Inter-Area Routing
To route a packet outside an area, a level 1 router sends the packet to the nearest level 2 capable router. The nearest router is always used, regardless of the destination. Level 1 routers don’t have enough information about other areas to make a decision.
When the packet reaches the level 2 router, it gets forwarded across the backbone to get to the correct area. It is then passed on to a level 1 router in the area for local delivery.
To optimise routing, it is possible to leak routes from the level 2 database into the level 1 database. This technique prevents suboptimal routing outside the area. When leaking routes, they have a flag added. This prevents advertising them back into the level 2 database elsewhere.
When routing the packet within an area, the routers use the System-ID of the router for delivery. When routing packets between areas, the Area-ID is the address. These addresses are part of the NET Value.
