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Chapter 2

Protocol Attributes

F I G U R E 2 . 1 4

Route updates vs. calculation

On the other hand, if the link between Router A and Router C becomes

congested, both routers have to calculate a new route metric. The change is
then advertised to Router B by both Routers A and C.

Topology and Route State Information

The topology database stores all routes and metrics known via adjacent
routers. Six routes can be stored for each destination network. From these
six routes, the router must select a primary and backup route; the primary
route will be added to the route table. While the best route is being chosen
for a destination, the route is considered to be in an active state. After the
route is chosen, the route status changes to passive.

Information given in Table 2.9 closely, though not exactly, represents

what is contained in an actual topology table. The Status field shows
whether a new route is being calculated or whether a primary route has been
selected. In our example, the route is in passive state because it has already
selected the primary route.

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