How SD-Access Works

In the previous course, you learned the five fabric roles.
Now you need to see how they work together when a user actually sends traffic.

Everything starts with one simple question: how does the fabric know where every user is?

The Topology You Will Follow

Throughout this lesson, you will follow a small fabric with three Fabric Edges.

PC1 sits behind Edge 1 on Floor 1.
You want it to reach PC3, which sits behind Edge 3 on Floor 3.

Figure 1 – The fabric topology

• PC1 has the IP address 10.1.1.10.

• PC3 has 10.3.3.30.

In LISP terms, these user IPs are called EIDs (Endpoint Identifiers).
Each Fabric Edge also has a routable IP, the RLOC (Routing Locator).

You already saw both terms in the LISP Fundamentals lesson.
SD-Access uses LISP for its control plane: every user is identified by its EID and reached through its Edge's RLOC.

RLOCs reach each other through the underlay, a routed Layer 3 network.
Catalyst Center provisions it for you, so you do not have to worry about it in this lesson.

Map-Register

When PC1 connects to Edge 1, the switch learns its MAC and IP address through ARP.

But that knowledge stays local to Edge 1.
The rest of the fabric does not know PC1 exists.

To fix this, Edge 1 sends a Map-Register message to the Control Plane Node.
It declares: "PC1 (10.1.1.10) is reachable through my RLOC (1.1.1.1)."

Figure 2 – Edge 1 registers PC1

The Control Plane Node stores this binding in its mapping database.

Map-Registers are sent every time a new endpoint connects.
They are also refreshed periodically to keep the entry alive.

The Mapping Database

Every Fabric Edge does the same thing for its own users.

Once they have all registered, the Control Plane Node holds a complete picture of the fabric.

Figure 3 – The mapping database after registration

This mapping database is the foundation of SD-Access.
The Control Plane Node is the single source of truth for endpoint location.

When a user moves from one floor to another, only the mapping database entry is updated.

If you have read the LISP Fundamentals lesson, the Fabric Edge plays the role of an xTR (ITR + ETR), and the Control Plane Node plays the role of MS/MR.

The mapping database is built.
Now PC1 wants to talk to PC3.

How does Edge 1 figure out where to send the traffic?

Cache Miss and Map-Request

PC1 sends a packet to 10.3.3.30.
It arrives at Edge 1 like on any switch.

Edge 1 checks its local cache first.
Does it already know where 10.3.3.30 is?

Figure 4 – Cache miss and Map-Request

The first time, the answer is no.
Edge 1 only knows about its own connected users.

So Edge 1 sends a Map-Request to the Control Plane Node.
It asks the same thing you would ask a directory: "Where does 10.3.3.30 live?"