Segment Routing SID Calculator
Derive prefix-SID labels from an SRGB base and index, and check SRGB utilisation across the domain.
Inputs
MPLS Label for This Prefix-SID
16,101
SRGB Range
16000–23999
SRGB Used by Prefix-SIDs
2.50%
SRGB Labels Free
7,800
Adjacency SIDs Domain-Wide
800
Index Fits the SRGB
Yes — index 101 maps to label 16101 inside 16000–23999
Step by step
Values used
SRGB base label = 16,000; SRGB size = 8,000 labels; Prefix-SID index for this node = 101; Nodes needing a prefix-SID = 200 nodes; Adjacency SIDs per node = 4 SIDs
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.
MPLS Label for This Prefix-SID
= 16,101
SRGB Range
= 16000–23999
SRGB Used by Prefix-SIDs
= 2.50
SRGB Labels Free
= 7,800
Adjacency SIDs Domain-Wide
= 800
Index Fits the SRGB
= Yes — index 101 maps to label 16101 inside 16000–23999
How it works
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.
Formulas
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.
- SRGB
- Segment Routing Global Block — the label range reserved for global SIDs, 16000–23999 by default on many platforms
- index
- Globally significant prefix-SID index advertised in the IGP
- adjacency SID
- Locally significant label for one link, allocated outside the SRGB from dynamic label space
Label derivation
label = SRGB base + prefix-SID index.
Frequently Asked Questions
How is Segment Routing SID calculated?
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.
Why does Segment Routing SID matter?
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.
What values do I need to enter?
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.
Must every router use the same SRGB?
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.