Lecture 2: Intro to BGP

Question 1

What is Border Gateway Protocol, (BGP)?

Answer 1

The protocol responsible for managing how traffic transits from AS to AS

Question 2

What is the primary purpose of BGP?

Answer 2

Allow routing systems to share information on what resources
can be reached and where

Question 3

Which protocol and port is used to send BGP information?

Answer 3

BGP information is sent via TCP (Transmission control protocol) using port 179

Question 4

How does BGP work?

Answer 4

• Router sends out updates of currently available routes (advertisements)
• Other routers use the advertisements to update their own mappings

Question 5

What do Path-Vector Routing Protocol do?

Answer 5

Define a route as the collection of autonomous systems that must be travelled from source to destination

Question 6

What is the AS-Path?

Answer 6

The AS-Path describes a given path from one AS to another

Question 7

How is the AS-Path generated?

Answer 7

An AS re-advertises routes it has received,and adds it’s own AS to the AS-PATH variable

The AS-PATH variable then identifies each AS that will be transited through for the route

Question 8

What is the purpose of the AS-Path?

Answer 8

To prevent loops in the
routing algorithm

Question 9

What is AS-Path Prepending?

Answer 9

AS adds its own AS multiple times to the AS-Path to discourage people from using it

Question 10

How do routers learn about their neighbours through BGP?

Answer 10

• BGP sends out hello messages
• BGP maintains regular communication to check if the neighbour is still alive
• BGP allows advertisements to be sent/received as routes change

Question 11

What are the 3 core activities in a BGP routing system.

Answer 11

1. Listen for paths from internal and
   external BGP sources
2. Pick the best path and put it into the routing table
3. Advertise the best path to your neighbours