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Question about EIGRP

Posted on 2011-10-26
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Last Modified: 2012-05-12
hello,

i have some doubt to answer some questions about EIGRP, I need a CCIE or CCNP to help me out

Thank you



Core1#show ip eigrp topology all-links
IP EIGRP Topology table for AS(65001) / ID (172.17.10.1)

Codes : P - Passive, A - Active, U - Update, Q - Query, R - Reply,
   r - reply Status, s - sia Status

P 172.17.3.128/25, 2 successors, FD is 30720, serno 9
    via 172.17.10.2 (30720/28160), FastEthernet0/1
    via 172.17.3.2 (30720/28160), FastEthernet0/3
P 10.140.0.0/24, 1 successors, FD is 156160, serno 16
    via 172.17.3.2 (156160/128256), FastEthernet0/3
    via 172.17.10.2 (157720/155160), FastEthernet0/1
P 172.17.10.0/24, 1 successors, FD is 28160, serno 1
    via Connected, FastEthernet0/1
P 172.17.0.0/30, 1 successors, FD is 20514560, serno 15
    via 172.17.1.1 (20514560/205122000), FastEthernet0/2
    via 172.17.10.2 (20516120/20513560), FastEthernet0/1
P 172.17.1.0/24, 1 successors, FD is 28160, serno 2
    via Connected, FastEthernet0/2
P 172.17.2.0/24, 1 successors, FD is 30720, serno 8
    via 172.17.10.2 (30720/28160), FastEthernet0/1
    via 172.17.3.2 (33280/30720), FastEthernet0/3
P 172.17.3.0/25, 1 successors, FD is 28160, serno 3
    via Connected, FastEthernet0/3
Core1#


Question 1 :

Which two networks does the Core1 device have feasible successors for? (choose two)

A- 172.17.0.0/30
B- 172.17.1.0/24
C- 172.17.2.0/24
D- 172.17.3.0/25
E- 172.17.3.128/24
F- 10.140.0.0/24

my answer is B & D.
please correct me if wrong


-------------------------------------------------------------------------------------

Question 2 :

Which three EIGRP routes will be installed for the 172.17.3.128/24 and 172.17.2.0/24 networks (choose Three)

A - 172.17.3.128/25 [90/28160] via 172.17.1 2, 01:26:35, FastEthernet0/2
B - 172.17.3.128/25 [90/30720] via 172.17.3.2, 01:26:35. FastEthemet0/3
C - 172.17.3.128/25 [90/30720] via 172.17.10.2, 01:26:35. FastEthernet0/1
D - 172.17.2.0/24 [90/30720] via 172.17.10.2, 02:10:11, FastEthernet0/1
E - 172.17.2.0/24 [90/28160] via 172.17.10.2, 02:10:11. FastEthernet0/1
F - 172.17.2.0/24 [90/33280] via 172.17.3.2, 02:10:11. FastEthernet0/3

My answer : B - C - D
please correct me if wrong

---------------------------------------------------------------------------------------

Question 3

Which three networks is the router at 172.17.10.2 directly connected to? (Choose three)

A - 172.17.0.0/30
B - 172.17.1.0/24
C - 172.17.2.0/24
D - 172.17.3.0/25
E - 172.17.3.128/25
F - 172.17.10.0/24

my answer is : F - B - D
please correct me if wrong

----------------------------------------------------------------------------------------
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Question by:World05
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17 Comments
 
LVL 17

Expert Comment

by:Marius Gunnerud
ID: 37036425
Was there a diagram that came along with these questions, if so please post it.
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Author Comment

by:World05
ID: 37039008
No, only the result of the command "show ip eigrp topology all-links"
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LVL 12

Expert Comment

by:jjmartineziii
ID: 37039647
Question 1 is A and E but Question 2 and 3 are correct
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Author Comment

by:World05
ID: 37042486
can you give me some explaination, even if my choices are correct, i wasn't sure of myself while selecting the answers
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Accepted Solution

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Marius Gunnerud earned 2000 total points
ID: 37044021
Question #1
Answer: A & F
Explenation: For the route to become a Feasible Successor it must meet  the feasibility condition.  The feasibility condition(FC) is a condition that is met if a neighbor's advertised distance to a destination is lower than the router's FD to that same destination.

P 172.17.0.0/30, 1 successors, FD is 20514560, serno 15
    via 172.17.1.1 (20514560/205122000), FastEthernet0/2
    via 172.17.10.2 (20516120/20513560), FastEthernet0/1


P 10.140.0.0/24, 1 successors, FD is 156160, serno 16
    via 172.17.3.2 (156160/128256), FastEthernet0/3
    via 172.17.10.2 (157720/155160), FastEthernet0/1

Question #2
Answer: B, C, D
Eplanation: The metrics that are equal to the FD will be placed in the routing table

P 172.17.3.128/25, 2 successors, FD is 30720, serno 9
    via 172.17.10.2 (30720/28160), FastEthernet0/1
    via 172.17.3.2 (30720/28160), FastEthernet0/3

P 172.17.2.0/24, 1 successors, FD is 30720, serno 8
    via 172.17.10.2 (30720/28160), FastEthernet0/1
    via 172.17.3.2 (33280/30720), FastEthernet0/3

Question #3
Answer: C, E, F
Explanation: Obviously router 172.17.10.2 is directly connected to the 172.17.10.0/24 network. So F is correct.  We can also see that the FD for the directly connected 172.17.10.0/24 network is 28160, so we can assume that all routes directly connected to 172.17.10.2 will have an AD of 28160. That would mean that answers C and E are also correct.
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Expert Comment

by:jjmartineziii
ID: 37045398
@MAG03 is correct. E isn't correct because its a /24. The topology table shows it as /25.

Tricky bastards.
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LVL 17

Expert Comment

by:Marius Gunnerud
ID: 37050884
/24? Topology shows /25 and the answer E shows /25. E is a correct answer
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Author Comment

by:World05
ID: 37108039
MAG03 : thank you for your response and sorry for this delay.

Ok, E for which question?

I don't have a very strong or deep knowledge of the eigrp topology table, i have some questions please,

When they ask for feasible successors do we have to compare FD against AD ? what are we looking for the FD must be equal to AD in order to consider it as a feasible successor?

P 172.17.0.0/30, 1 successors, FD is 20514560, serno 15
    via 172.17.1.1 (20514560/205122000), FastEthernet0/2
    via 172.17.10.2 (20516120/20513560), FastEthernet0/1


P 10.140.0.0/24, 1 successors, FD is 156160, serno 16
    via 172.17.3.2 (156160/128256), FastEthernet0/3
    via 172.17.10.2 (157720/155160), FastEthernet0/1

is that correct or am i missing something here??


2nd question:

when they ask which three EIGRP routes will be installed for the 172.17.3.128/24 and 172.17.2.0/24 networks, Do we have to compare FD to the successor FD?

3rd question:

what about the last question which i don't understand, i see that you include every successor that has 172.17.10.2,

What about this one:

P 10.140.0.0/24, 1 successors, FD is 156160, serno 16
    via 172.17.3.2 (156160/128256), FastEthernet0/3
    via 172.17.10.2 (157720/155160), FastEthernet0/1

why we shouldn't include this also??


Last question, What does mean :

P 172.17.3.128/25, 2 successors, FD is 30720, serno 9
    via 172.17.10.2 (30720/28160), FastEthernet0/1
    via 172.17.3.2 (30720/28160), FastEthernet0/3
P 10.140.0.0/24, 1 successors, FD is 156160, serno 16

2 successors means that the route is connected to 2 routers?? and 1 successors only connected to 1 router??

thank you
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LVL 17

Expert Comment

by:Marius Gunnerud
ID: 37108837
The answer E is correct for question 3 in your original post.

Question 1: The feasibility condition(FC) is a condition that is met if a neighbor's advertised distance (AD) to a destination is lower than the router's FD to that same destination.  So yes you are correct.  As long as the AD is lower than the FD the route is eligible to become a Feasible Successor.

Question 2:  First we need to determine if the route meets the Feasibility Condition by comparing the AD to FD. Then we need to determine which of these will become the Successor which is done by comparing the Metrics.  The route with the lowest metric will be chosen as the Successor.  Feasible Distance is the best metric to the destination, and the Successor is the route that meets the Feasibility Condition and has the lowest metric, since the FD is the path with the lowest metric and the metric of the route is equal to the FD then that path has the best route to the destination and will become the Successor and entered into the routing table...hope that made sense.

Question 3:  The answer lies in the Administrative Distance (or Reported Distance if you like).  
The router's ID is 172.17.10.1 which means router 172.17.10.2 is connected to the same switch (or is directly connected to) as 172.17.10.1.  the AD for 172.17.10.0 is 28160.  That would mean that all via routes that have an AD of 28160 are directly connected to 172.17.10.2.
The route 10.140.0.0 is only accessed via two routes and neither route has an AD of 28160 so this network is not directly connected to the 172.17.10.2 router.

Question 4:  2 Successors means that the router is sending traffic to the same destination over 2 routes. This means that loadbalaning is in effect.  1 Successor means that there is only one route that the router is using to send traffic to a given destination.
0
 

Author Comment

by:World05
ID: 37109205
thank you for your explaination

I'm somehow confused now about question 1

Why we didn't include the answer C :

P 172.17.2.0/24, 1 successors, FD is 30720, serno 8
    via 172.17.10.2 (30720/28160), FastEthernet0/1
    via 172.17.3.2 (33280/30720), FastEthernet0/3

the FD > AD
which means 30720 > 28160 and equal to 30720

i think  this one must be a successor also.

and also this one:

P 172.17.0.0/30, 1 successors, FD is 20514560, serno 15
    via 172.17.1.1 (20514560/205122000), FastEthernet0/2
    via 172.17.10.2 (20516120/20513560), FastEthernet0/1

what do you think?
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LVL 17

Expert Comment

by:Marius Gunnerud
ID: 37109944
First of all, question 1 only asks for 2 answers. 172.17.3.2 is not included as an answer because it does not meet the Feasibility Condition.  The AD of 172.17.3.2 route is equal to the FD. The Feasibility Condition states that the AD of a route must be less than the FD to be eligible to be a Feasible Successor.

172.17.10.2 is a Feasible Successor to get to 172.17.0.0.  This one was included in my answer a few posts above.  The AD of 172.17.10.2 is less than the FD and therefore meet the Feasibility Conditions.
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Author Closing Comment

by:World05
ID: 37111184
You deserve 9000 points not only 2000, thank you very much
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Author Comment

by:World05
ID: 37113930
do you have excercices like these ones?
0
 
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Expert Comment

by:Marius Gunnerud
ID: 37114013
Sorry I do not. I am sure there are a few sites that have some questions posted though.
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Author Comment

by:World05
ID: 37124004
no problem, just 1 last question and sorry to disturb you, i want to confirm my knowledge:

FD is the total distance from the source to the destination right? Example:


(Please check the topology image file i uploaded)


as i can see the FD from R1 via R2 to network A = 30
                        FD from R1 via R4 to network A = 20

so R4 will be the successor and R1 will be the feasible successor, because the FD>AD which means 30>20 then R1 will choose the path via R4 it has a lower AD

                       R4's AD = 15
                       R2's AD = 25


Please correct me if wrong.
eigrp-topology.JPG
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LVL 17

Expert Comment

by:Marius Gunnerud
ID: 37126630
Yes FD is the total distance to reach the destination (Reported Distance (aka. AD) + distance to reach neighbor).

Remember that FD is the best metric along a path to the destination. You are correct that R4 will be chosen as the Successor. FD of 20 is the lowest metric so R2 will not qualify as a Feasible Successor as its Reported Distance (AD) is 25.

All in all you are on the right track.
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Author Comment

by:World05
ID: 37127271
thank you very much, it seems easy to understand just with some practise, but still have problems to understand the eigrp topology in first sight :D
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