BGP-LS Extension of SRv6 Flex-Algo

In scenarios where a controller dynamically orchestrates an SRv6 TE Policy based on Flex-Algo, forwarders need to report information such as node attributes, link attributes, and SRv6 SIDs to the controller through BGP-LS. BGP-LS information is encapsulated in Node NLRI/Link NLRI information and advertised to the controller.

BGP-LS is extended to implement the preceding functions, as described in Table 1.

**Table 1** BGP-LS extensions for SRv6 Flex-Algo
Attribute	TLV	Sub-TLV
Node attributes	FAD	Flex-Algo Exclude Any Affinity
		Flex-Algo Include Any Affinity
		Flex Algo Include All Affinity
		Flex-Algo Definition Flags
	SR-Algorithm TLV	None
Link attributes	Application Specific Link Attributes TLV	TE Metric
		Min/Max Unidirectional Link Delay
		Extended Administrative Group (color)
		Unidirectional Link Loss TLV
	SRv6 End.X SID TLV	None
	SRv6 LAN End.X SID TLV	None
SRv6 SID attributes	SRv6 Endpoint Function TLV	None

FAD TLV

The FAD TLV is carried in the Node NLRI. Figure 1 shows the format of the FAD TLV.

Figure 1 FAD TLV format

Figure 1 describes the fields in the FAD TLV.

**Table 2** Fields in the FAD TLV
Field	Length	Description
Type	16 bits	Type of the TLV.
Length	16 bits	Total length of the TLV (excluding the Type and Length fields).
Flex-Algo	8 bits	Flex-Algo ID, which is an integer ranging from 128 to 255.
Metric-Type	8 bits	Metric type used during calculation. This field can be set to an IGP metric, minimum unidirectional link delay, or TE metric value.
Calc-Type	8 bits	Calculation type, which can currently only be SPF and does not need setting.
Priority	8 bits	Priority of the TLV.
Sub-TLVs	Variable	Optional sub-TLVs, which can define some constraints.

The sub-TLVs of the FAD TLV include Flex-Algo Exclude Any Affinity, Flex-Algo Include Any Affinity, Flex-Algo Include All Affinity, and Flex-Algo Definition Flags.

The Flex-Algo Exclude Any Affinity, Flex-Algo Include Any Affinity, and Flex-Algo Include All Affinity sub-TLVs have the same format, as shown in Figure 2.

Figure 2 Affinity sub-TLV format

Figure 3 shows the format of the Flex-Algo Definition Flags sub-TLV.

Figure 3 Format of the Flex-Algo Definition Flags sub-TLV

SR-Algorithm TLV

The SR-Algorithm TLV is used to advertise the Flex-Algos supported by a node. Figure 4 shows the format of the SR-Algorithm TLV.

Figure 4 Format of the SR-Algorithm TLV

Table 3 describes the fields in the SR-Algorithm TLV.

**Table 3** Fields in the SR-Algorithm TLV
Field	Length	Description
Type	16 bits	Type of the TLV.
Length	16 bits	Total length of the TLV (excluding the Type and Length fields).
Algorithm	8 bits	Algorithm.

Application Specific Link Attributes TLV

Figure 5 shows the format of the Application Specific Link Attributes TLV.

Figure 5 Format of the Application Specific Link Attributes TLV

Table 4 describes the fields in the Application Specific Link Attributes TLV.

**Table 4** Fields in the Application Specific Link Attributes TLV
Field	Length	Description
Type	16 bits	Type of the TLV.
Length	16 bits	Total length of the TLV (excluding the Type and Length fields).
SABML	8 bits	Standard Application Identifier Bit Mask Length, in octets. The value must be 0, 4, or 8. If the Standard Application Identifier Bit Mask field does not exist, the SABML field must be set to 0.
UDABML	8 bits	User-Defined Application Identifier Bit Mask Length, in octets. The value must be 0, 4, or 8. If the User-Defined Application Identifier Bit Mask field does not exist, the UDABML field must be set to 0.
Standard Application Identifier Bit Mask	Variable	Standard Application Identifier Bit Mask, where each bit represents a standard application.
User-Defined Application Identifier Bit Mask	Variable	User-Defined Application Identifier Bit Mask, where each bit represents a user-defined application.
Link Attribute Sub-TLVs	Variable	Sub-TLVs contained in the TLV.

The Application Specific Link Attributes TLV carries sub-TLVs, such as TE Metric, Min/Max Unidirectional Link Delay, Extended Administrative Group (color), and Unidirectional Link Loss.

Figure 6 shows the format of the TE Metric sub-TLV.

Figure 6 Format of the TE Metric sub-TLV

Figure 7 shows the format of the Min/Max Unidirectional Link Delay sub-TLV.

Figure 7 Format of the Min/Max Unidirectional Link Delay sub-TLV

Figure 8 shows the format of the Extended Administrative Group (color) sub-TLV.

Figure 8 Format of the Extended Administrative Group (color) sub-TLV

Figure 9 shows the format of the Unidirectional Link Loss sub-TLV.

Figure 9 Format of the Unidirectional Link Loss sub-TLV

SRv6 End.X SID Sub-TLV

The SRv6 End.X SID sub-TLV is used to advertise SRv6 End.X SIDs associated with adjacencies. Figure 10 shows the format of the SRv6 End.X SID sub-TLV.

Figure 10 Format of the SRv6 End.X SID sub-TLV
Click to enlarge

Table 5 describes the fields in the SRv6 End.X SID sub-TLV.

**Table 5** Fields in the SRv6 End.X SID sub-TLV
Field	Length	Description
Type	16 bits	Type.
Length	16 bits	Length.
Endpoint Function	16 bits	Type of an SRv6 endpoint behavior.
Flags	8 bits	Flags: B: Backup flag. If this flag is set, the End.X SID has a backup path. S: Set flag. If this flag is set, the End.X SID is associated with a group of adjacencies. P: Persistent flag. If this flag is set, the End.X SID is permanently allocated. The End.X SID can remain unchanged after peer relationship flapping, a protocol restart, or a device restart occurs.
Algorithm	8 bits	Algorithm ID: 0: SPF 1: Strict SPF 128–255: used in Flex-Algo scenarios
Weight	8 bits	Weight of the End.X SID used for load balancing.
SID	16 octets	Advertised SRv6 SID.
Sub-TLVs (variable)	Variable	Included sub-sub-TLVs.

SRv6 LAN End.X SID Sub-TLV

The SRv6 LAN End.X SID sub-TLV is used to advertise SRv6 End.X SIDs associated with adjacencies on a LAN. Figure 11 shows the format of the SRv6 LAN End.X SID sub-TLV.

Figure 11 Format of the SRv6 LAN End.X SID sub-TLV

Compared with the SRv6 End.X SID sub-TLV, this sub-TLV has one more ISIS System-ID (6 octets) or OSPFv3 Router-ID (4 octets) of the neighbor 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
ISIS System-ID (6 octets) or OSPFv3 Router-ID (4 octets) of the neighbor	48/32 bits	System ID of an IS-IS process or router ID of an OSPFv3 neighbor.

SRv6 Endpoint Function TLV

The SRv6 Endpoint Function TLV is an SRv6 SID NLRI attribute used to carry the instructions associated with each type of SID. Each SRv6 SID is associated with a network function instruction. Figure 12 shows the format of the SRv6 Endpoint Function TLV.

Figure 12 Format of the SRv6 Endpoint Function TLV

Table 7 describes the fields in the SRv6 Endpoint Function TLV.

**Table 7** Fields in the SRv6 Endpoint Function TLV
Field	Length	Description
Type	16 bits	Type of the TLV.
Length	16 bits	Length of the Value field in the TLV, in bytes.
SRv6 Endpoint Function	16 bits	Code value of the associated network function.
Flags	8 bits	Flags, which are not in use currently.
Algorithm	8 bits	Algorithm ID: 0: SPF 1: strict SPF 128–255: used in Flex-Algo scenarios