Routing Areas

routing areas allows domains to grow larger without need of complex inter-domain routing protocols (intra-domain “simpler”)
routing area
- are subdomains/partitions of a domain/Autonomous System
- intra-domain routing protocols stays within each routing area
backbone area - a special routing area also known as area 0
area border router (ABR)
- a router that is a member of both the backbone area and non-backbone area
- ABRs are distinct to routers at the edge of an AS, which are referred to as AS border routers
List indent undo

problem: how does a router in one area determine the right next hop to another area
solution: to forward packet from one non-backbone area to another, there are three parts:
- packet travels from source network to backbone area
- then crosses the backbone area
- then travels from backbone area to destination network
to make this work
- ABRs summarizes routing information of an area and make it available in their advertisements to other areas
- example, R1 receives link-state advertisements from all the routers in area 1 and can thus determine the cost of reaching any network in area 1. When R1 sends link-state advertisements into area 0, it advertises the costs of reaching the networks in area 1 much as if all those networks were directly connected to R1. This enables all the area 0 routers to learn the cost to reach all networks in area 1. The area border routers then summarize this information and advertise it into the non-backbone areas. Thus, all routers learn how to reach all networks in the domain.

problem: dividing a domain into areas makes a tradeoff between scalability and optimality of routing. the use of routing areas forces all packets traveling in one ares to another to go through the backbone area
solution: flexibly decide which routers go in area 0
- virtual link - allows this to happen and helps improve optimality of routing

／var／log marcus chiu