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IP Routing Questions

March 16th, 2020 Go to comments

Question 1

Question 2

Explanation

The source and destination IP addresses of the packets are unchanged on all the way. Only source and destination MAC addresses are changed.

Question 3

Question 4

Explanation

Both the line “O 172.16.0.128/25” and “S 172.16.0.0/24” cover the host 172.16.0.202 but with the “longest (prefix) match” rule the router will choose the first route.

Note: Network 172.16.0.192/29 does not cover host 172.16.0.202. For network 172.16.0.192/29:

Increment: 8
Network address: 172.16.0.192
Broadcast address: 172.16.0.200

Question 5

Explanation

The default AD of static route is 1 so we need to configure another number for the static route.

Question 6

Explanation

From the output, we see R1 will use the entry “O 10.10.13.0/25 [110/4576] via 10.10.10.1, …” to reach host 10.10.13.10. This is a network route.

Note: “B* 0.0.0.0/0 …” is a default route.

Question 7

Explanation

Our new static default route has the Administrative Distance (AD) of 120, which is bigger than the AD of OSPF External route (O*E2) so it will not be pushed into the routing table until the current OSPF External route is removed.

For your information, if you don’t type the AD of 120 (using the command “ip route 0.0.0.0 0.0.0.0 10.13.0.1”) then the new static default route would replace the OSPF default route as the default AD of static route is 1. You will see such line in the routing table:

S* 0.0.0.0/0 [1/0] via 10.13.0.1

Question 8

Question 9

Question 10

Explanation

The destination IP addresses match all four entries in the routing table but the 192.168.16.0/27 has the longest prefix so it will be chosen. This is called the “longest prefix match” rule.

Question 11

Explanation

The prefix with “longest prefix” will be matched first, in this case is “/29”.

Question 12

Explanation

The Administrative distance (AD) of EIGRP is 90 while the AD of OSPF is 110 so EIGRP route will be chosen to install into the routing table.

Comments (8) Comments
  1. Anonymous
    March 28th, 2020

    Hi! Anyone can teach me. why is that everytime I click the question it appears the explanation only not the question?

  2. marie
    April 23rd, 2020

    @9tut where does those answers are coming from on question 10?
    B. 102.168.16.0/24
    C. 102.168 26.0/26
    D. 102.168.16.0/27

  3. 9tut
    April 24th, 2020

    @marie: They should be 192, not 102. Thanks for your detection, we have just fixed them!

  4. indeed
    June 8th, 2020

    @9tut – are You sure?
    Both the line “O 172.16.0.128/25” and “S 172.16.0.0/24” cover the host 172.16.0.202 but with the “longest (prefix) match” rule the router will choose the first route.

    I belive that administrative distance for static routes is 1 so Static will be taken as a first…?
    I would be true with useing only ospf or static routes…?
    Am I right?

  5. 9tut
    June 9th, 2020

    @indeed: In this case the static route will not be chosen as the “O 172.16.0.128/25” route is more specific than the static route (longest prefix wins).

  6. Anonymous
    July 10th, 2020

    There is a variation to Question 6 asking to route to the Internet.

  7. jonathan
    August 10th, 2020

    @9tut
    Question 4: Can someone please help me if the longest prefix rule wins. Then why dosen’t / 29 (D) become the preferred choice.

  8. 9tut
    August 11th, 2020

    @jonathan: Network 172.16.0.192/29 does not cover host 172.16.0.202. For network 172.16.0.192/29:

    Increment: 8
    Network address: 172.16.0.192
    Broadcast address: 172.16.0.200

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