Home > OSPF Questions

OSPF Questions

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

Quick OSPF Overview:

OSPF forms neighbor relationship with other OSPF routers on the same segment by exchanging hello packets. The hello packets contain various parameters. Some of them should match between neighboring routers. These include:

+ Hello and Dead intervals
+ Area ID
+ Authentication type and password
+ Stub Area flag
+ Subnet ID and Subnet mask

When OSPF is run on a network, two important events happen before routing information is exchanged:
+ Neighbors are discovered using multicast hello packets.
+ DR and BDR are elected for every multi-access network to optimize the adjacency building process. All the routers in that segment should be able to communicate directly with the DR and BDR for proper adjacency (in the case of a point-to-point network, DR and BDR are not necessary since there are only two routers in the segment, and hence the election does not take place).
For a successful neighbor discovery on a segment, the network must allow broadcasts or multicast packets to be sent.

In an NBMA network topology, which is inherently nonbroadcast, neighbors are not discovered automatically. OSPF tries to elect a DR and a BDR due to the multi-access nature of the network, but the election fails since neighbors are not discovered. Neighbors must be configured manually to overcome these problems

Each OSPF area only allows some specific LSAs to pass through. Below is a summarization of which LSAs are allowed in each OSPF area:

Area Restriction
Normal None
Stub No Type 5 AS-external LSA allowed
Totally Stub No Type 3, 4 or 5 LSAs allowed except the default summary route
NSSA No Type 5 AS-external LSAs allowed, but Type 7 LSAs that convert to Type 5 at the NSSA ABR can traverse
NSSA Totally Stub No Type 3, 4 or 5 LSAs except the default summary route, but Type 7 LSAs that convert to Type 5 at the NSSA ABR are allowed

Question 1

Explanation

LSA Type 7 is 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 -> LSA Type 7 only exists in an NSSA area.

Question 2

Question 3

Explanation

Answer B is not correct because using “passive-interface” command on ASW1 & ASW2 does not prevent DSW1 & DSW2 from sending routing updates to two access layer switches.

Question 4

Explanation

From the output above, we see the following LSAs:

+ Router Link States (Area 0): LSA Type 1 (Area 0)
+ Net Link States (Area 0): LSA Type 2 (Area 0)
+ Summary Net Link States (Area 0): LSA Type 3 (Area 0)
+ Router link States (Area 4): LSA Type 1 (Area 4)
+ Net Link States (Area 4): LSA Type 2 (Area 4)
+ Summary Net Link States (Area 4): LSA Type 3 (Area 4)

There are two areas represented on this router, which are Area 0 & Area 4. So we conclude this is an ABR router.

Just for your information, from the Router Link States (Area 0) part, we only see one entry 15.15.15.33. It is both the Link ID and ADV Router so we can conclude this is an IP address of one of the interfaces on the local router.

Question 5

Question 6

Questions 7

Explanation

When OSPF is run on a network, two important events happen before routing information is exchanged:
+ Neighbors are discovered using multicast hello packets.
+ DR and BDR are elected for every multi-access network to optimize the adjacency building process. All the routers in that segment should be able to communicate directly with the DR and BDR for proper adjacency (in the case of a point-to-point network, DR and BDR are not necessary since there are only two routers in the segment, and hence the election does not take place).
For a successful neighbor discovery on a segment, the network must allow broadcasts or multicast packets to be sent.

In an NBMA network topology, which is inherently nonbroadcast, neighbors are not discovered automatically. OSPF tries to elect a DR and a BDR due to the multi-access nature of the network, but the election fails since neighbors are not discovered. Neighbors must be configured manually to overcome these problems -> C is not correct while D is correct.

In Point-to-Multipoint network: This is a collection of point-to-point links between various devices on a segment. These networks also allow broadcast or multicast packets to be sent over the network. These networks can represent the multi-access segment as multiple point-to-point links that connect all the devices on the segment. -> A is correct.

Question 8

Explanation

OSPF forms neighbor relationship with other OSPF routers on the same segment by exchanging hello packets. The hello packets contain various parameters. Some of them should match between neighboring routers. These include:

+ Hello and Dead intervals
+ Area ID
+ Authentication type and password
+ Stub Area flag
+ Subnet ID and Subnet mask

So there are three correct answers in this question. Maybe in the exam you will see only two correct answers.

Question 9

Explanation

Let’s have a quick review of LSAs Type 4 & 5:

Summary ASBR LSA (Type 4) – Generated by the ABR to describe an ASBR to routers in other areas so that routers in other areas know how to get to external routes through that ASBR. For example, suppose R8 is redistributing external route (EIGRP, RIP…) to R3. This makes R3 an Autonomous System Boundary Router (ASBR). When R2 (which is an ABR) receives this LSA Type 1 update, R2 will create LSA Type 4 and flood into Area 0 to inform them how to reach R3. When R5 receives this LSA it also floods into Area 2.

OSPF_LSAs_Types_4.jpg

In the above example, the only ASBR belongs to area 1 so the two ABRs send LSA Type 4 to area 0 & area 2 (not vice versa). This is an indication of the existence of the ASBR in area 1.

Note:
+ Type 4 LSAs contain the router ID of the ASBR.
+ There are no LSA Type 4 injected into Area 1 because every router inside area 1 knows how to reach R3. R3 only uses LSA Type 1 to inform R2 about R8 and inform R2 that R3 is an ASBR.

External Link LSA (LSA 5) – Generated by ASBR to describe routes redistributed into the area and point the destination for these external routes to the ASBR. These routes appear as O E1 or O E2 in the routing table. In the topology below, R3 generates LSAs Type 5 to describe the external routes redistributed from R8 and floods them to all other routers and tell them “hey, if you want to reach these external routes, send your packets to me!”. But other routers will ask “how can I reach you? You didn’t tell me where you are in your LSA Type 5!”. And that is what LSA Type 4 do – tell other routers in other areas where the ASBR is!

OSPF_LSAs_Types_5.jpg

Each OSPF area only allows some specific LSAs to pass through. Below is a summarization of which LSAs are allowed in each OSPF area:

Area Restriction
Normal None
Stub No Type 5 AS-external LSA allowed
Totally Stub No Type 3, 4 or 5 LSAs allowed except the default summary route
NSSA No Type 5 AS-external LSAs allowed, but Type 7 LSAs that convert to Type 5 at the NSSA ABR can traverse
NSSA Totally Stub No Type 3, 4 or 5 LSAs except the default summary route, but Type 7 LSAs that convert to Type 5 at the NSSA ABR are allowed

Reference: http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13703-8.html

Therefore there are two OSPF areas that prevent LSAs Type 4 & 5: Totally Stub & NSSA Totally Stub areas

Comments
Comment pages
1 2 1390
  1. Begemot
    September 5th, 2019

    Careful with Q7:

    A network engineer enables OSPF on a Frame Relay WAN connection to various remote sites, but no OSPF adjacencies come up Which two actions are possible solutions for this issue? (Choose Two)

    A. Change the network type to point-to-multipoint under WAN interface
    B. Enable virtual links
    C. Change the network type to nonbroadcast multipoint access
    D. Configure the neighbor command under OSPF process for each remote site
    E. Ensure that the OSPF process number matches among all remote sites

    Answer: A D

    Some say C and D is the correct answer, I thought so too, therefore did some research and finally got it. In fact A and D are the correct answers. Here’s why.

    OSPF can run on non-broadcast media such as Frame Relay in 2 modes:
    1. Nonbroadcast Multiaccess (NBMA) mode by _simulating_ a broadcast model. There are 2 ways to simulate broadcast model on an NBMA network:
    a) change network type to broadcast (per interface): ip ospf network broadcast
    b) configure neighbors manually under: router ospf xx
    2. Point-to-multipoint mode by treating non-broadcast network as collection of point-to-point links. You do that by changing network type to Point-to-multipoint: ip ospf network point-to-multipoint.

    So, to recap, we have 3 options in total. First 2 are to simulate broadcast model and 3rd is to simulate a point-to-multipoint network:
    1A) switch network type to broadcast: ip ospf network broadcast
    1B) define neighbors manually under router ospf xx: neighbor xx.xx.xx.xx
    2) switch network type to point-to-multipoint: ip ospf network point-to-multipoint

    Let’s read the question again: “Which two actions are possible solutions?”, which means EITHER/OR.

    We have 3 options as per Cisco, but look at the multiple choice answers – do you see “switch network type to broadcast” in the list? Answer “C” suggests switching to network to Non-broadcast which is quite the opposite, and an invalid solution.

    I know, it’s a dick question from Cisco, but I am glad I’ve figured it out.
    I hope this helps someone.

  2. Wit
    September 10th, 2019

    @ Begemot
    it did help for sure.. thanks for your explanation I needed this clarification

  3. IP HELPER
    October 16th, 2019

    who needs updated dump for ROUTE 300-101 can contact me at: cisco4career @ gmail . com
    Good luck guys!

  4. RRRRRMMMM
    November 5th, 2019

    Q9, I agree is Stub and totally stub, answer is B and C.

    NSSA areas don’t prevent type 5 LSAs. The ASBR creates a type 7 LSA and flood in the NSSA area, the ABR in that area receives this LSA type 7, and reacts creating a TYPE 5 LSA on the backbone area (and eventually other areas). Therefore, if you have a NSSA (or totally NSSA), you are not preventing the creation of type 5 LSAs in your OSPF domain.
    The only LSA you are preventing is the type 4 LSA, as there is no need to tell other areas how to reach the ASBR as the type 5 LSA created by the ABR and flooded actually points to HIS OWN router-id and not the ASBR router -id)

    Stub and Totally Stub areas prevent redistribution, obviously there will be no type 5 nor type 4 in your ospf domain as a consequence of these types of areas specifically. Configuring a specific area as stub (or totally stub) prevents type 5 and type 4 LSAs.

  5. LuciasDA
    November 11th, 2019

    @CertPrep

    can you amend Q9 – definitely isn’t correct.

    Even the Cisco info states this Stubby and Totally Stubby

Comment pages
1 2 1390
  1. No trackbacks yet.