How do routers use administrative distance values to determine the preferred route when learning the same route from multiple sources?
Routers, in the context of dynamic routing protocols, utilize administrative distance (AD) values to determine the preferred route when learning the same route from multiple sources. Administrative distance is a numerical value assigned to different routing protocols or static routes, representing their trustworthiness or preference. When a router learns the same route from different sources, it compares the AD values associated with each source to decide which route to include in the routing table and use for forwarding packets.
In networking, each routing protocol is assigned a default administrative distance value based on its reliability or trustworthiness. Lower AD values indicate higher preference, meaning that routes with lower AD values are considered more reliable and preferred over routes with higher AD values. For instance, a directly connected route typically has an AD of 0, making it the most preferred route. Static routes usually have an AD of 1, which is higher than directly connected routes but lower than most dynamic routing protocols.
When a router receives routing information about the same destination from multiple sources, it compares the AD values of the sources. The router selects the route with the lowest AD value as the preferred route and installs it in the routing table. If the router receives updates for the same route with different AD values, it will choose the route with the lowest AD, assuming it is the most reliable source for that route.
For example, consider a scenario where a router is running both OSPF (AD of 110) and RIP (AD of 120). If both routing protocols advertise a route to the same destination network, the router will choose the OSPF-learned route due to its lower AD value, making it the preferred route for forwarding packets.
In cases where the router learns the same route from different sources with the same AD value, tie-breaking mechanisms such as metric values or route preferences within the same protocol may come into play to determine the best path. These additional criteria help routers make more granular decisions when selecting routes from multiple sources with identical AD values.
In essence, routers use administrative distance values as a primary metric to determine the trustworthiness and preference of routing information received from various sources. By comparing AD values, routers can select the most reliable route to populate their routing tables and forward traffic effectively in dynamic routing environments.
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