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Linkstate Protocols

Question of CCNA2 Chapter 10 (introducing to linkstate protocols)

Can someone Explain to me why there are no 4 correct answers?

To achieve network convergence, what three steps does each link state router take? (Choose three.)

use automatic summarization to reduce the size of routing tables

build a Link State Packet (LSP) containing the state of each directly connected link

flood the LSP to all neighbors, who then store all LSPs received in a database

discover neighbors and establish adjacencies using the hello packet sent at regular intervals

construct a complete map of the topology and compute the best path to each destination network

use the DUAL FSM to select efficient, loop-free paths, and insert routes into the routing table

chapter 11 says:

Before an OSPF router can flood its link-states to other routers, it must first determine if there are any other OSPF neighbors on any of its links. In the figure, the OSPF routers are sending Hello packets on all OSPF-enabled interfaces to determine if there are any neighbors on those links.

So for convergence HELLO-packets are necessairy?

Greetz

12 REPLIES

Re: Linkstate Protocols

Hellos are sometimes necessary for convergence particularly when the loss of hellos lead to SPF recalculation

Say for eg you have a topology R1 -- SW -- R2

If the link between R2 and the SW fails, R1 will not directly come to know about the status of the link.

It senses that it is not receiving hellos from the neighbor and after missing 4 hellos, it declares the neighbor dead causing convergence.

HTH

Narayan

Re: Linkstate Protocols

hi there Simon

I think the correct answers to that question where numbers 2,3 and 5 but if you have a closer look to number 4 you'll see that's also a correct anwser. So i think it's a little mistake of cisco.

regards

Rein

New Member

Re: Linkstate Protocols

Hi RAIN,

Are you sure about it? Cause, if you say it's a fault of cisco you can risk your head!

Becarefull for that!

(friendly) regards

Cisco man

New Member

Re: Linkstate Protocols

hi Simon,

Once the link is down the router will

1. build a Link State Packet (LSP) containing the state of each directly connected link

2. Flood the LSP to all neighbors, who then store all LSPs received in a database

3.construct a complete map of the topology and compute the best path to each destination network

hello Packets are only used to check if the neighbor is UP/DOWN.

It wont help if any of the link goes down.

So, Only 3 answers for this question.

HTH.

Cheers,

Nikhil E.

Re: Linkstate Protocols

hi

I will like to add one more point here.There is a catch in statement no. 4 .

It says that hellos are used to discover neighbours and establish ADJACENCIES which is not correct.

Even though hellos are discovered and 2 way communication has been established the neighbours can not become adjacent as there are other parrameters as well such as network type etc.

so 4TH point is not correct and only 3 statements are true

HTH

regards,

shri :)

Silver

Re: Linkstate Protocols

Good point by Shri.

Instead of "discover neighbors and establish adjacencies using the hello packet sent at regular intervals"

It should say "discover neighbors and detect indirect link failures using the hello packet sent at regular intervals"

New Member

Re: Linkstate Protocols

the question is:

To achieve network convergence, what three steps does each link state router take? (Choose three.)

So if a network converge, each router has to know who the neighbors are? So hello packets are correct also?

Then I have 4 correct answers?

Bronze

Re: Linkstate Protocols

In terms of exam point of view ,cisco looks for accurate answers.Now the question is regarding the convergence.

Let me tell u network convergance means all the routers in the network agreeing to a same topology.Take an network for example,consider it has found its neighbor through hello and then got all link states,ran spf and build the routing table.Now every router had did the same and network is stable..

Now in link state protocol whenever a change occurs its not by the hello that the router in network come to know that a state of a link is down.In link state an immediate state change lsa is send to all,and then again spf gets calculated and finally converges.

So if u look here there is not much role played by hello for convergence.also as shrikar explain ur adjacency is not completed with hello parameter only,it depends on network type also.So i think more accurate answer to the question is 2,3 and 5.

Re: Linkstate Protocols

So is it correct that we can say that hello packets are only necessary for convergence during startup?

tanx

New Member

Re: Linkstate Protocols

I think the same, anyone aggree with that?

Re: Linkstate Protocols

HELLO exchange is required for keeping in touch with neighbors, while LSA exchange is required for convergence. Pls dont mix it up.

Another thing, all neighbors are not adjancies but all adjancies are neighbors.

to summarise: option 2, 3 & 5 are rite.

Re: Linkstate Protocols

Hi,

It seems there is lot of confusion around the hello packets!!!!

Hello packets do not directly help you to converge the network they are only helpfull in discovering neighbours.The hello packet do not contain any LSA.The LSAs are the thing which help you to converge the network.As arun said the LSA are exchanged only when adjacency is formed.LSA are not exchanged with all neighbours(as all neighbours need not to be adjacent).

Take an example R1 has 2 connections to R2 & R3.Now if the link between R1-R2 goes down,Hello packets between R1 & R3 will not help R3 to kno that R2 is down its LSAs exchanged between R1 and R3 which helps R3 to know that link between R1-R2 is down.

HTH

regards,

shri :)

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