# The SPB vocabulary, sorted

> I-SID, B-VLAN, B-MAC, BEB, BCB, L2VSN, L3VSN. Which of those is a service, which is transport, and which is a role a switch plays — and the one fact that makes the rest obvious.

Source: https://ronutz.com/en/learn/spb-fabric-vocabulary  
Updated: 2026-08-13  
Related tools: https://ronutz.com/en/tools/fabric-connect-spb-explainer, https://ronutz.com/en/tools/voss-fabric-id, https://ronutz.com/en/tools/voss-exos-translator

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## The difficulty is the words

Shortest Path Bridging is simpler than what it replaces. It does not feel that
way on first contact, because seven unfamiliar terms arrive in one paragraph and
nothing signals which are the same kind of thing.

Sort them into three piles and the technology stops resisting.

**Transport:** the backbone VLANs and the backbone MACs. **Service:** the I-SID,
and the L2VSN or L3VSN it represents. **Role:** what a given switch does, edge
or core.

## The one fact that reorients everything

**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 has provisioned from shortest-path trees.

There is no spanning tree. Nothing is blocked. Every link carries traffic,
because the forwarding decision came from a computed topology rather than from
watching frames arrive and guessing.

Once that lands, the rest follows: if forwarding is computed, the core does not
need to learn anything, and if the core does not learn, it does not need to know
your services exist.

## The service identifier

The **I-SID** is a twenty-four-bit number carried in the 802.1ah backbone
header. Twenty-four bits is about sixteen million, against a VLAN tag's four
thousand — which is why a fabric can give every service its own identifier
instead of rationing them.

An I-SID is not a VLAN and not a subnet. It becomes one or the other by what you
bind it to:

- Bind it to an **edge VLAN** and it is an **L2VSN**. Two sites configure the
  same I-SID against their local VLAN and share a broadcast domain, with nothing
  configured in between.
- Bind it to a **VRF** and it is an **L3VSN**, carrying routed traffic for one
  routing instance.

Many deployments encode the VLAN into the I-SID — 20010 for VLAN 10 — so the
mapping reads at a glance. That is convention, not requirement.

## Two backbone VLANs, and why

A fabric normally runs **two** B-VLANs, 4051 and 4052 by default on Extreme
equipment. Services are distributed between them by the low bit of the I-SID:
even on one, odd on the other. Two shortest-path trees, each carrying roughly
half the services.

That distribution is the default rather than a rule of the standard. When one
service behaves differently from its neighbours, **which tree it is on is the
first thing to check.**

## Edge and core

A **backbone edge bridge** is where a customer VLAN or VRF meets an I-SID, and
where MAC-in-MAC encapsulation is added and removed. Services are provisioned
here.

A **backbone core bridge** forwards on backbone MACs alone and never sees a
customer MAC address.

That is the operational claim SPB is actually making: **adding a service means
touching the two or more edge switches where it appears, and nothing in
between.** The core is untouched, not because of a design convention, but
because it genuinely does not carry the information.

The same property sizes the core's forwarding table by the number of fabric
nodes rather than by the number of end stations, which is why the encapsulation
exists at all.