BGP
Autonomous systems and their numbers, interior against exterior routing, eBGP and iBGP sessions over TCP 179 and their four messages, path vector and AS_PATH, best-route choice in short form, route hijacking and leaks.
- 28 cards
- 6 topics
- Free
The outline, topic by topic
Each topic is weighted the way the exam weighs it.
- Concepts
- Questions
- Sort and link
- On the diagram
- Scenarios
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A few cards from the deck
BGP: what is it?
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The Internet's exterior routing protocol: border routers exchange prefixes over TCP sessions and choose their routes according to policy, not distance alone.
An IGP looks for the best path; BGP looks for the path that business agreements allow. The shortest is not necessarily the one chosen: a higher local preference wins.
A router receives three announcements for 203.0.113.0/24. It applies the highest local preference, then the shortest AS_PATH. Which does it keep?
```text
Saut suivant Pref. locale AS_PATH
192.0.2.1 200 65010 65001
192.0.2.5 100 65020
192.0.2.9 100 65030 65040 65001`- The one from 192.0.2.1, despite its longer path
- The one from 192.0.2.5, with the shortest path
- The one from 192.0.2.5: as with a distance, the lowest preference wins
- None alone: it keeps all three and shares the traffic
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The one from 192.0.2.1, despite its longer path
Local preference is compared before AS_PATH length: the highest wins, 200 beats 100, even with two ASes instead of one. Path length only matters when the preferences are equal.
BGP sets up its sessions over TCP, on port …; between two different autonomous systems the session is …, inside a single system it is ….
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BGP sets up its sessions over TCP, on port 179; between two different autonomous systems the session is eBGP, inside a single system it is iBGP.
The “e” or “i” prefix only says whether the two peers are in the same AS. 520 is RIP's port, and OSPF has no port: it runs over IP.
Autonomous system (AS): what is it?
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A set of networks under a single routing policy, identified by a number (16 bits originally, 32 bits today).
An operator, a university or a large site has its own. Numbers 64512 to 65534 are reserved for private use, like 10.0.0.0/8 for addresses.