Lesson 37 · Routing

BGP: the internet's map

The internet is thousands of separate networks, and none of them sees the whole map. Follow one address block, 203.0.113.0/24, as its owner announces it with BGP: every network adds its own number to the AS_PATH, a loop is spotted and dropped, your ISP picks a path by policy, and when a link breaks the route is withdrawn and traffic finds another road.

10 min12 steps5 questionsBeginner

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01B receives an announcement for 203.0.113.0/24 with AS_PATH 64496 and passes it on to D. What AS_PATH does D see?

Over eBGP, the sender puts its own AS number in front of the path. B is AS64497, so D hears 64497 64496. D adds its own number only when it passes the route on.

02A receives an update whose AS_PATH is 64497 64496. A is AS64496. What does it do?

Finding your own AS number in the AS_PATH means the route has gone round in a loop. BGP routers ignore such routes (RFC 4271 §9.1.2).

03D hears two routes of the same length: one from B, a peer it does not pay, and one from C, a provider it pays. Which does it usually pick, and why?

Local preference is checked before AS_PATH length. Most networks prefer customer routes, then peer routes, then provider routes, because that keeps their costs down.

04Which transport does BGP use between two routers?

BGP-4 runs over a TCP connection to port 179. TCP gives it reliable, in-order delivery, so BGP only has to send changes.

05The A-B link breaks and B has no other route to 203.0.113.0/24. What does B send to D?

An UPDATE message can carry withdrawn routes as well as new ones. D removes the route through B and falls back to its route through C.

Key terms

Glossary

Autonomous system (AS): A network run by one organisation with one routing policy, such as an ISP, a cloud provider or a university.
AS number (ASN): The number that identifies an AS in BGP. Numbers 64496 to 64511 are reserved for examples like this lesson.
BGP: Border Gateway Protocol, version 4: how ASes tell each other which address blocks they can reach.
eBGP / iBGP: External BGP runs between two different ASes; internal BGP shares those routes between routers inside one AS.
Prefix: A block of IP addresses written with its length, like 203.0.113.0/24. BGP announces prefixes, not single addresses.
AS_PATH: The list of AS numbers an announcement has passed through, newest first. Used to spot loops and to compare routes.
Local preference: A number a network gives its own routes to rank them by policy. Higher wins, and it is checked before path length.
Withdrawal: The part of an UPDATE message that says a prefix can no longer be reached through the sender.

Fine print

Notes and sources

AS numbers 64496-64511 (RFC 5398) and the prefixes 192.0.2.0/24, 198.51.100.0/24 and 203.0.113.0/24 (RFC 5737) are reserved for documentation.
"More than 75,000 ASes" is the number seen in the global routing table; it grows every week.
Local preference values 200 / 100 / 50 are illustrative. Each network sets its own, usually customer > peer > provider.
Inside an AS, iBGP passes routes between its own routers without adding the AS number again.
A router that carries the full internet table holds about a million IPv4 routes, all learned this way.

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