IS-ISv6 for BIERv6

To forward BIERv6 packets based on a BitString, each node must first generate a BIFT. The nodes obtain the information required to generate a BIFT by using IS-ISv6 extensions designed for BIERv6, as described in Table 1.

**Table 1** IS-ISv6 extensions for BIERv6
Type	Name	Function	Carried In
TLV	Extended IS Reachability TLV (IPv6)	Advertises BFR-prefixes and floods BFR node information in a sub-domain. The BFR-prefix is a loopback interface IPv6 address of a BFR in a sub-domain.	IS-IS packets
Sub-TLV	BIER Info Sub-TLV	Advertises information such as sub-domain IDs and BFR-IDs.	TLV (Type = 237) in IS-IS packets
Sub-Sub-TLV	End.BIER Info Sub-sub-TLV MPRA Info Sub-sub-TLV	Advertises End.BIER SIDs or MPRAs.	BIER Info Sub-TLV
Sub-Sub-TLV	BIERv6 Encapsulation Sub-sub-TLV	Advertises the Max SI (short for set ID), BSL, and start BIFT-ID.	BIER Info Sub-TLV

Extended IS Reachability TLV (IPv6)

The Extended IS Reachability TLV (IPv6) is used to advertise BFR-prefixes. Figure 1 shows its format.

Figure 1 Extended IS Reachability TLV (IPv6)

Table 2 describes the fields.

**Table 2** Fields in the Extended IS Reachability TLV (IPv6)
Field	Length	Description
Type	8 bits	TLV type. The value is 236 or 237. The value 236 indicates that this TLV is used if the IPv6 capability is enabled for an IS-IS process in standard topology mode using the ipv6 enable topology standard command. The value 237 indicates that this TLV is used if the IPv6 capability is enabled for an IS-IS process in IPv6 topology mode using the ipv6 enable topology ipv6 command. Currently, only the type value 237 is supported.
Length	8 bits	Length.
Metric Information	32 bits	Metric information of the IPv6 prefix.
U	1 bit	U flag, indicating Up or Down.
X	1 bit	External bit.
S	1 bit	S flag, indicating whether a sub-TLV is present.
Resv	5 bits	Reserved field.
Prefix Len	8 bits	The value ranges from 0 to 128, and is 128 when BIERv6 information is carried.
Prefix	128 bits	BFR-prefix, which is a loopback interface IPv6 address of a BFR in a sub-domain.
BIER Info Sub-TLV	Variable	This field is optional and used to carry BIER information.

BIER Info Sub-TLV

The BIER Info Sub-TLV is used to advertise information such as sub-domain IDs and BFR-IDs. Figure 2 shows its format.

Figure 2 BIER Info Sub-TLV

Table 3 describes the fields.

**Table 3** Fields in the BIER Info Sub-TLV
Field	Length	Description
Type	8 bits	Type. The value is 32.
Length	8 bits	Length.
BAR	8 bits	BIER algorithm used to calculate a path to a BFER. BIER algorithms are defined by the Internet Assigned Numbers Authority (IANA).
IPA	8 bits	IGP algorithm, which can be used by the BIER algorithm defined in the BAR field.
Sub-domain-id	8 bits	Sub-domain ID, which uniquely identifies a sub-domain.
BFR-ID	16 bits	BFR-ID of a node in the sub-domain. If no BFR-ID is configured, 0 (invalid BFR-ID) is carried in packets.
Sub-sub-TLV	Variable	This field is optional. Whether the sub-sub-TLV is present is determined by the Length field. The BIER Info Sub-TLV may include End.BIER Info Sub-sub-TLV and BIERv6 Encapsulation Sub-sub-TLV.

End.BIER Info Sub-sub-TLV

The End.BIER Info Sub-sub-TLV is used to advertise End.BIER SIDs. Figure 3 shows its format.

Figure 3 End.BIER Info Sub-sub-TLV

Table 4 describes the fields.

**Table 4** Fields in the End.BIER Info Sub-sub-TLV
Field	Length	Description
Type	8 bits	The value is 3, indicating that an End.BIER SID is carried.
Length	8 bits	Length. The value is 16.
End.BIER IPv6 Address	128 bits	End.BIER SID.

MPRA Info Sub-sub-TLV

The MPRA Info Sub-sub-TLV is used to advertise MPRAs. Figure 4 shows its format.

Figure 4 MPRA Info Sub-sub-TLV

Table 5 describes the fields.

**Table 5** Fields in the MPRA Info Sub-sub-TLV
Field	Length	Description
Type	8 bits	The value is 3, indicating that an MPRA is carried.
Length	8 bits	Length. The value is 16.
Multicast Policy Reserved Address(MPRA)	128 bits	MPRA.
Extension	Not limited	This field is optional and is not filled in. If a received packet contains the Extension field, this field is ignored.

BIERv6/G-BIER Encapsulation Sub-sub-TLV

The BIERv6/G-BIER Encapsulation Sub-sub-TLV is used to advertise the Max SI, BSL, and start BIFT-ID. Figure 5 shows the format of the sub-sub-TLV.

Figure 5 BIERv6/G-BIER Encapsulation Sub-sub-TLV

Table 6 describes the fields.

**Table 6** Fields in the BIERv6 Encapsulation Sub-sub-TLV
Field	Length	Description
Type	8 bits	If the value is 6, BIERv6 encapsulation information is carried. If the value is 2, G-BIER encapsulation information is carried.
Length	8 bits	Length.
Max SI	8 bits	Maximum set ID in a specific <BIERv6/G-BIER sub-domain, BSL>.
BSL	4 bits	Coded value of the BitString length. The available coded values are as follows: 0001: indicates that the BSL is 64 bits long. 0010: indicates that the BSL is 128 bits long. 0011: indicates that the BSL is 256 bits long. Other values are currently reserved according to an RFC and unsupported.
BIFT-ID	20 bits	This field is set to 0 during packet encapsulation and is ignored when the packet is received. This field can be considered as a reserved field.