IS-IS for SR-MPLS

IS-IS SID TLV Extensions

Prefix-SID Sub-TLV

The Prefix-SID sub-TLV carries IGP-Prefix-SID information. Figure 2-10 shows the format of the Prefix-SID sub-TLV.

Figure 2-10 Prefix-SID Sub-TLV format

Table 2-6 Meanings of fields in the Prefix-SID Sub-TLV

Field Name	Length	Description
Type	8 bits	Unassigned. The recommended value is 3.
Length	8 bits	Packet length.
Flags	8 bits	Flags field. Figure 2-11 shows its format. Figure 2-11 Flags field The meaning of each flag is as follows: R: re-advertised flag. If this flag is set, a prefix is imported from another protocol or is penetrated from another level (for example, a prefix is penetrated from an IS-IS Level 1 area to a Level 2 area). N: node SID flag. If this flag is set, a prefix SID identifies a node. If a prefix SID is set to a loopback interface address, this flag bit is set. P: no-PHP flag. If this flag is set, PHP is disabled so that the penultimate node sends a labeled packet to the egress. E: explicit null label flag. If this flag is set, the explicit null label function is enabled. An upstream neighbor must replace an existing label with an explicit null label before forwarding a packet. V: value flag. If this flag is set, a prefix SID carries a value, instead of an index. By default, the flag is not set. L: local flag. If this flag is set, the value or index carried in a prefix SID is of local significance. By default, the flag is not set. A node must compute an outgoing prefix label based on the P and E flags in a prefix SID advertised by a next hop, regardless whether the optimal path to the prefix SID passes through the next hop. When a node advertises reachability messages (for example, from Level-1 to Level-2 or from Level-2 to Level-1) generated by another IS-IS Speaker, the local node must set the P flag and clear the E flag in a prefix SID. The following behavior is related to P and E flags: If the P flag is not set, any upstream node of the prefix SID producer must strip off the prefix SID, which is similar to PHP in MPLS forwarding. The MPLS EXP bit is also cleared. In addition, if the P flag is not set, the received E flag bit is ignored. If the P flag is set, the following situations occur: If the E flag is not set, any upstream node of the prefix SID producer must reserve the prefix SID on the top of the label stack. This method is used in path stitching. For example, a prefix SID producer may use this label to forward a packet to another MPLS LSP. If the E flag is set, any upstream node of the prefix SID producer must replace the prefix SID label with an explicit null label. In this mode, the MPLS EXP flag is retained. If the prefix SID producer is the destination, the node can receive the original MPLS EXP field value. The MPLS EXP flag can be used in QoS services.
Algorithm	8 bits	Algorithm: 0: Shortest Path First 1: Strict Shortest Path First
SID/Index/Label (variable)	Variable length	This field contains either of the following information based on the V and L flags: 4-byte label offset value, within an ID/label range. In this case, V and L flags are not set. 3-byte local label: The rightmost 20 bits are a label value. In this case, the V and L flags must be set.

Adj-SID Sub-TLV

An Adj-SID Sub-TLV is optional and carries IGP Adjacency SID information. Figure 2-12 shows its format.

Figure 2-12 Adj-SID Sub-TLV format

Table 2-7 Meanings of fields in the Adj-SID Sub-TLV

Field Name	Length	Description
Type	8 bits	Unassigned. The recommended value is 31.
Length	8 bits	Packet length.
Flags	8 bits	Flags field. Figure 2-13 shows its format. Figure 2-13 Flags field The meaning of each flag is as follows: F: address family flag. 0: IPv4 1: IPv6 B: backup flag. If the flag is set, an Adj-SID is used to protect another node. V: value flag. If this flag is set, an Adj-SID carries a label value. The flag is set by default. L: local flag. If this flag is set, the Adj-SID value or index is of local significance. The flag is set by default. S: sequence flag. If this flag is set, an Adj-SID is an adjacency sequence. P: permanent label. If this flag is set, an Adj-SID is a permanently assigned SID, which is unchanged, regardless of a device restart or interface flapping.
Weight	8 bits	Weight. The Adj-SID weight is used for load balancing.
SID/Index/Label (variable)	Variable length	This field contains either of the following information based on the V and L flags: 3-byte local label: The rightmost 20 bits are a label value. In this case, the V and L flags must be set. 4-byte label offset value, within an ID/label range. In this case, V and L flags are not set.

A designated intermediate system (DIS) is elected as a medium during IS-IS communication on a LAN. On the LAN, an NE merely needs to advertise a link message to the DIS and obtain all link information from the DIS, but does not need to exchange link information between NEs.

In segment routing implementation, each NE advertises Adj-SIDs to all neighbors. On the LAN, each NE advertises only an IS-IS Extended IS reachability TLV-22 to the DIS and encapsulates neighbors' Adj-SIDs in a new TLV, which is a LAN-Adj-SID Sub-TLV. The TLV contains all Adj-SID that the NE allocates to all LAN neighbors.

Figure 2-14 shows the format of the LAN-Adj-SID Sub-TLV.

Figure 2-14 LAN-Adj-SID Sub-TLV format

SID/Label Sub-TLV

A SID/Label Sub-TLV includes a SID or an MPLS label. The SID/Label Sub-TLV is a part of the SR-Capabilities Sub-TLV.

Figure 2-15 shows the format of the SID/Label Sub-TLV.

Figure 2-15 SID/Label Sub-TLV format

SID/Label Binding TLV

The SID/Label Binding TLV is used in communication between SR and LDP. It defines the mapping between a prefix and a SID.

Figure 2-16 shows the SID/Label Binding TLV format.

Figure 2-16 SID/Label Binding TLV format

Table 2-9 Meanings of fields in the SID/Label Binding TLV

Field Name	Length	Description
Type	8 bits	Unassigned. The recommended value is 1.
Length	8 bits	Packet length.
Flags	8 bits	Flags field. +-+-+-+-+-+-+-+-+ |F|M|S|D|A| | +-+-+-+-+-+-+-+-+
Range	16 bits	Prefix address and range of SIDs associated with the prefix.
Prefix Length	8 bits	Prefix length
Prefix	Variable length	Prefix.
SubTLVs	Variable length	Sub-TLV, such as SID Sub-TLV and Label Sub-TLV

IS-IS SR Capability TLV Extension

SR-Capabilities Sub-TLV

In segment routing, each NE must be able to advertise its SR capability and global SID range (or global label index). To meet the preceding requirement, an SR-Capabilities Sub-TLV is defined and embed in the IS-IS Router Capability TLV-242 for transfer. The SR-Capabilities Sub-TLV can be propagated only within the same IS-IS level area.

Figure 2-17 shows the format of the SR-Capabilities Sub-TLV.

Figure 2-17 SR-Capabilities Sub-TLV format

Table 2-10 Meanings of fields in the SR-Capabilities Sub-TLV

Field Name	Length	Description
Type	8 bits	Unassigned. The recommended value is 2.
Length	8 bits	Packet length.
Flags	8 bits	Flags field. Figure 2-18 shows its format. Figure 2-18 Flags field The meaning of each flag is as follows: I: MPLS IPv4 flag. If the flag is set, SR MPLS IPv4 packets received by all interfaces can be processed. V: MPLS IPv6 flag. If the flag is set, SR MPLS IPv6 packets received by all interfaces can be processed.
Range	24 bits	SRGB range. The advertising end releases the following SR-Capabilities in the following ranges. SR-Capability 1:Range: 100, SID value: 100 SR-Capability 2: Range: 100, SID value: 1000 SR-Capability 3: Range: 100, SID value: 500 The receive end links the preceding ranges and generates an SRGB. SRGB = [100, 199] [1000, 1099] [500, 599] Different label indexes may span multiple ranges. Index 0: label 100 ... Index 99: label 199 Index 100: label 1000 Index 199: label 1099 ... Index 200: label 500 ...
SID/Label Sub-TLV (variable)	Variable length	See SID/Label Sub-TLV. The SRGB start value is included. When multiple SRGBs are configured, ensure that the SRGB sequence is correct and the SRGBs do not overlap.

SR-Algorithm Sub-TLV

NEs use different algorithms, for example, the SPF algorithm and various SPF variant algorithms, to compute paths to the other nodes or prefixes. The newly defined SR-Algorithm Sub-TLV enables an NE to advertise its own algorithm. The SR-Algorithm Sub-TLV is also carried in the IS-IS Router Capability TLV-242 for transfer. The SR-Algorithm Sub-TLV can be propagated within the same IS-IS level.

Figure 2-19 shows the format of the SR-Algorithm Sub-TLV.

Figure 2-19 SR-Algorithm Sub-TLV format

SR Local Block Sub-TLV

The SR Local Block Sub-TLV contains a label range that an NE reserves for local SIDs. The local SIDs are used as adjacency SIDs or are allocated by the other components. For example, an application (App) or a controller instructs the NE to assign a special local SID. To notify the App or controller of available local SIDs, the NE must advertise an SR local block SRLB.

Figure 2-20 shows the format of the SR Local Block Sub-TLV.

Figure 2-20 SR Local Block Sub-TLV format

The SRLB TLV advertised by the NE may contain a label range that is out of the SRLB. Such a label range is assigned locally and is not advertised in the SRLB. For example, an adjacency SID is assigned a local label, not a label within the SRLB range.

IS-IS SR LSP Creation

An intra-IGP-area SR LSP is created.

In Figure 2-21, devices run IS-IS. Segment routing is used and requires each device to advertise the SR capability and supported SRGB. In addition, the advertising end advertises a prefix SID offset within the SRGB range. The receive end computes an effective label value to generate a forwarding entry.

Figure 2-21 IS-IS SR LSP creation

Devices A through F are deployed in areas of the same level. All Devices run IS-IS. An SR tunnel originates from Device A and is terminated at Device D.

An SRGB is configured on Device D. A prefix SID is set on the loopback interface of Device D. Device D encapsulates the SRGB and prefix SID into a link state protocol data unit (LSP) (for example, IS-IS Router Capability TLV-242 containing SR-Capability Sub-TLV) and floods the LSP across the network. After another device receives the SRGB and prefix SID, it uses them to compute a forwarding label, uses the IS-IS topology information, and runs the Dijkstra algorithm to calculate an LSP and LSP forwarding entries.

An inter-IGP area SR LSP is created

In Figure 2-22, to establish an inter-area SR LSP, the prefix SID must be advertised across areas by penetrating these areas. This overcomes the restriction on IS-IS's flooding scope within each area.

Figure 2-22 Inter-IGP area SR LSP

Devices A and D are deployed in different areas, and all devices run IS-IS. An SR tunnel originates from Device A and is terminated at Device D.

An SRGB is configured on Device D. A prefix SID is set on the loopback interface of Device D. Device D generates and delivers forwarding entries. It encapsulates the SRGB and prefix SID into an LSP (for example, IS-IS Router Capability TLV-242 containing SR-Capability Sub-TLV) and floods the LSP across the network. Upon receipt of the LSP, Device C parses the LSP to obtain the prefix SID, calculates and delivers forwarding entries, and penetrates the prefix SID and prefix address to the Level-2 area. Device B parses the LSP to obtain the prefix SID, calculates and delivers forwarding entries, and penetrates the prefix SID and prefix address to the Level-1 area. Device A parses the LSP and obtains the prefix SID, uses IS-IS to collect topology information, and runs the Dijkstra algorithm to compute a label switched path and tunnel forwarding entries.