Network Design and Operations
A router has learned routes to the same destination, 192.168.50.0/24, from both OSPF and RIP. On this router, the administrative distance, which expresses the preference for each route source (lower is preferred), is 110 for OSPF and 120 for RIP. Which route goes into the routing table?
- ABoth routes are installed and used alternately for each packet
- BOnly the RIP route is installed
- COnly the OSPF route is installed
Show answer and explanation
Answer: C. Only the OSPF route is installed
Routing protocols express route quality (the metric) in different ways, such as OSPF cost and RIP hop count, so their metrics cannot be compared directly. When routes to the same destination with the same prefix length are learned from several sources, the router therefore installs the one with the lower administrative distance, a measure of how much each source is trusted. Here OSPF's 110 is lower than RIP's 120, so the OSPF route goes into the routing table. Directly connected networks commonly get 0 and static routes 1, giving lower values to what people configure, but the name and default values differ between equipment vendors.
- A. Routes from different protocols have metrics with different meanings, so they are not installed side by side and used alternately.
- B. RIP has the higher value and therefore the lower preference, so its route is not installed.
- C. Correct. The OSPF route, with the lower administrative distance, is installed.