# Announcing a prefix with BGP

You have a BGP router. It speaks to two neighbouring autonomous systems, and
whatever you announce will travel around a loop and come back to you, so you
can see exactly what the network did with it.

Open the router's status page (your instructor will give you the address,
usually `http://<host>:8080/statusz`). Everything on the page is live.

**Your prefix:** `10.100.<N>.0/24` where `N` is the number you were given.
Use your name as *owner*: only you can withdraw what you announce.

## The topology

```
 you ──announce──▶ AS65000 (ferrix) ──▶ AS65001 (r1) ──▶ AS65002 (r2) ──▶ back to AS65000
```

Everything under **Received routes** came in from a neighbour. A yellow row
means it is one of the prefixes announced locally: it went around the loop.

## Exercises

1. **Plain announcement.** Announce your prefix with no options. Watch the
   event log, then find your prefix in *Received routes*. What is its AS
   path, and why is it that long? Which ASN appears first, and what does that
   tell you about the direction the route travelled?

2. **Read the bytes.** Click *bytes* on your route under *Local routes*.
   Hover the coloured segments. Find the 16 bytes of marker, the length, and
   the AS_PATH attribute. How many bytes does your ASN take up in the path,
   and why?

3. **NO_EXPORT.** Announce with the community `NO_EXPORT` (use the
   well-known picker). Wait ten seconds. Does your prefix come back? Look at
   the topology and explain which router stopped it and why it still
   accepted it.

4. **Asking a neighbour for help.** Announce with the community
   `65001:100`. r1 has been configured to act on that. What changed in the AS
   path this time, and who changed it? This is how real operators ask an
   upstream to prepend on their behalf; look up your ISP's "BGP communities"
   page later.

5. **Blackholing.** Announce with `BLACKHOLE`. What happens? When would an
   operator want this?

6. **Withdraw.** Withdraw your prefix and watch the event log. How many
   messages did it take, and what did the withdrawal look like on the wire
   compared to the announcement? (Click *bytes* before withdrawing to compare.)

## Things to notice

- Your router never sent anyone else's routes onward. Only the two FRR
  routers re-advertise. What is a router that re-advertises called, and what
  would happen to the AS path if ferrix did it too?
- The AS path is BGP's loop prevention. Your prefix came back with your own
  ASN in the path and the router accepted it anyway. What should it have
  done, and why is this lab allowed to cheat?
- Communities are just 32-bit tags with no built-in meaning except a few
  well-known ones. Everything r1 did with `65001:100` was policy someone
  wrote. Whose job is it to document what a community means?