In the figure,SR-MPLSis enabled onR1, R2, and R3.

Understanding SR-MPLS Label Calculation

SR-MPLS usesSegment Routing Global Block (SRGB)andPrefix SIDsto assign labels dynamically. TheMPLS labelfor a node’s loopback interface is calculated as follows:

MPLS Label=SRGB Base+Prefix SID Index

Where:

SRGB Base=20000(as given in the figure)

Prefix SID Index=30(configured for R3’s Loopback1)

Label Calculation:

20000+30=20030

Thus, theMPLS label corresponding to the loopback1 interface of R3 is 20030.

Why the Answer is 20030?

✔The SRGB defines the base label range.

✔Each router is assigned a unique Prefix SID index.

✔The label is dynamically computed by adding the Prefix SID Index to the SRGB Base.

Why Other Numbers Are Incorrect?

❌Any value outside the range 20000-21000

Would be incorrect because it must be withinR3’s SRGB range.

❌Using the Prefix SID Index directly (30)

Incorrect becauseMPLS labels are not assigned directly using Prefix SIDs; they are computed using the SRGB.

????Reference:Huawei HCIE Datacom – SR-MPLS Label Calculation and SRGB