Refer to the exhibit. R1 has just received a packet from host A that is destined to host B. Which route in the routing table is used by R1 to reach host B?
B is correct; it uses the lowest AD out of all the routes presented that go to the 10.10.13.0/25 subnet. A is a default route and would only be used if there wasn't a route to that subnet in the routing table.
A is not listed in the routing table
Based on the default route entry configured on R1 from the exhibit, a default route would look like this:
0.0.0.0/0 [1/0] via 10.10.10.2
B is a valid entry + it has the lowest AD (given that all route entries have the same prefix length)
Longest prefix match is preferred over any AD value.
Here, although the default route uses AD of 1 ,it isn't the proper match for the destination.
Destination address is 10.10.13.10 and the longest prefix match for that is 10.10.13.0/25.
There are currently 3 routes configured for that destination each with different AD value and also OSPF is running.
Now when you first have the longest prefix match then only you check the AD value. So that would make the route with AD of 108 the perfect path for the destination.
Router make decision based on the following checklist in order.
1. Longest prefix match
2. AD
3. Metric
This is just a high level overview but for the CCNA I think this will suffice.
The default route 0.0.0.0 via 10.10.10.2 is not chosen by R1 to reach Host B because specific routes take precedence over the default route. In routing decisions, routers always prefer the most specific match for a destination in the routing table. SO Routers always choose a route with the longest prefix length (the most specific match) over a less specific one.
A is the answer because it is a static route indicated by an administrative distance of 1, being the lowest of all routes to Host B (others being dynamically learnt via OSPF), it is chosen as the preferred route.
Just so blooming sneaky isn't it. You think you are getting it and it seems like you are not as there are just so many tricky/ trap questions.
So read the questions fully and make sure you are aware of the keywords in the question.
If choice 'A' was in the routing table, that would be the route used.
However it is not so the answer is B!
A is not even listed in the routing table.
B is a valid route in the routing table and it is the one with the lowest AD , therefore; it will be used to reach host B
Since the network 10.10.10.8/30 is not advertised in the OSPF configuration and there is no specific route to 10.10.10.10 in the routing table, R1 wouldn't have a direct route to reach 10.10.10.10. In this scenario, R1 would typically follow the default behavior of selecting the best match in its routing table. The best match among the routes listed would be the default route
A default route is a route that takes effect when no other route is available for an IP destination address. In our case, we have a route that matches the destination and thus the default route won't be used.
The correct answer is B
OSFP routing, indicated by area 0 routing command, has AD of 110. There is a floating static route configured with 108 AD. As the configured static route's AD is lower (108) than the OSPF's default AD (110) it will route the traffic via 10.10.10.10 because it has the lowest AD and thus will be put into the routing table.
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