Lecture 4: iBGP and eBGP

Question 1

What is Internal BGP (iBGP)?

Answer 1

iBGP is used to share external routes with peers of the gateway router

Question 2

What is External BGP (eBGP)?

Answer 2

eBGP is used for communication between different autonomous systems

Question 3

How does iBGP and eBGP differ?

Answer 3

• Information shared
• Who info is shared to
• Next-hop behaviour

Question 4

How does the AS-Path work in iBGP?

Answer 4

iBGP does not append the ASN to the AS-Path of a path/route

Question 5

How does the next hop work in iBGP?

Answer 5

Routes shared via iBGP will not update the next-hop attribute

Question 6

What is the Interior Gateway Protocol (IGP)?

Answer 6

Used to distribute internal routing information

Allows internal routers to determine where traffic needs to go

Question 7

How do we prevent loops in iBGP?

Answer 7

Full mesh routing

Question 8

How does full mesh routing work?

Answer 8

Every iBGP router is networked with every other iBGP router

Advertisements only make a single trip between router-router, no readvertising necessary

Question 9

What is the problem with Full Mesh Networking?

Answer 9

Doesn’t scale well

Number of links are close to n^2 (Increase exponentially)

Question 10

What is used for internal routing instead of full mesh iBGP to deal with the scaling problem?

Answer 10

IGP

Question 11

What are the 2 types of IGP routing?

Answer 11

• Link-State protocols
• Distance-Vector protocols

Question 12

What is Link-State protocols?

Answer 12

Routers know the entire network topology and flood updates to all other routers

Question 13

What is Distance-Vector protocols?

Answer 13

Routers only advertise to neighbours, and do not know the entire network topology

Question 14

What are examples of Distance-Vector protocols?

Answer 14

RIP, RIPv2, RIPng
IGRP

Question 15

What are examples of Link-State protocols?

Answer 15

OSPF, IS-IS

Question 16

When choosing an IGP protocol who makes the decisions and what are the factors in the decisions?

Answer 16

• It is up to the administrators of the
  autonomous system
• Choice based on cost, hardware
  vendor, network size, and specialty requirements

Question 17

What are the limitations of iBGP?

Answer 17

• The lack of re-advertisement
• Scalability issues with full mesh networks

Question 18

___ is used to handle routing within the AS?

Answer 18

IGP

Question 19

__ is used to handle the distribution of external routes

Answer 19

eBGP and iBGP

Question 20

How does iBGP and IGP work together to allow communication between the internal router, the local gateway router and the external gateway router in the destination AS?

Answer 20

• Gateway router learns path via eBGP and advertises it internally in the AS via iBGP
• Internal router learns path via iBGP and uses an IGP based routing table to determine the path to the local gateway router