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EIGRP autosummarization

Posted on 2011-03-20
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Last Modified: 2012-05-11
I am trying to understand this practice question on cisco Adv test:

 10.10.10.0/24----E0 RTR1 S0----172.16.10.0/24---S0 RTR2 S1---192.168.20.0/24---S1 RTR3---E0

RTR1 config:
router eigrp 4
  network 10.0.0.0
  network 172.16.0.0
  no auto-summary

Auto-summarization is disabled on RTR1 but is enabled on RTR2 and RTR3. Which
two routes will be seen on the routing table of RTR3:
A. 10.0.0.0/8
B. 10.10.0.0/16
C. 10.10.10.0/24
D. 172.16.0.0/16
E. 172.16.0.0/24
F. 172.16.10.0/24
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Question by:totaram
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14 Comments
 
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Expert Comment

by:Don Johnston
Comment Utility
A & D

When auto-summary is enabled, routes will be summarized to the classfull boundaries.
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Author Comment

by:totaram
Comment Utility
Thats what I thought, answer is C,D.
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Expert Comment

by:Don Johnston
Comment Utility
Says who? :-)
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Expert Comment

by:gcl_hk
Comment Utility
10.10.10.0/24 is advertise by RTR1, even RTR2 have enabled auto summary, as the 10.10.10.0/24 is learn from neighbor, so auto summarization will not modify it and advertise to others by default
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Author Comment

by:totaram
Comment Utility
The answer provided with the question bank...

Could it be that since the two interfaces on RTR1 are directly connected, the no auto summarization  advertises them as what they are tp both RTR2 & RTR3.??
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Expert Comment

by:gcl_hk
Comment Utility
I have tried on lab.... and the result for your reference.

en
conf t
host R3
int g0/1
no sh
ip add 10.10.10.3 255.255.255.0
int fa0/0/1
no sh
ip add 172.16.10.3 255.255.255.0
router eigrp 4
network 10.0.0.0
network 172.16.0.0
no auto-summary

en
conf t
host R4
int fa0/0
no sh
ip add 172.16.10.4 255.255.255.0
int s0/0/0
ip add 192.168.20.4 255.255.255.0
router eigrp 4
network 172.16.0.0
network 192.168.20.0

en
conf t
host R2
int s0/0
clockrate 64000
ip add 192.168.20.2 255.255.255.0
router eigrp 4
network 192.168.20.0

10.10.10.0/24 - G0/1 --- R3 --- F0/0/1 - 172.16.10.0/24 - F0/0 --- R4 --- S0/0/0 - 192.168.20.0/24 - S/0 --- R2

R2#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

D    172.16.0.0/16 [90/2172416] via 192.168.20.4, 00:22:03, Serial0/0
C    192.168.20.0/24 is directly connected, Serial0/0
     10.0.0.0/24 is subnetted, 1 subnets
D       10.10.10.0 [90/2174976] via 192.168.20.4, 00:21:04, Serial0/0
0
 

Author Comment

by:totaram
Comment Utility
You gave the output on R2, what is 'sh ip route' on R3??

I think you are right if command 'no passive int' is defined on R1. I guess default is passive interface...
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Accepted Solution

by:
gcl_hk earned 63 total points
Comment Utility
R3????

R3#sh ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, + - replicated route

Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 3 subnets, 3 masks
D        10.0.0.0/8 [90/30720] via 172.16.10.4, 00:04:11, FastEthernet0/0/1
C        10.10.10.0/24 is directly connected, GigabitEthernet0/1
L        10.10.10.3/32 is directly connected, GigabitEthernet0/1
      172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
C        172.16.10.0/24 is directly connected, FastEthernet0/0/1
L        172.16.10.3/32 is directly connected, FastEthernet0/0/1
D     192.168.20.0/24
           [90/2172416] via 172.16.10.4, 00:40:14, FastEthernet0/0/1

R4#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
C       172.16.10.0/24 is directly connected, FastEthernet0/0
D       172.16.0.0/16 is a summary, 00:41:17, Null0
C    192.168.20.0/24 is directly connected, Serial0/0/0
     10.0.0.0/24 is subnetted, 1 subnets
D       10.10.10.0 [90/30720] via 172.16.10.3, 00:40:16, FastEthernet0/0

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Author Comment

by:totaram
Comment Utility
Sorry, didn't realize that you had named them differently.. the last one does have the interface address in routing table.
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Assisted Solution

by:Don Johnston
Don Johnston earned 62 total points
Comment Utility
Now I see what's going on. The auto-summary feature only works on directly connected routes. Since R2 learned the 10.10.10.0 route, it won't summarize when it sends it to R3.

And since R2 is connected to the 172.16.10.0 network, it will summarize that to R3.

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Author Comment

by:totaram
Comment Utility
"Since R2 learned the 10.10.10.0 route, it won't summarize when it sends it to R3."

How did R2 learn 10.10.10.0/24 route, it is not directly connected to R2, 172.16.0.0/16 is connected to R2 and R1.

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Expert Comment

by:Don Johnston
Comment Utility
>How did R2 learn 10.10.10.0/24 route,

R1 advertised it via EIGRP.
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Author Comment

by:totaram
Comment Utility
I see... R1 is no-auto summary and the other subnet (172.16.0.0) is directly connected, so auto-summary :-)
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Expert Comment

by:Don Johnston
Comment Utility
Correct.
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