Lecture 3 - Border Gateway Protocol Flashcards
In the context of BGP, what is a path vector?
A description of the ASes that a packet will have to traverse to get to its destination
What does Network Layer Reachability Information convery?
Prefix and path vector
Given a path vector, how are loops detected?
They are detected if an AS number apears more than once in a non consecutive manner. Note, if a AS number appears in a consecutive manner, it means the path ins prepending
What is the key difference between iBGP and eBGP?
iBGP is used between BGP speakers that belong to the same AS. eBGP is used between ISP’s
in BGP why is a prepended path used?
It is used to artificially increase the length of a path vector in the hopes that a neighbouring AS will no take that path
Name the methods that allow an ISP/AS to influence the policies of a peer ISP/AS
Path prepending
Advertise longer prefix length
MED
Community String
Why does an ISP use hot potato routing as apposed to the highest capacity path?
To ensure packets exit an AS/ISP as quickly as possible, and thereby, minimise the time in which these packets consume network resources; buffer and bandwidth
The information in a routers routing table can come from?
information from OSPF’s LSA’s
announcements from iBGP or eBGP speakers
What is the BGP used for?
for exchanging routing information between gateway hosts in a network of autonomous systems
What is an AS?
Autonomous System
A connected group of one or more IP prefixes by one or more network operators which has a single and clearly defined routing policy
What is a SLA
Service Level Agreement
a contract between service provider and the customer
What is a IXP?
Internet Exchange Point
Where internet networks come together to peer or exchange traffic between their networks.
Direct connection between ISP’s as opposed to via 3rd party
What is a MED?
Multi-Exit Discriminator
Is a hint to external neighbours about the preferred path into an AS
A lower MED value is preferred over a higher value
What happens in AS Path Prepending?
The BGP router adds its own AS number to the front (left side) of the AS path.
The AS path lists all the AS’s that need to be traversed to reach the location where the prefix that the path is attached to is advertised from.
Avoids loops.
