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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 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
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.