6 - BGP

Question 1

Why use EGP to link ASes?

Answer 1

Scaling to large networks (hierarchy, limit scope of failure)

Define admin boundaries (hide info, make own routing choices)

Policy (Control network reachability)

Question 2

Reasons not to use IGP to link ASes

Answer 2

If the other ISP has routing problems:
- Instability will ripple (backbones)
- Leaked prefixes leak into your backbone
- Won’t scale

Hard to filter routes.

Question 3

BGP

Answer 3

Border Gateway Protocol

Question 4

BGP focuses on…

Answer 4

Routing policy, not topology

Binds network groups

Question 5

BGP pros

Answer 5

Good route filtering capabilities
Ability to isolate from other’s problems

Question 6

BGP can be used to keep traffic local. Why is this useful?

Think about cost

Answer 6

Traffic doesn’t have to go over your expensive line but is instead peered locally

Question 7

BGP Policy

Answer 7

Control how you accept and send routing updates to neighbours

Question 8

Peering

Answer 8

COnnecting to other local ISPs directly.

Question 9

Transit

Answer 9

Connecting through ISPs to other ISPs.

At a cost

Question 10

Why Peer?

Answer 10

Reduce upstream transit costs
Local Traffic stays local
Increase International bandwidth
Can increase performance
May be the only way to connect customers to some part of the internet (tier 1)

Question 11

Why not Peer?

Answer 11

• Peers are usually competition
• Requires periodic renegotiation

Question 12

What provides internal connectivity in an AS?

Answer 12

IGP

Question 13

BGP OPEN

Answer 13

Opens TCP conn. to peer and authenticates

Question 14

BGP UPDATE

Answer 14

“Announcement”: prefix is reachable
“Withdraw”: prefix is not reachable

Question 15

BGP KEEPALIVE

Answer 15

keeps connection alive in absence of UPDATES
serves as ACK to OPEN request

Question 16

BGP NOTIFICATION

Answer 16

reports errors in previous msg
closes connection

Question 17

BGP basics

2 items. Updates and Protocol

Answer 17

Incremental Updates
- Send one copy of RIB then changes

Path Vector protocol
- Keeps track of the AS path of routing information

Question 18

neighbor

Answer 18

BGP peer

Question 19

NLRI

Answer 19

NLRI - Network Layer Reachability Info

Question 20

Router ID

Answer 20

32bit int to uniquely id router
Comes from loopback or highest IP on router

Question 21

What is “Default”?

Answer 21

Where to send traffic when there is no explicit route in the table

Question 22

What happens in a BGP Announcement with NLRIs?

Answer 22

Each AS originates a set of NLRI.
These are exchanged

Question 23

What if there are multiple paths to a given prefix?

Answer 23

BGP chooses the best and installs in the IP forwarding table

Question 24

What are withdrawn routes composed of?

Answer 24

Network Prefix
Mask Length

Question 25

iBGP mesh level

Answer 25

Full mesh, every router talks to every router

Question 26

Route Reflectors

What do they do? What elements are required to prevent loops?

Answer 26

Pass on the best iBGP route updates
ORIGINATOR_ID and CLUSTER_LIST required to prevent loops.