What it does

Give an I-SID, an SPB nickname or a switch role, and the tool sorts the Shortest Path Bridging vocabulary for you: which backbone VLAN carries the service and why, what an L2VSN and an L3VSN actually bind to, what a B-VLAN is and is not, and what changes between a backbone edge bridge and a backbone core bridge. Deterministic and offline — it explains identifiers, it does not query a fabric.

The behaviour worth checking

Even I-SIDs ride one backbone VLAN and odd ones the other. With the Extreme defaults of 4051 and 4052, I-SID 20010 is carried on 4052 and 20011 on 4051. Two shortest-path trees, each carrying roughly half the services.

The tool says this is the default distribution rather than a rule of the standard, because a deployment can assign differently — and when one service behaves unlike its neighbours, which tree it is on is the first thing to check.

The fact the tool repeats on every result

A B-VLAN is not a VLAN. It does not flood unknown unicast, broadcast or multicast. It forwards only on backbone MAC tables that IS-IS provisioned from shortest-path trees. No spanning tree, nothing blocked.

Everything else follows: if forwarding is computed, the core does not learn; if the core does not learn, it does not need to know your services exist. Which is why adding a service touches only the edge switches where it appears.

Validation it performs

An I-SID must be in the 24-bit range, and the tool refuses values outside it rather than explaining a number that cannot exist. A nickname must be in x.xx.xx form, and a malformed one warns — a duplicate or malformed nickname is one of the few errors here that breaks things far from where it was typed.

What it will not do

It cannot see your fabric. It does not know your topology, your adjacencies or whether IS-IS is actually up. It explains what your numbers mean inside a fabric, not what your fabric is doing.

How this differs from the VOSS fabric identifier decoder

Both take an I-SID and a nickname, and they answer different questions:

  • The fabric identifier decoder validates an identifier. Paste something and it works out whether it is a 24-bit I-SID, a 20-bit nickname or a system-id, and whether it is in range.
  • This tool explains what the identifier means in a fabric. Which backbone VLAN carries it and why, what an L2VSN and an L3VSN bind to, what a B-VLAN is and is not, and what changes between an edge bridge and a core bridge.

Decode the value there; understand the arrangement here.