SRGB & Prefix-SID Planner

Segment routing carries the path in the packet instead of holding state in the network. There is no LDP and no RSVP — the label for a destination is calculated.

Label = SRGB base + prefix SID index. A router with SRGB 16000–23999 and prefix SID 5 is reached with label 16005.

The SRGB used is that of the router being reached, not the one imposing the label. This is the detail that catches people. If every router uses 16000–23999 then the label for a destination is the same everywhere, and a packet trace reads sensibly. If the SRGBs differ, each router imposes a different label for the same destination — which works perfectly and makes troubleshooting considerably harder.

That is why using one SRGB across the domain is the near-universal convention. It is a convention rather than a requirement, and the reason for it is legibility.

A prefix SID is global. It identifies one destination across the whole domain, so two routers sharing one is a real fault — traffic for both follows whichever the routing decides.

Labels pop as segments complete. A segment list of two routers is a stack of two labels; the top one is removed when the packet reaches that router, and the next takes over. So the stack shrinks along the path rather than being carried whole.