Solved: OSPF Question

pauloroque · ‎09-21-2009

This is a question from Wendell Odom CCIE Certification Guide book, 3td edition, chapter 9, question 9. I not sure if the asnwer in the appendix is correct. So I post it here to see if someone agrees with me. There might be more than one right answers.

Thx.

Paulo Roque

R1 is an OSPF ASBR that injects an E1 route for network 200.1.1.0/24 into the OSPF

backbone area. R2 is an ABR connected to area 0 and to area 1. R2 also has an Ethernet

interface in area 0, IP address 10.1.1.1/24, for which it is the designated router. R3 is a

router internal to area 1. Enough links are up and working for the OSPF design to be

working properly. Which of the following are true regarding this topology? (Assume no

other routing protocols are running, and that area 1 is not a stubby area.)

* a. R3's cost for the route to 200.1.1.0 will be the cost of the route as it was injected into the

OSPF domain by R1, without considering any internal cost.

* b. R3's cost for the route to 200.1.1.0 will include the addition of R3's cost to reach R1,

plus the external cost listed in the LSA.

* c. R3's cost for the route to 10.1.1.0/24 will be the same as its cost to reach ABR R2.

* d. R3's cost for the route to 10.1.1.0/24 will be the sum of its cost to reach ABR R2 plus

the cost listed in the type 3 LSA created for 10.1.1.0/24 by ABR R2.

* e. It is impossible to characterize R3's cost to 10.1.1.0/24 because R3 uses a summary type

3 LSA, which hides some of the costs.

10. R1 and R2 each connect via Fast Ethernet interfaces to the same LAN, which should be in

Peter Paluch · ‎09-21-2009

Hello Paulo,

You are completely correct! I was too hasty about the d) option - I haven't noticed it talks about an inter-area route, not about the external network. Good catch! Thanks!

Best regards,

Peter

View solution in original post

Peter Paluch · ‎09-21-2009

Hello Paulo,

The correct answer is b) only. For an E1 external network, the R3's cost will be given by the sum:

From R3 to ABR (R2) + from ABR (R2) to ASBR (R1) + from ASBR (R1) to the external route (the seed metric)

The first component of the distance (from R3 to R2) is computed using the information in LSA1 and LSA2. The second component (from R2 to R1) is known to R3 by means of LSA4 that is created on the R2 and sent into the area 1. The third component (the metric of the E1 route redistributed by R1 into the backbone) is known to R3 by means of LSA5 originated by the R1 itself.

The a) answer would be correct if the redistributed route would be an E2 route. The c) answer is completely incorrect in this case. The d) is incorrect because in computation of metrics for external routes, the LSA3 are not used. The e) is obviously also incorrect.

You're welcome to ask further if you have any more questions.

Best regards,

Peter

pauloroque · ‎09-21-2009

Hello Peter.

I agree on letter 'b'. But 'd' is also correct, because the route in question (10.1.1.0/24) is an IA route.

Thx.

Paulo Roque

Peter Paluch · ‎09-21-2009

Hello Paulo,

You are completely correct! I was too hasty about the d) option - I haven't noticed it talks about an inter-area route, not about the external network. Good catch! Thanks!

Best regards,

Peter

pauloroque · ‎09-21-2009

Thank you Peter.

The ansewer in Odom's is letter 'C'.

Paulo Roque