Refer to the exhibit. A router received these five routes from different routing information sources. Which two routes does the router install in its routing table? (Choose two.)
Ref: Route Selection in Cisco Routers - Cisco
“…
Summary
The LONGEST PREFIX MATCH always wins among the routes actually installed in the routing table, while the routing protocol with the LOWEST ADMINISTRATIVE DISTANCE always wins when installing routes into the routing table.
…”
You have 5 routes from 4 different routing protocols. Two of these routes are from OSPF: the router must choose (between them) which one to install in the routing table, and here the longest prefix match criteria wins (router chooses OSPF route 10.0.0.0/30).
Now, you have 4 routes. 3 routes are all /30 routes (same prefix length): the router chooses on the basis of lowest administrative distance, and OSPF wins (administrative distance for these protocols is OSPF 110, which is lower than RIP 120, which is lower than BGP 200).
The last route is an EIGRP one, and this is also a host route (look at the /32 prefix). There can’t be a longest prefix than a host route. Also, this route doesn’t have to compete with any other route to be installed in the routing table. So, this route is the router’s second choice.
Answers A and D are correct.
Googling your reference found me this link:
https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/8651-21.html
From your reference:
Let's look at another scenario to see how the router handles another common situation: varying prefix lengths. Assume, again, that a router has four routing processes running on it, and each process has received these routes:
EIGRP (internal): 192.168.32.0/26
RIP: 192.168.32.0/24
OSPF: 192.168.32.0/19
Which of these routes will be installed in the routing table? Since EIGRP internal routes have the best administrative distance, it's tempting to assume the first one will be installed. However, since each of these routes has a different prefix length (subnet mask), they're considered different destinations, and they will all be installed in the routing table.
We have 3 correct answers not 2 !
One route to 10.0.0.0/30 selected from the 3 routes IBGP/OSPF and RIP (OSPF is correct option A)
One route to a host 10.0.0.1/32 (D) EIGRP
One route to 10.0.0.0/16 OSPF (C)
A D and C are correct ?
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Hi GreadDane; are you sure that OSPF route 10.0.0.0/30 will be installed intead of OSPF 10.0.0.0/16, including the other one? This way, the router would ignore some 65k valid destinations!
Packet tracer result is CD
10.0.0.0/16 has absorbed 10.0.0.0/30.
This question has nothing to do with longest prefix match. Longest prefix match is for choosing which route to use to reach a destination address. This question is about choosing which routes to install. In a real router, it should install every route with a unique network ID and mask. The correct answer for real hardware should be ACD.
This question is BAF and should have three correct answers A C D
riddle me this riddle me that:
If every host route would "win" from a non host (subnet) route in the routing table, how do you think you will be able to ping other hosts in the subnets those host routes are in?
there will be three correct answers. One will be selected among the three /30 mask, other one will be /16 mask and last one will be /32 mask.
among the three /30 mask routes, Ospf will be selected since Ospf has lowest AD which is 110 whereas RIP has AD value of 120 and iBGP has AD value of 200.
source: https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/8651-21.html
according to cisco, 3 routes will be installed, because different subnets mean different destinations. Moot question
A&D
The longest-matching route is preferred first.
In the event, there are multiple routes to a destination with the same prefix length, the route learned by the protocol with the lowest administrative distance is preferred.
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