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OSPF Questions 3

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

Question 1


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


The wildcard mask should be instead of the subnet mask

Question 4


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


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


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


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.


  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


    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.

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