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.
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