RIP Redistribution Metric 15 Leaves the Route Unreachable?

Why the Answer Is Correct

RIP uses hop count as its only metric, and a value of 16 is defined as unreachable, so the seed metric used when a routing protocol is redistributed into RIP decides how far that prefix can travel. The exhibit shows the router running router rip with redistribute ospf 1 metric 15, and because each RIP router increments the hop count when it advertises, the redistributed prefix is already sitting at the 15 ceiling on the next router; one more hop takes it to 16, RIP's infinity, so the far router refuses to install 192.168.250.0 and traffic from 172.16.3.16/28 toward 192.168.5.16/28 fails. Option C is the configuration that removes that ceiling problem by lowering the seed metric to 14, leaving exactly the headroom the remaining hop needs. As Pietjeplukgeluk reasoned, "we should reduce hop count to allow adjacent router" to stay under the limit rather than equal it, and dapardo points out that "Any value greater than 15 will be considered infinite." That is the whole test: a seed of 15 is valid only if the prefix is never advertised again.

Why the Other Options Are Wrong

Option A keeps the redistribution metric at 15 and instead depends on poison reverse (split horizon with poisoned reverse) to protect the prefix, but poison reverse only stops a router from advertising a route back out the interface it learned it on — it cannot resurrect a route whose hop count has reached 16. That is why Coffee_bean_master's "because of poison reverse" rationale does not survive the arithmetic, even though chaba7654321 reports that "A works for me in a lab." Options B and D configure router ospf 1 (OSPF network statements or OSPF-side redistribution), and no OSPF change can reduce a RIP hop count, so neither can restore the missing path to 192.168.5.16/28. Ivan_AGC argues the opposite way, saying "if the metric is configured in 15 reducing it to 14 will not help to deliver the route," but that ignores the additional increment needed to reach 16 — at a seed of 14 the last router still sees 15, which is usable. Keegom's reminder that "The maximum number of hops allowed for RIP is 15" is precisely the rule that separates A from C.

Community Comment Notes

Community sentiment runs strongly toward C (roughly 70 to 30), and the reasoning is metric arithmetic rather than memorised answer keys. dapardo backs the verdict with a Cisco redistribution document, while bk989 reads the exhibit step by step and concludes that "So we need to reduce the metric to 14," so the downstream router receives 15 instead of 16. Pietjeplukgeluk and Keegom both frame the choice around RIP's hop limit, and the dissenters are instructive: Ivan_AGC reads R4 as never receiving the route and Coffee_bean_master prefers poison reverse, while chaba7654321 says A worked in a lab. Those lab results usually mean the prefix was never re-advertised far enough to hit 16, so the seed metric was never actually exercised.