint Function.
SID	16 octets	Advertised SRv6 SID.
Sub-sub-tlv-len	8 bits	Sub-sub-TLV length.
Sub-sub-TLVs (variable)	Variable length	Included sub-sub-TLVs.

SRv6 LAN End.X SID Sub-TLV

The SRv6 LAN End.X SID sub-TLV is used to advertise an SRv6 End.X SID associated with a LAN adjacency. Figure 7 shows its format.

Figure 7 SRv6 LAN End.X SID sub-TLV

Compared with the SRv6 End.X SID sub-TLV, the SRv6 LAN End.X SID sub-TLV simply has an additional System ID field. Table 6 describes the fields in this sub-TLV.

**Table 6** Fields in the SRv6 LAN End.X SID sub-TLV
Field	Length	Description
System ID	48 bits	IS-IS system ID

Node MSD Sub-TLV

The Node MSD sub-TLV is used to advertise the MSD supported by a device. Figure 8 shows its format.

Figure 8 Node MSD sub-TLV

Table 7 describes the fields in the Node MSD sub-TLV.

**Table 7** Fields in the Node MSD sub-TLV
Field	Length	Description
Type	8 bits	Type.
Length	8 bits	Length.
MSD-Type	8 bits	MSD type: Maximum Segments Left MSD Type: specifies the maximum value of the Segments Left (SL) field in the SRH of a received packet before the SRv6 endpoint function associated with a SID is applied. Maximum End Pop MSD Type: specifies the maximum number of SIDs in the top SRH in an SRH stack. Value 0 indicates that the local node cannot perform the penultimate segment pop of the SRH (PSP) or ultimate segment pop of the SRH (USP) operation. Maximum H.Insert MSD Type: specifies the maximum number of SIDs that can be inserted as part of the H.Insert operation. Value 0 indicates that the local node cannot perform an H.Insert operation. Maximum H.Encaps MSD Type: specifies the maximum number of SIDs that can be included as part of the H.Encaps operation. Maximum End D MSD Type: specifies the maximum number of SIDs in an SRH when performing decapsulation associated with End.D (for example, End.DX6 and End.DT6) functions.
MSD Value	8 bits	MSD value.

SRv6 Endpoint Function

SRv6 uses endpoint functions to define the behaviors associated with different SIDs. In addition to advertising locator routes, IS-IS advertises SRv6 SIDs and their associated SRv6 endpoint functions through various SID sub-TLVs for path/service programming units to perform network programming. Table 8 shows the SRv6 endpoint functions that can be advertised by IS-IS. In this table, "Y" indicates that the corresponding sub-TLV can advertise the specified type of SRv6 endpoint function, and "N" indicates that the corresponding sub-TLV cannot advertise the specified type of SRv6 endpoint function.

**Table 8** SRv6 endpoint functions that can be advertised by IS-IS
SRv6 Endpoint Function	SRv6 End SID Sub-TLV	SRv6 End.X SID Sub-TLV	SRv6 LAN End.X SID Sub-TLV
End (no PSP, no USP)	Y	N	N
End (with PSP)	Y	N	N
End (with USP)	Y	N	N
End (with PSP & USP)	Y	N	N
End.X (no PSP, no USP)	N	Y	Y
End.X (with PSP)	N	Y	Y
End.X (with USP)	N	Y	Y
End.X (with PSP & USP)	N	Y	Y
End.DT4	Y	N	N
End.DT6	Y	N	N
End.DX4	N	Y	Y
End.DX6	N	Y	Y
End.OP	Y	N	N