What Gets Advertised, and What Speaks
A passive interface, and the difference between the two halves of OSPF
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 what it does not
- ▸Distinguish an interface that is in OSPF from one that sends hellos
- ▸Say why a user-facing LAN should be advertised but never spoken on
Before this lesson — it builds directly on:
A router's job on a link between two routers and its job on a LAN full of laptops are completely different, and OSPF has one switch that gets confused for both.
On a router-to-router link you want a conversation: hellos, an adjacency, an exchange of databases. On a LAN full of end hosts you want none of that — but you still want everyone else in the network to know the LAN is there, because otherwise nobody can reach the laptops.
A passive interface is the setting that gives you the second without the first. It exists because the alternatives are both wrong: run OSPF normally and you invite anything on that LAN to become a router; take the interface out of OSPF and the LAN disappears from everybody's routing table.
Watch these fields in the lab: ospf.type · ospf.routerId
- Advertised
- The network on an interface appears in this router's Router-LSA, so every router in the area learns a route to it. Nothing about conversations.
- Adjacencyglossary
- Two routers that have exchanged hellos, agreed on their parameters and synchronised databases. Only forms where hellos are sent.
- Passive interface
- An interface that is advertised and never sends a hello. Both halves are deliberate: the network stays reachable and nothing on it can become a neighbour.
✓ 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.
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.