Techniques for exposing maximum node and/or link segment identifier depth utilizing IS-IS

1. A method in a device implementing a network element—in a Segment Routing (SR) network, comprising:

transmitting, by the network element, a Type Length Value (TLV) element including a Maximum Segment Identifier Depth (MSD) value, wherein the TLV element was advertised using an Intermediate System to Intermediate System (IS-IS) protocol, wherein the MSD value identifies a maximum number of segment identifier (SID) labels that the network element is able to push into packet headers of received packets to enable forwarding of the received packets through the SR network, and wherein the MSD value is associated with a first link of the network element and the TLV element further includes a second MSD value associated with a second link of the network element; and

receiving, from a controller, data for a path to be utilized by the network element for forwarding the received packets through the SR network, wherein the data includes a set of one or more SID labels to be pushed into the received packets associated with the path, and wherein the set of SID labels has fewer than or equal to the MSD value number of SID labels, and

wherein the controller and the network element do not utilize the Path Computation Element Protocol (PCEP) over a southbound interface.

2. The method of claim 1 , wherein the TLV element is an IS-IS Router Capability TLV.

3. The method of claim 2 , wherein the MSD value is carried within a node MSD sub-TLV carried by the IS-IS Router Capability TLV.

4. The method of claim 2 , further comprising:

determining, by the network element, the MSD value as a smallest one of one or more MSD values of one or more line cards of the network element.

5. The method of claim 2 , wherein the MSD value is carried within a link MSD sub-TLV carried by the TLV element.

6. The method of claim 5 , wherein the TLV element is a Type 22 IS-IS Extended IS Reachability TLV.

7. The method of claim 5 , wherein the TLV element is a Type 23 IS-IS IS Neighbor Attribute TLV.

8. The method of claim 5 , wherein the TLV element is a Type 222 IS-IS Multi-Topology IS TLV.

9. The method of claim 5 , wherein the TLV element is a Type 223 IS-IS Multi-Topology IS Neighbor Attribute TLV.

10. A method in a controller communicatively coupled with a plurality of network elements of a Segment Routing (SR) network, the method comprising:

receiving, from a first network element of the plurality of network elements, a Type Length Value (TLV) element including a Maximum Segment Identifier Depth (MSD) value, wherein the TLV element was advertised using an Intermediate System to Intermediate System (IS-IS) protocol, wherein the MSD value identifies a maximum number of segment identifier (SID) labels that the first network element is able to push into packet headers of received packets to enable forwarding of the received packets through the SR network, and wherein the MSD value is associated with a first link of the first network element and the TLV element further includes a second MSD value associated with a second link of the first network element;

determining, based upon the MSD value, a path from the first network element to a second network element of the plurality of network elements for an identifiable set of traffic that utilizes a set of one or more SID labels, wherein a number of SID labels in the set of SID labels is less than or equal to the MSD value; and

sending, to the first network element, data for the path to be utilized by the first network element for the forwarding of the received packets through the SR network, wherein the data includes the set of SID labels to be pushed into received packets of the identifiable set of traffic,

wherein the controller and the first network element do not utilize the Path Computation Element Protocol (PCEP) over a southbound interface.

11. The method of claim 10 , wherein the TLV element is an IS-IS Router Capability TLV.

12. The method of claim 11 , wherein the MSD value is carried within a node MSD sub-TLV carried by the IS-IS Router Capability TLV.

13. The method of claim 10 , wherein the MSD value is carried within a link MSD sub-TLV carried by the TLV element.

14. The method of claim 13 , wherein the TLV element is a Type 22 Extended IS Reachability TLV.

15. The method of claim 13 , wherein the TLV element is a Type 23 IS Neighbor Attribute TLV.

16. The method of claim 13 , wherein the TLV element is a Type 222 Multi-Topology IS TLV.

17. The method of claim 13 , wherein the TLV element is a Type 223 Multi-Topology IS Neighbor Attribute TLV.

18. A non-transitory computer-readable storage medium having instructions which, when executed by one or more processors of a device, cause the device to implement a network element to act in a Segment Routing (SR) network and perform operations comprising:

transmitting a Type Length Value (TLV) element including a Maximum Segment Identifier Depth (MSD) value, wherein the TLV element was advertised using an Intermediate System to Intermediate System (IS-IS) protocol, wherein the MSD value identifies a maximum number of segment identifier (SID) labels that the network element is able to push into packet headers of received packets to enable forwarding of the received packets ( 150 B) through the SR network, and wherein the MSD value is associated with a first link of the network element and the TLV element further includes a second MSD value associated with a second link of the network element; and

receiving, from a controller, data for a path to be utilized by the network element for forwarding the received packets through the SR network, wherein the data includes a set of one or more SID labels to be pushed into the received packets associated with the path, and wherein the set of SID labels has fewer than or equal to the MSD value number of SID labels, and

wherein the controller and the network element do not utilize the Path Computation Element Protocol (PCEP) over a southbound interface.

19. The non-transitory computer-readable storage medium of claim 18 , wherein the TLV element is an IS-IS Router Capability TLV.