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IP Routing 2

May 12th, 2017 Go to comments

Question 1

Question 2

Explanation

The administrative distance of each protocol is compared to see if that route is better or not.

Question 3

Explanation

Letter “D” is used to symbolize for EIGRP (with a default AD of 90). Letter “E” is not used for EIGRP because it has been used for Exterior Gateway Protocol (EGP). It is a obsolete routing protocol now.

Question 4

Question 5

Explanation

If there is a default route then surely the router will forward all unknown traffic to there. But if there is no default route in the routing table then the router will discard the packet.

Question 6

Question 7

Question 8

Explanation

Making a forwarding decision actually consists of three sets of processes: the routing protocols, the routing table, and the actual process which makes a forwarding decision and switches packets. The longest prefix match always wins among the routes actually installed in the routing table, while the routing protocol with the lowest administrative distance always wins when installing routes into the routing table.

routing_process.gifReference: http://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/8651-21.html

Therefore in this question, the process will be: longest bit match > lowest AD > lowest metric > equal load balancing.

Question 9

Explanation

In fact this question is not clear. If we understand that “router is unable to find a known route in the routing table” and there is no default route in the routing table then the router will surely discard the packet -> A is correct. But we are not sure if there is a default route or not so let learn more about gateway of last resort.

A Gateway of Last Resort is a route used by the router when no other known route exists to send the IP packet. For CCNA level, when ip routing feature is enabled, a gateway of last resort is usually created by:
+ The “ip default-network” command (but dynamic routing protocols have different behaviors). But in general, the “ip default-network” cannot set the gateway of last resort without a known route in the routing table.
+ Creating a static route to network 0.0.0.0 0.0.0.0 is another way to set the gateway of last resort on a router. This is the reason why this question is not clear as it does not tell us if a default route exists or not.

Maybe in this question a default route does not exist. Otherwise the author would notice and indicate it in the question.

For more information about Gateway of Last Resort, please read: http://www.cisco.com/c/en/us/support/docs/ip/routing-information-protocol-rip/16448-default.html

Question 10

Explanation

The default administrative distance value of static route is 1 which is always smaller than any dynamic routing protocol so it is always preferred to dynamic routing protocols.

Question 11

Explanation

Floating static routes are static routes that have an administrative distance greater than the administrative distance (AD) of another static route or dynamic routes. By default a static route has an AD of 1 then floating static route must have the AD greater than 1. Floating static route has a manually configured administrative distance greater than that of the primary route and therefore would not be in the routing table until the primary route fails.

Comments (15) Comments
  1. Chad
    June 13th, 2017

    Passed with the 747q dumps, all questions were from there.

  2. Yammato
    June 13th, 2017

    Question 7
    If a route is not present in the routing table for a particular destination, what would the router do?

    If there is no route in the routing table the router should not discard the package? For at no time the enunciation of the question signals the present-day representation of a standard route. What if a default route was not configured on this router? The right thing would be to dispose of the correct package?

  3. Becky
    June 21st, 2017

    Question 9
    Answer should be D

  4. Anonymous
    June 24th, 2017

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  5. OSC
    June 26th, 2017

    In Q8 When a device learns multiple routes to a specific network, it installs the route with?

    The response longest bit match (highest subnet mask) is not correct because it sais ” it installs the route with?” not which route the router choose. If the router are installed allredy, the router choose the longest prefix match, but if it has to install a new route, it will install the one with the best AD.
    So best suitable response will be B Lowest AD.
    What do you think?
    Does anyone agree?

    Acording to: http://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/8651-21.html
    Making Forwarding Decisions

  6. JD
    June 26th, 2017

    Agreed to OSC. 9tut please correct. Thank You.

  7. JD
    June 26th, 2017

    Q8 is about installing the route in routing table not choosing a route from the routing table to route the packet.

  8. JD
    June 26th, 2017

    @Becky: And what if the router has no gateway of last resort configured? The question does not say anything about it.

  9. OSC
    June 27th, 2017

    And respect to Q5&9 at last we can make it at head or tail, it’s an anbiguos question and depends on how luky you are.
    Wha Q5 D and Q9 A?
    Why not the opposite?
    I don’t know, toss a coin and good luck!!

  10. @Anon
    July 8th, 2017

    I think in start configuration in routing table you don’t have any default of last resort.
    We should also point if router didn’t found any match means that you don’t have any Default Gateway, because if it exist we will not give information “router unable to find know destination”

    ip route 0.0.0.0 0.0.0.0
    0.0.0.0 any adres
    0.0.0.0 any mask

  11. Anony
    July 12th, 2017

    I agree with the above comments that say that Q8 is B.
    The device learns about multiple routes to a network and must choose one to install – it’s not choosing an already installed route to use. It’s installing a new one. So it will use the lowest AD.

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