Stuff from example questions

Question 1

Count to infinity problem

Answer 1

A->B->C
If B->C broken, there’s a chance it will advertise to A that it can reach C in 3 (via A then B), A updates and advertises C in 4 hops (via B then A then B) and so on for infinity.

Question 2

Distinguish interior and exterior routing protocols

Answer 2

Interior used to connect same AS routers. Routers broadcast info which is shared freely as routers are trusted. May be connected dynamically.

Exterior connect different AS and only BGP exists as option. Neighbors are defined statically and policies are used to control flow.

Question 3

How can community BGP be used to apply policy? Customer, peer and provider

Answer 3

Routes can belong to communities and the second part of the community value refers to the community number. eg users 100, peer 200, prov 300. Import and export rules can be used to block or allow flow of traffic for each community

Question 4

Two ways of setting up BGP primary and backup links

Answer 4

The local preference attribute so that one exit is preferred

You could also use Weight. Highest weight = preferred

Question 5

BGP router learns prefix from two locations, how to decide path in RIB?

Answer 5

Local preference
shortest AS path
External over internal

ROutes first validated (ie next hop available) then BGP best path selection. A tie breaker is the closest next hop.