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Rayhaan JaufeerallyandClaude Fable 5.1 9d6996c6c1 slides: no step-by-step reveals; UPDATE example is 2a0d:d740::/48 at SwissIX

- Removed every fragment and stepped code highlight: a slide now shows
  everything at once. Two code blocks keep a static highlight on one line.
- Dropped "What stands between you and an outage".
- The UPDATE walked through in section 3 is now 2a0d:d740::/48 as AS210036
  announces it to the SwissIX route servers: next hop 2001:7f8:24::c9
  (pr01's port on the peering LAN), AS path 210036, MED 0 and 210036:10:2,
  matching what pr01's export filter puts on routes to peers. Encoded with
  the packet crate and parsed back to the identical message; the slide
  generator asserted every quoted number, including the next hop, against
  the bytes. Adds a MULTI_EXIT_DISC slide to the walkthrough.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

2026-09-20 21:48:12 +02:00

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Transit and exchanges

Notes: Section 4. Two very different ways of getting routes, and what an Internet exchange physically is.

Two ways to reach other networks

IP transit

You pay a provider
They give you routes to everyone
They announce you to everyone

Peering

Usually free, by mutual benefit
You exchange your own routes, and your customers'
Traffic between the two of you only

Notes: Transit is a customer-provider relationship; peering is between equals. A peer does not carry your traffic to third parties. Most networks have both: transit as the fallback, peering to keep traffic cheap and close.

An exchange is a big Ethernet switch

Notes: Every member plugs one router port into the same layer-2 network and gets one address in a shared subnet. No routing inside the exchange: just Ethernet. Dashed blue is a BGP session, green is where the packets actually go. a5ec is 42476 in hex: the route server's address spells its AS number.

What that implies

Your neighbour is on your LAN: one hop, found by neighbour discovery
A route's next hop is the peer's LAN address, so traffic goes directly member to member
The exchange never routes your packets. It switches frames
So the LAN is policed hard: one MAC address per port, no stray broadcasts, no proxy ARP

Notes: A misbehaving port can hurt every member, which is why exchanges publish strict rules for what may appear on the peering LAN and quarantine ports that break them.

Route servers

Without one, peering with 200 members is 200 BGP sessions.

One session to the route server brings routes from everyone who uses it
It is not in the traffic path, and leaves the next hop alone
It does not add its own AS to the path
You steer it with communities: 42476:0:<as> "not to them", 42476:1:<as> "only to them"

Notes: A route server is a BGP speaker that only redistributes. Because it is transparent, the AS path looks exactly as if you peered with each member directly. We will use those communities in the lab.

Our upstream router, for real

Where	BGP sessions	Neighbour networks
Transit (CommunityRack)	2	1
SwissIX, Zürich	28	12
DE-CIX, Frankfurt	26	8
France-IX, Paris	23	10
NL-ix, Amsterdam	22	6

One small router, pr01.rue, AS210036. IPv4 and IPv6 are separate sessions.

Notes: Counted from its BIRD configuration. It is also on two smaller Swiss exchanges. The same network (Cloudflare, Apple, Hurricane Electric) often appears at several exchanges, which matters when you choose where to announce.

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