Hi All

Sadly the quiz engine isn’t working as it should. However the questions below are still the same and should still help you if you want to dive deep into OSPF.


Question 1
RouterA's ethernet interface is configured using the network type BCAST and the interface is up/up.  There are no other routers on that network segment.  RouterA has the DR role for that segment.  What LSA(s) will the router send to describe this ethernet segment?
Type-1 Router LSA with Transit Link - As all networks are described using Type-1 Tranit link if they are configured as BCAST.
Type-2 Network LSA. Because the advertising router is the designated router (DR) for that network segment.
Type-1 Router LSA with Stub-Link The router won't send a Network LSA unless the interface is a DR and the router forms an adjacency on that interface.
Type-1 Router LSA with Transit Link AND a Type-2 Network LSA. Because the advertising router is the designated router (DR) for that network segment and the interface is BCAST.
Question 1 Explanation: 
To generate a Type-2 LSA, an interface It must be of type “broadcast”, AND have an established adjacency. Even though a “broadcast” interface may show a role of “DR” (as it would if it had found no other OSPF router on the link), it would not advertise a type-2 unless there is a full adjacency formed!! Read more here: http://thenetworksherpa.com/ospf-broadcast-interfaces-and-its-lsas/
Question 2
What is the first LSA sequence number OSPF chooses on startup?
Question 2 Explanation: 
OSPF uses signed 32-bit sequence number space. The sequence numbering starts with negative numbers - set sets the most significant bit to 1 to indicate negative, thus the 0x8. 0x80000000 is skipped, so the first valid sequence number is 0x80000001 Read more here: http://thenetworksherpa.com/ospf-sequence-numbers/
Question 3
Is OSPF a link-state or distance vector protocol?
Always link state
Always distance vector
Both. It link-state within each area, and distance vector between area's
Question 3 Explanation: 
When the ABR generates type-3's it is dropping all of that link and node foo and essentially says "If you need get to this prefix, come to me". This is classic distance vector behaviour. Note that the topology of Area 0 itself is still described and maintained using the link state-approach. Jeff Doyle explains this very well in his book Routing TCP/IP Volume 1, see the OSPF section.
Question 4
When does the ABR originate a type-4 summary on a Cisco router?
Never, only the ASBR generates a type-4 LSA.
The ABR generates a type-4 only if it receives a type-4 from the ASBR?
The moment you add 'redistribute' command to ASBR.
The first time the ABR sees the first Type-5 prefix arriving from the ASBR.
Question 4 Explanation: 
Adding the redistribution command tells the router it is an ASBR. When the ABRs sees an updated Router-LSA with the ASBR flag, it generates a type-4 on it's behalf. This happens if if you have no valid routes to re-distribute. More details here: http://thenetworksherpa.com/ospf-what-really-triggers-type-4-lsa-origination/
Question 5
What is the numerical value of LSInfinity?
0 (all zeros)
Both 65535 and 16777342
Question 5 Explanation: 
Okay, so this could get contentious. But based on the overloaded definitions, I believe LSInfinity refers to both values. Read my justification here: http://thenetworksherpa.com/ospf-lsinfinity-not-equal-lsinfinity/
Question 6
An area-internal router has two OSPF enabled (passive) interfaces configured with subnets and These subnets have been installed in the ABRs routing table (no tricks). How many Type-3 Summary LSAs will a cisco router generate by default to represent these links?
Two Prefixes - One for each stub-network link in the router LSA
One Prefix - A single aggregate of both prefixes, as they are contiguous.
Three prefixes - One for each stub-network link in the router LSA AND the aggregate.
Question 6 Explanation: 
The type-3 LSA name of 'summary' is confusing. When OSPF generates a type-3 LSA it is "summarizing" by removing all of the area topology information and just sending prefix information to Area 0 and beyond. Sometimes we talk about grouping two contiguous subnets together as a summary but I like to call this "route aggregation". By default OSPF will generate a separate type-3 LSA describing every reachable prefix it can glean from the type1 and type-2 LSAs (and install in it's routing table!). For Cisco routers you can aggregate contiguous subnets to a single super net using the 'area x range y' command, but this isn't automatic.
Question 7
How many links will a Router LSA use to describe an IP-addressed local interface with an established point-to-point adjacency?
Question 7 Explanation: 
A Router will describe this interface in it's router LSA with a stub-link to describe the prefix and a point-to-point link to describe the topology. More details here: http://thenetworksherpa.com/ospf-making-sense-of-router-lsa-point-to-point-links/
Question 8
Q: If a router is stuck in EXSTART, which of the following issues is the most likely root cause?
area type mismatch
mtu mismatch
authentication type mismatch
authentication key mismatch
hello timer mismatch
Question 8 Explanation: 
Any of the others answer would stop you getting to 2-WAY state, but they must be good if you're getting to EXSTART.
There are 8 questions to complete.

23 thoughts on “OSPF Quiz

  1. Pingback: A quiz on OSPF - The Network Sherpa

  2. Great quiz! I got three wrong (same as Daniel and #7 as well. Was confused how I got 85%, but it seems you award partial credit for getting the right answer on your second try.

    Well done!

  3. Really nice interface, smooth, runs well. You going to do more?
    I got tripped up on the 65535 vs 16777342 one too, went with 16777342

    • Hey Jeff,
      I probably will do another one, not sure when though. It’s hard to generate good questions. Actually it’s making sure I have the answers right is the hard part. No promises, but what would be a valuable space to cover, more OSPF or other topics?

  4. Q1 : There are no other routers on that network segment. RouterA has the DR role for that segment.

    The question is misleading. How come the RouterA has the DR role without any neighbor ? The DR role is negotiated during the adj process.

    Q4 : When does the ABR originate a type-4 summary on a Cisco router?

    I think that we can go deeper. Based on what is the ABR generating the LSA4 ?


    • Hi Dan,
      Thanks for the comment. I’m not sure that I’m misleading people about the DR role in Q1. You are correct that the DR is negotiated during the ADJ process, but what if there are no other speakers on that LAN segment when an OSPF interface starts-up.

      A router with an interface type of BCAST will correctly nominate itself as the DR if it hears from no other OSPF speakers before the expiry of the WAIT timer. Another speaker can then start up on the wire, learn of the existing DR (and move on the the subsequent steps of the adj formation process).

      I take your point on Q4. I guess I’m interested in the timing of the type-4 generation. I like your angle too.

      /John H

    • Hi Srinivasa,
      Thanks for the feedback. It was indeed theory based. My general philosophy is that you can handle most scenarios if you know the theory. To start, I wanted to create a quiz where the answers were provided in my previous theory-based posts.

      That said, I’m always happy to get feedback. Do you have any particular scenarios in mind? Which practical aspects of OSPF do you think are particularly tricky or worth covering? I can’t guarantee I’ll have the time to address your concerns, but always love suggestions from readers.

  5. you hit the nail with q: 4,5 and 7 – makes me think that I need to spend some more time on your website! Never knew about this site, I wonder how did I miss 🙂

    cheers John, hopefully your new role gives you some time to get back to this website? 🙂

    If you are looking for suggestions on future topics, I would love to see a similar series on the nitty-gritty of BGP and also some firewall/network security concepts! combined with use cases of the type of issues you have seen with designs of ospf/bgp and network security.

    take care,

    • Hi Muddassir,
      Thanks for taking the time to comment. The OSPF topics came from a period where I was testing the protocol for Amazon.

      I’m doing a fair bit of work on VPNs at the moment, so I might have some posts coming on those topics.

      John Harrington

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