Everyone Draws the Same Map

What routers exchange, and why nobody has to be trusted

Step 1 of 2 — understand the idea. Read this first and the packet trace will confirm what you already expect, instead of teaching you two things at once.

What you'll be able to do

▸Explain what a Router-LSA describes, and why no router can misrepresent another
▸Trace an SPF calculation by hand and get the same answer the router did
▸Explain why raising a cost moves traffic without any route being configured

Before this lesson — it builds directly on:

How Two Routers Become Neighbours

The obvious design is for routers to tell each other about routes: *I can reach network X, it costs me 5.* That is how the earliest protocols worked, and it has a nasty flaw. When you hear that, you cannot tell whether the path they are describing comes back through you. If it does, and you both believe each other, you have a loop and no way to see it. The two routers politely increase their cost estimates forever, which is why the failure has a name: counting to infinity. Link-state protocols avoid it by never advertising a conclusion. Each router describes only what it is connected to, everyone builds the same map, and a loop is simply visible on the map.

✗ Common misunderstanding: OSPF routers tell each other which routes to use.

Why that's wrong: They exchange descriptions of their own links and nothing else. No OSPF packet ever contains a route. Each router builds the same map and then computes its own routes from it.

Correct model: Distribute the inputs, not the conclusions. Everyone runs the same algorithm over the same data and independently reaches the same answer.

The packets prove it: Open any Link State Update in the trace: it contains a router id and a list of that router's own links with costs. There is no destination-and-next-hop pair anywhere in it.

Watch these fields in the lab: ospf.lsa[0].advertisingRouter

LSAglossary: Link State Advertisement: one router's description of its own links, with a cost on each. Never a route.
LSDBglossary: The collection of every LSA in the area. Identical on every router once converged — and where an OSPF fault is actually diagnosed.
Sequence number: Increases each time a router re-describes itself. It is how a receiver tells a newer copy from the one it holds, and it is what stops flooding circulating forever.
Costglossary: The price of sending out an interface, conventionally derived from bandwidth. One-way, so a link can legitimately cost different amounts in each direction.
SPFglossary: Shortest Path First — Dijkstra's algorithm, run over the database, rooted at the router doing the running.

Watch these fields in the lab: ospf.lsa[0].sequenceNumber

✓ Concept check — before you open the packets

These test the idea, not the trace. You should be able to answer them from the explanation above.

What is inside a Link State Update packet?

Path A crosses two routers costing 10 each. Path B crosses five costing 1 each. Which does OSPF install?

An OSPF route looks wrong. Where do you look first?

Now that you understand the concept, observe how it appears in the packet exchange.

The lab runs a real simulated capture — pause, step, click any packet, and inspect every byte.