5.2 Configuring OSPF Operation Across Multiple Areas
5.2.1 Forwarding packets in a multi-area network
This section summarizes how routers flood information, and how they build their routing tables when operating within a multi-area environment. There are some differences when flooding is used across multiple areas versus its use within an area. Before reviewing how ABRs and other router types process route information, you should know how a packet makes its way across multiple areas. In general, the path a packet must take is as follows:
  • If the packet is destined for a network within an area, it is forwarded from the internal router through the area to the destination internal router.
  • If the packet is destined for a network outside the area, as shown in the main figure, it must go through the following path:
  • The packet goes from the source network to an ABR.
  • The ABR sends the packet through the backbone area to the ABR of the destination network. All packets must cross the backbone when being forwarded from one area to another.
  • The destination ABR then forwards the packet through the area to the destination network.