Derive prefix-SID labels from an SRGB base and index, and check SRGB utilisation across the domain.
Advertising an index rather than a label is what lets Segment Routing work without a label distribution protocol: every node computes its neighbour's label from that neighbour's advertised SRGB. Adjacency SIDs are local and come from the dynamic range, so they do not consume SRGB space. A consistent SRGB across the domain makes labels predictable and troubleshooting straightforward, because the same prefix has the same label everywhere; inconsistent SRGBs are legal but turn every traceroute into an arithmetic exercise.
Segment Routing SID
A prefix-SID is an index, not a label: the local label is SRGB base + index, so the same index resolves to a different label on a node with a different SRGB.
Label derivation
label = SRGB base + prefix-SID index.
A prefix-SID is an index, not a label: the local label is SRGB base + index, so the same index resolves to a different label on a node with a different SRGB. Advertising an index rather than a label is what lets Segment Routing work without a label distribution protocol: every node computes its neighbour's label from that neighbour's advertised SRGB. Adjacency SIDs are local and come from the dynamic range, so they do not consume SRGB space.
A consistent SRGB across the domain makes labels predictable and troubleshooting straightforward, because the same prefix has the same label everywhere; inconsistent SRGBs are legal but turn every traceroute into an arithmetic exercise.
This calculator takes 5 inputs: SRGB base label, SRGB size, Prefix-SID index for this node, Nodes needing a prefix-SID, Adjacency SIDs per node. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
Not strictly — SR works with different SRGBs because each node advertises its own — but use the same block everywhere anyway. Uniform SRGBs make labels globally meaningful, which is worth far more than the flexibility of varying them.