An engineer has implemented EIGRP in the data center with a large OSPF routed environment.
All EIGRP routes were functioning properly until enabling EIGRP route summarization. Legacy
OSPF routes are now being used for route selection. Why have the routes changed?
A.
EIGRP summary routes have a higher administrative distance than EIGRP specific routes
B.
reduce the administrative distance for the EIGRP summarized routes
C.
OSPF has a more specific route with EIGRP summarization enabled
D.
increase the administrative distance for the OSPF route process
E.
modify the bandwidth and delay metrics to be preferential over OSPF
C
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Why C? Why is the answer C as compared to what the site says the answer is?
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Hi,
Answer A is wrong, because of this (5 vs 90):
only possible answer is C.
Routing Protocol Administrative distance
Directly connected interface 0
Static route out an interface 1
Static route to next-hop address 1
DMNR – Dynamic Mobile Network Routing 3
EIGRP summary route 5
External BGP 20
Internal EIGRP 90
IGRP 100
OSPF 110
IS-IS 115
Routing Information Protocol (RIP) 120
Exterior Gateway Protocol (EGP) 140
On Demand Routing (ODR) 160
External EIGRP 170
Internal BGP 200
Next Hop Resolution Protocol (NHRP) 250
Floating Static Route (ex. DHCP-learned) 254
Unknown 255
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More specific routes always wins from AD. So the best answer is: C.
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Guys, I think you missed Something in the Question and specifically the answer A!
Maybe the EIGRP summary routes have changed manually to a higher AD than the default value!
Ex: ip summary-address eigrp 10 0.0.0.0 0.0.0.0 150
considering this, then the answer A is correct!;-)
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found this
Lists the administrative distance default values of the protocols that Cisco device supports below;
Routing protocol /Route source
Default Distance Values
Connected interface 0
Static route 1
Enhanced Interior Gateway Routing
Protocol (EIGRP) summary route 5
External Border Gateway Protocol (eBGP) 20
Internal EIGRP 90
IGRP 100
OSPF 110
Intermediate System-to-Intermediate System (IS-IS) 115
Routing Information Protocol (RIP) 120
Exterior Gateway Protocol (EGP) 140
On Demand Routing (ODR) 160
External EIGRP 170
Internal BGP 200
Unknown 255
http://cisconet.com/routing/routing-general/116-list-of-routing-protocol-administrative-distance-value.html
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so if it was summarized, it should be more believable considering an ad of 5 comparing to ad of 110 of ospf or eigrp 90…right?
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Ans: C
Example
Making Forwarding Decisions
Let’s look at the three routes we just installed in the routing table, and see how they look on the router.
router# show ip route
….
D 192.168.32.0/26 [90/25789217] via 10.1.1.1
R 192.168.32.0/24 [120/4] via 10.1.1.2
O 192.168.32.0/19 [110/229840] via 10.1.1.3
….
If a packet arrives on a router interface destined for 192.168.32.1, which route would the router choose? It depends on the prefix length, or the number of bits set in the subnet mask. Longer prefixes are always preferred over shorter ones when forwarding a packet.
In this case, a packet destined to 192.168.32.1 is directed toward 10.1.1.1, because 192.168.32.1 falls within the 192.168.32.0/26 network (192.168.32.0 to 192.168.32.63). It also falls within the other two routes available, but the 192.168.32.0/26 has the longest prefix within the routing table (26 bits verses 24 or 19 bits).
Likewise, if a packet destined for 192.168.32.100 arrives on one of the router’s interfaces, it’s forwarded to 10.1.1.2, because 192.168.32.100 doesn’t fall within 192.168.32.0/26 (192.168.32.0 through 192.168.32.63), but it does fall within the 192.168.32.0/24 destination (192.168.32.0 through 192.168.32.255). Again, it also falls into the range covered by 192.168.32.0/19, but 192.168.32.0/24 has a longer prefix length.
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