How does SR policy operate in Segment Routing Traffic Engineering?
A.
An SR policy for color and endpoint is deactivated at the headend as soon as the headend learns a valid candidate path for the policy.
B.
When "invalidation drop" behavior occurs, the SR policy forwarding entry is removed and the router drops all traffic that is steered into the SR policy.
C.
When a set of SID lists is associated with the SR policy designated path, traffic steering is ECMP-based according to the qualified cost of each SID-list.
D.
An active SR policy installs a BSID-keyed entry in the forwarding table to steer the packets that match the entry to the SR policy SID-list.
Correct answer is D.
https://datatracker.ietf.org/doc/html/draft-ietf-spring-segment-routing-policy-02
6.3. Forwarding Plane
A valid SR Policy installs a BSID-keyed entry in the forwarding plane
with the action of steering the packets matching this entry to the
selected path of the SR Policy.
Agree on D... more info on why the others are incorrect:
A is incorrect, the SR policy is not deactivated, and can be shared by other prefixes with the same color and endpoint:
https://www.cisco.com/c/en/us/td/docs/routers/asr9000/software/asr9k-r7-8/segment-routing/configuration/guide/b-segment-routing-cg-asr9000-78x/configure-sr-te-policies.html
"
After SR-PCE provides the compute path, an intent-driven SR policy is instantiated at the head-end router. Other prefixes with the same intent (color) and destined to the same egress PE can share the same on-demand SR policy.
"
B is incorrect, the SR policy stays up in the control plane even while invalid
"
By default, if an SR Policy becomes invalid, traffic would fall back to the native SR forwarding path.
In some scenarios, a network operator may require that certain traffic be only carried over the path associated with an SR policy and never allow the native SR LSP to be used. The SR-TE Path Invalidation Drop feature is introduced to meet this requirement.
With the Path Invalidation Drop feature enabled, an SR policy that would become invalid (for example, no valid candidate path available) is programmed to drop traffic. At the same time, the SR policy stays up in the control plane to prevent prefixes mapped to the SR policy from falling back to the native SR LSP.
"
C is incorrect (W-ECMP is for SRv6, and steering is based on the weight, not the qualified cost of each SID-list):
https://www.cisco.com/c/en/us/td/docs/routers/asr9000/software/711x/segment-routing/configuration/guide/b-segment-routing-cg-asr9000-711x/configure-srv6-traffic-engineering.html
"
If a set of segment lists is associated with the active path of the policy, then the steering is per-flow and weighted-ECMP (W-ECMP) based according to the relative weight of each segment list.
"
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