Rayhaan JaufeerallyandClaude Fable 5.1
eb77090d01
- The status page has a collapsible "Configuration" section showing the config file as read at startup, with its path. Served by the new GET /api/config; RouterInfo::with_config carries the text. - Removed the AS-path prepends field and the SwissIX prepend presets (42476:101..103) from the form to keep the exercise focused. The API still accepts `prepend`. - Docs follow: the handout loses its prepending exercise, the lab README its prepend row, and the BIRD config notes that the 42476 namespace is still admitted for hand-typed values. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
2.8 KiB
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
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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?
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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?
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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. -
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. -
Blackholing. Announce with
BLACKHOLE. What happens? When would an operator want this? -
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
65001:100was policy someone wrote. Whose job is it to document what a community means?