How Two Routers Become Neighbours

Why saying hello takes more than one packet

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 hello packet proves, and what it cannot prove on its own
▸Read a neighbour table and say what each state means about the link
▸Explain why hello and dead intervals must match, and how that failure looks in a capture

Before this lesson — it builds directly on:

How a Route Gets Into the Table

Static routes work, and for a small network they are fine. The trouble starts when something breaks. A static route keeps pointing at a path that no longer exists. It has no way to know. Somebody has to notice, work out what changed, and reconfigure — on every affected router, correctly, under pressure, usually at night. A routing protocol removes the human from that loop. Routers watch their own links, tell each other what they see, and recompute when it changes. The point is not that it saves typing; it is that the network can respond to a failure faster than a person can be woken up.

✗ Common misunderstanding: Receiving a hello proves the link works.

Why that's wrong: It proves the link works in ONE direction. Unidirectional failures are real — a damaged fibre pair, a failing transmitter, a filter applied one way — and would produce a black hole neither router could see from its own state.

Correct model: Two-way is proven by finding your OWN router id inside the neighbour's hello. Until then the adjacency waits in Init.

The packets prove it: The trace shows Init on the first hello received, and 2-Way only when the neighbour's hello lists this router's id back.

Watch these fields in the lab: ospf.routerId

Router IDglossary: A router's name inside OSPF, written as a dotted quad. It is an identifier, not an address — nothing is ever sent to it. It must be unique in the area.
Hello interval: How often a router announces itself on a link, and one of the values both ends must agree on exactly.
Dead interval: How long silence is tolerated before a neighbour is declared gone. Conventionally four hello intervals.
Adjacencyglossary: A relationship two routers have agreed to, in which they synchronise databases. Not the same as merely seeing each other's hellos.
Designated routerglossary: On a shared segment, the router everyone forms an adjacency with, so the segment does not need one between every pair. It has no authority over routing.

Watch these fields in the lab: ospf.helloInterval · ospf.routerDeadInterval

✓ 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 does receiving a hello from a neighbour prove?

Why does a shared segment elect a designated router?

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.