A path in SR Policy can be either explicitly defined or instantiated automatically using ODN (On-demand Next-hop). Please refer to my another article on SR Policy , below is the link for that,
Basically, ODN functionality in SRTE automatically instantiate SR Policy paths by headend node. These paths are based on ODN templates , each template specifies the requirement of the path. ODN template specifies the characteristics of the candidate path such as metric or preference or link affinity etc.
Coloring plays very important role in ODN solution. Ingress PE node is pre-configured with ODN templates for each desired SRTE SLA requirement. Egress PE node colors the service routes (VPN routes) , advertises to ingress PE with colors that signifies SRTE SLA requirement.
Ultimately, goal of ODN is to instantiate candidate path. For example, say ingress PE node is configured with ODN template with color 100 and it received service routes with color 100 and next hop N from egress PE, ingress PE node automatically instantiates ODN candidate path for SR Policy (100,N) where 100 is the color and N is the endpoint (N is the next hop advertised by egress PE node , typically loopback address of the egress PE).
When ingress (headend node) node receives withdrawal for all service routes having color 100, it tears down the ODN instantiated candidate paths for SR policy (100,N). And if none of ODN candidate paths exists, then headend node tears down SR policy as well.
lets understand whole concept of ODN using example, refer below figure 1,
In the above figure, say Node S1 is the ingress PE and S4 is the egress PE node. Requirement is to have L3VPN between S1 and S4 using ODN with SLA optimization path with metric as delay.
Node S4 advertises service routes x.x.x.x/y using MP-BGP with color 100 (color is exchanged using BGP extended communities) and Next-hop as S4 towards Node S1. When service routes received by Node S1, since ODN template exists with color 100 on Node S1, BGP request SR-TE process to instantiate candidate path for the SR Policy with endpoint S4 and color 100 based on ODN template ( ODN template with color 100). This is ODN functionality.
Now, SRTE computes candidate path to endpoint S4 with SLA optimized path with metric as delay (ODN template with color 100 defines this). SR Policy is created (100,S4) with dynamic candidate path.
In Summary, SR candidate path can be either statically configured or instantiated automatically using ODN. It drastically reduces complex configuration on service PE nodes. the ODN can be applied to both intra or inter domain networks. I hope this article is useful to understand ODN concept.