09 - Exploring EBGP

Question 1

External BGP provides ___ between ___.

Answer 1

inter-domain routing, autonomous systems

Question 2

BGP uses TCP ___.

Answer 2

179

Question 3

What are the three main characteristics of BGP?

Answer 3

• Reliable – periodic TCP keepalives, incremental, triggered updates
• Scalable – 600k+ internet routes, TCP sliding window ACKs
• Stable – Batched updates, slower convergence

Question 4

BGP is a ___ protocol

Answer 4

path vector (modified distance vector)

Question 5

What does BGP advertise?

Answer 5

• a path of autonomous systems

* networks at the end of the path

Question 6

What are the BGP data structures?

Answer 6

• BGP neighbour table – list of BGP peers
• BGP table – list of all BGP learned routes
• IP routing table – list of best path routes

Question 7

External BGP routes (routes from an external AS) have an AD of __.

Answer 7

20

Question 8

Internal BGP routes (routes on the same AS) have an AD of ___.

Answer 8

200

Question 9

What are the BGP message types?

Answer 9

• Open
• Keepalive
• Update
• Notification

Question 10

A router that runs BGP is known as a BGP ___.

Answer 10

speaker

Question 11

A BGP ___ is when a BGP speaker is configured to form a neighbour relationship with another BGP speaker

Answer 11

peer

Question 12

What are the requirements for establishing an EBGP neighbour relationship?

Answer 12

• Different AS number
• Defined neighbours
• Reachability

Question 13

BGP chooses a single best path based upon __, not link characteristics like bandwidth or delay.

Answer 13

policy

Question 14

BGP is not designed for _____.

Answer 14

load balancing

Question 15

What is the BGP route selection process?

Answer 15

Route selection process, routing policy is based on factors called attributes:

1. Prefer highest WEIGHT attribute (local to router)
2. Prefer highest LOCAL PREFERENCE attribute (global with AS)
3. Prefer route ORIGINATED by local router (next hop = 0.0.0.0)
4. Prefer shorted AS path (least number of AS in AS-path attribute)
5. Prefer lowest ORIGIN attribute (IGP < EGP < incomplete)
6. Prefer lowest MED attribute (exchanged between autonomous systems)
7. Prefer an EBGP PATH over an IBGP path
8. (IBGP route) prefer PATH through closest IGP neighbour
9. (EBGP route) prefer oldest EBGP PATH (neighbour with longest uptime)
10. Prefer path with lowest neighbour BGP ROUTER ID
11. Prefer path with lowest neighbour IP address