Question about OSPF

Alceo

Can you please explain why answer B is incorrect in the image below?


This is not from a ****.

Thanks

OfWolfAndMan

There is not enough information in this diagram to tell. Obviously the router ID is highest of all, however what is another part of the DR/BDR election? Highest ospf priority, which trumps highest RID in the DR/BDR election. While the default is 1, they did not show you the configuration for each router, therefore there's not enough information to determine if R3 is the DR or not.

EdTheLad

As per the description, ospf has a bare bones config therefore the correct answers are B and E.

A. While this statement would be true if there were 3 areas, there is nothing in the topology or wording to indicate the elaborate config.
B. R3 has the highest ip address so it will win the DR election. We have to assume minimal config as per the wording of the question.
C. This could be true as the interfaces will be broadcast by default, but due to us knowing that the ptp link should really be configured under ospf as ptp it's lower on the list of probable answers.
D. Just wrong, timers need to match to form an adjacency.
E. OSPF will use the BW of an interface to calc ospf cost, FE will have a cost of 1, therefore R5 and R6 will have an additional cost of 1 than R4 to reach subnet 11.
F. LSAs are not send every 30 seconds.

So in summary, A,B,C,E could all be right, but the two most logical answers from what we have been given are B and E.

Hondabuff

Alceo wrote: »

Can you please explain why answer B is incorrect in the image below?


This is not from a ****.

Thanks

That's got to be a Boson or Transcender exam. They are always assume this and assume that.

tecnodog7

Since we are on the OSPF topic, I was wondering why is it important to assign a router-id.
What is wrong with them choosing its own(the highest physical interface)

gorebrush

Because how do you know in the future if you add a new interface with a higher-IP and the device is booted?

Can lead to unpredictable results, which is why you should *always* set them.

Good fun when you start thinking about NSSA T7-T5 translator elections and what not...

tecnodog7

Hmmm so we as admin rather control everything so if tomorrow something breaks we aren't playing guessing games?
How are network diagrams documented in the company? Good documentations?

Hondabuff

tecnodog7 wrote: »

Since we are on the OSPF topic, I was wondering why is it important to assign a router-id.
What is wrong with them choosing its own(the highest physical interface)

So your DR is sending out the LSA's about the network and just not from a router in the middle of your topology. Picture having your Core router running OSPF and you have 1000 branch offices. You left the routers to decide who is the DR and BDR. Your corp office is in Texas and your new DR is in Japan because of the higher interface won the election. A VPN goes down connecting Corp office to Hawaii. You have to wait for your DR router in Japan to notify all the other routers back through your VPN to the Corp office then out to all the branch offices. You can see where this could blow up if you now saturate the VPN link to Japan and the tunnel drops. Now your BDR kicks in that's in Australia and floods its VPN with the new LSA and LSD. Now your network is buzzing like a nest or hornets! Houston we have a problem!!

tecnodog7

Hmmmm makes sense

theodoxa

EdTheLad wrote: »

B. R3 has the highest ip address so it will win the DR election. We have to assume minimal config as per the wording of the question.

The question doesn't specify "default" configuration, only that the RIDs have not been manually configured and that there are no Loopback Addresses. Based on the wording, the priority could have been changed.

[EDIT] D and F are outright wrong. B and C could be correct, but only if you assume default configuration. A is definitely correct. Interestingly, E is correct, but again only if you assume "default" configuration (They could have manually set the cost or have changed the auto-cost reference-bandwidth). I assume the reason they chose E is because tuning OSPF metrics is a CCNP topic (unless that has changed with the new CCNA) and would beyond the scope of CCNA.