In the following diagram, Gateway router is configured with EIGRP routing peering with Core2, in addition to 2 static routes. For client workstation to access server, 1) which router table is correct for Gateway router? 2) Which path will client machine take?
Select a route table for Gateway router:
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Answer: Route Table B and the purple path.
Explanation: There are three possible routes from the client to the server: the static route to 20.0.0.0/16, the floating static route to 20.0.0.0/24 and EIGRP route to 20.0.0.0/24. The floating static route and EIGRP route have the identical destination subnet IP address and mask, but have different Administrative distance (AD): The AD of EIGRP is 90 and AD of the floating route is 110. The route with smaller AD will be selected and therefore EIGRP route is in the route table. The static route 20.0.0.0/16 has different subnet mask and so appears in the route table as a route to the destination different than 20.0.0.0/24.
The router will select the route with maximum network mask length to forward the packet if there are two route entries to the destination. In this case, the EIGRP route is selected.
In the above case, if the link between Core2 and server goes down, which path should the client take to access the server, the blue one or the purple one?
Answer: The purple path.
Explanation: When the interface Serial1/0 of Core1 is down，Gateway is unable to learn EIGRP route to 20.0.0.0/24 and therefore the route to 20.0.0.0/24 is replaced by the floating static route. The route table of Gateway will be：
Gateway will select the floating static route to forward the traffic to Core2（23.1.1.2）. This can be confirmed via the trace route result as following:
This case demonstrates that the static route cannot detect the topology change and leads to the communication failure. And this is the primary reason that the dynamic routing is deployed in the modern IP network.