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OSPF Questions 3

July 28th, 2017 in ROUTE 300-101 Go to comments

Question 1

Explanation

LSAs Type 8 (Link LSA) have link-local flooding scope.  A router originates a separate link-LSA for each attached link that supports two or more (including the originating router itself) routers.  Link-LSAs should not be originated for virtual links.

Link-LSAs have three purposes:
1.  They provide the router’s link-local address to all other routers attached to the link.
2.  They inform other routers attached to the link of a list of IPv6 prefixes to associate with the link.
3.  They allow the router to advertise a collection of Options bits in the network-LSA originated by the Designated Router on a broadcast or NBMA link.

LSAs Type 9 (Intra-Area Prefix LSA) have area flooding scope. An intra-area-prefix-LSA has one of two functions:
1.  It either associates a list of IPv6 address prefixes with a transit network link by referencing a network-LSA…
2.  Or associates a list of IPv6 address prefixes with a router by referencing a router-LSA.  A stub link’s prefixes are associated with its attached router.

LSA Type 9 is breaking free of LSA Type 1 and LSA Type 2 as they were used in IPv4 OSPF to advertise the prefixes inside the areas, giving us a change in the way the OSPF SPF algorithm is ran.

Reference (and for more information): http://packetpushers.net/a-look-at-the-new-lsa-types-in-ospfv3-with-vyatta-and-cisco/

Question 2

Question 3

Explanation

The wildcard mask should be 0.0.0.255 instead of the subnet mask 255.0.0.0.

Question 4

Explanation

Route aggregation can be performed on the border routers to reduce the LSAs advertised to other areas. Route aggregation can also minimize the influences caused by the topology changes.

Question 5

Question 6

Explanation

IS-IS is an interior gateway protocol (IGP), same as EIGRP and OSPF so maybe they are the best answers. Although RIP is not a wrong choice but it is not widely used because of many limitations (only 15 hops, long convergence time…).

Question 7

Explanation

This command affects all the OSPF costs on the local router as all links are recalculated with formula: cost = reference-bandwidth (in Mbps) / interface bandwidth 

Note: The default reference bandwidth for OSPF is 10^8 bps or 100Mpbs so the “auto-cost reference-bandwidth 100” is in fact the default value so answer A may be  a correct answer.

Question 8

Explanation

NSSA External LSA (Type 7) – Generated by an ASBR inside a Not So Stubby Area (NSSA) to describe routes redistributed into the NSSA. LSA 7 is translated into LSA 5 as it leaves the NSSA. These routes appear as N1 or N2 in the routing table inside the NSSA. Much like LSA 5, N2 is a static cost while N1 is a cumulative cost that includes the cost upto the ASBR.

OSPF_LSAs_Types_7.jpg

Question 9

Comments
  1. RCKar@
    September 7th, 2017

    Thank you very much

  2. keesie
    October 20th, 2017

    answer to Q7 is wrong:
    Default ref-BW is 100Mbps, so changing it to ref-BW 100Mbps will change nothing!
    Answer A

  3. Vojta
    October 21st, 2017

    @keesie

    You are right, at least in IOS version Version 12.4(13b) the value in the “auto-cost reference-bandwidth value” is in Mbps not kbps.

  4. Night King
    November 8th, 2017

    “Question 7
    Note: The default reference bandwidth for OSPF is 10^8 bps or 100Mpbs so the “auto-cost reference-bandwidth 100” is in fact the default value so answer A may be a correct answer.”

    Yeah, 100 is already the default, so I would say ‘A’ is definitely the correct answer. The explanation says this and multiple people calling it out… please update the PDF if this is the agreed answer.

  5. salman
    November 14th, 2017

    Hello Everyone,
    Yes for Question 7 your theory is correct but think if there was some other reference BW before and now he changed to “auto-cost reference-bandwidth 100”. Then obviously the statement “The data path changes for all links” i.e C will be the answer.

  6. tell
    January 2nd, 2018

    Q6
    IS-IS is a “Link-State” routing protokoll
    OSPF is also an “Link-State” routing protokoll
    EIGRP is an hybrid but CISCO say this is an “distance vector” protocol

    https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/13669-1.html#igrp

    i think OSPF is the right anwser.

  7. Anonymous
    January 2nd, 2018

    on question 7, notice the question never mentions ospf, C is the correct answer. We only are assuming they are talking about OSPF because the question is in the OSPF section of the PDF. But it does NOT actually say OSPF anywhere.

  8. Anonymous
    January 2nd, 2018

    er nvm, just dawned on me that’s an OSPF only command. hmm so A could be right

  9. Tasdevil
    January 6th, 2018

    Question 7

    The answer is all paths changes for all links. Since you only are coding in the value (although default) on ONE router in the network. Whats to say the other routers on the network are not configured on defaults (including this one). By changing one router only you will get strange routing updates if the configurations do not match the other routers on the network.
    https://supportforums.cisco.com/t5/network-infrastructure-documents/how-to-configure-ospf-cost/ta-p/3133153

  10. Anonymous
    January 12th, 2018

    what is the answer, C or A? in explanation, the said answer is A.

    Question 7

    If you configure one router in your network with the auto-cost reference-bandwidth 100 command, which effect on the data path is true?
    A. The data path remains the same for all links
    B. The data path changes for 10Mbps links only
    C. The data path changes for all links
    D. The data path changes for 10Gbps links only

    Answer: C

    Explanation

    This command affects all the OSPF costs on the local router as all links are recalculated with formula: cost = reference-bandwidth (in Mbps) / interface bandwidth

    Note: The default reference bandwidth for OSPF is 10^8 bps or 100Mpbs so the “auto-cost reference-bandwidth 100” is in fact the default value so answer A may be a correct answer.

    Thanks.

  11. Anonymous
    January 15th, 2018

    Q3 area id , if u type subnet mask instead of wildcaed ios bu default convert it to wildcard

  12. Anonymous
    January 15th, 2018

    one approach for migrating from one routing protocol to another is to make sure new routing protocol has higher AD, ….
    Q6 internal bgp and rip

  13. Marcus
    March 16th, 2018

    About Q6. The migration to RIP is meaningless. I think we have just an incomplete question.

    Could anyone explain Q2? I don’t understand two issue there.
    1. The routers in the area and their priority doesn’t matters for Router ID. Why it mentioned in question?
    2. With case in the absence of a loopback interface the OSPF process uses the highest ip address of nonloopback interface. Does it matter physical or logical interface have a highest ip?

  14. Marcus
    April 4th, 2018

    Q2 is an incorrect question, I guess. I tested it in UNL – it doesn’t matter there is physical or logical interface, because a router just select highest IP from any available interface (I didn’t configure loopback in this lab).

  15. Marcus
    April 10th, 2018

    I have tested Q7 in UNL:
    R1(config)#router ospf 1
    R1(config-router)#au
    R1(config-router)#auto-cost reference-bandwidth 100
    R1(config-router)#auto-cost reference-bandwidth 1000
    % OSPF: Reference bandwidth is changed.
    Please ensure reference bandwidth is consistent across all routers.

    As you can see, nothing happens after implementing first command. I think correct answer is A: “The data path remains the same for all links.”

    P.S. By default, Reference-Bandwidth is 100 (100, 000 in kbps) and SVI BW is 1000000 Kbit, so we need auto-cost reference-bandwidth 10000 to see meaningful changes. In that case the cost of 1Gb links will be 10 (10,000,000 / 1,000,000 = 10).

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