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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.

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