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Rayhaan JaufeerallyandClaude Fable 5.1 2910a2a800 slides: light theme, final edits, single-file bundle

As presented. The palette is light on request of the reviewer; code blocks
and byte dumps keep a dark ground so the byte colours still work. The OPEN
walk lights the hovered bytes as an inverted block and its table cells are
dark text on a tint of their group colour.

build.py inlines reveal.js, the stylesheets, scripts and Markdown sections
into slides/dist/hands-on-bgp.html for uploading.

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

2026-10-01 20:40:55 +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 locally connected parties 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.

What that implies

Peering participants in an exchange are all one hop away, found by neighbour discovery
A route's next hop is the peer's LAN address, so traffic goes directly member to member
The exchange is a simple layer 2 switch, allowing for a large capacity at low cost
Strict(ish) rules on what traffic is allowed on the peering lan (e.g. no DHCP or broadcast packets)

Notes:

IPv6 neighbour discovery and ARP are used to discover the MAC addresses of peers
after which traffic can flow directly

Route servers

Are a piece of shared infrastructure that simplifies peering.

Most participants peer with the route server and annouce their prefixes there
The route server can then distribute these routes to every other participant
Simplifes from n² to n sessions
Not in the traffic path and does not add its own AS into the path
Allows controlling behaviour with communities, e.g. 42476:0:<as> "not to <as>", 42476:1:<as> "only to <as>"

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.

Looking glasses

Are a useful tool to inspect the state of another network or internet exchange

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