CCNA practise tests - answers unclear

[LetMePass]

Hello,

I'm writing to ask if you could please assist me in understanding how to arrive at these answers.

1/ Maximum hop count is 15, anything more than that is considered unreachable. Here I suppose

192.168.40.1

is 15 hops away then it takes an extra hop to send packets to

10.0.0.0..

2/ What information should lead the student to answer B? Is that because of the O (OSPF) before

10.10.13.10/25

? Why does

10.10.13.10/25

turned into

10.10.13.10/32

in the question?

3/

In the picture, there is no

10.10.13.208/29

why are there differences between exhibit and CLI info? Something to do here with longest match? May I get a more precise explanation?

4/ Here I suppose both switches belong to the same network

162.168.1.0

5/ It does not appear to be anything related to metric or AD, I assume something related with longest match again. Could you please elaborate about that answer?

Any help will be greatly appreciated. 

Best Regards,

Chris

[Flavio Miranda]

Hi @LetMePass 

 If you be more specific I can try to help you. What exactly you did not understand?

[LetMePass]

Hello Flavio,

I'm merely troubled by the lack of explanations for the exhibits; it appears to be geared toward those with strong foundations. These responses strike me as opaque at best. As a result, in order to gain a better comprehension, I would have needed a full explanation of every answers.

Concerning /4

a minor portion of the explanation is missing, preventing me from getting the full picture. Fortunately, I eventually got both switches belong to 

162.168.1.0

Best Regards,

Chris

[Flavio Miranda]

@LetMePass 

 Do you have any trouble understanding the question 4 ?

[LetMePass]

No, I got it, the problem is there is no demonstration. I would have formulated the answer in a clearer way.

[paul driver · Accepted solution]

Hello
1) The question asks which route will not be placed into  the neighbouring routers rib table, as the routing process being used is RIP, by default that uses a hop count (metric) of maximum of 15 to calculate its routing decision, any higher is deemed unreachable, so that route won’t be accepted in any other routers rib table shown in the diagram as it would mean the hop has incremented by 1 to 16 (unreachable)

2) The ospf route is the preferred route due to the longest match in the route table, and any

/32 address

is classed as a

host

route, so that why the pc

(host)

with that ip address  is referred to as

10.10.13.10/32

3) Even though you see nothing for

1010.13.208/29

it is assumed that the switch is advertising that

prefix

over ospf with a

next hop of its 10.10.10.1

interface

4) Correct the router cannot have two interfaces in the same routing table associated with the same subnet

192.180.1.0/24

5) The answer is based on the static route entry of

rtr1 
ip route 10.10.130.0 255.255.255.128 10.10.10.10 108

and it assigned lowest metric value 108, so the router will choose that route to forward traffic towards host

10.10.13.10/32

[LetMePass]

Hello Paul, 

Thank you for responding so quickly. If it's not too much bother, I'd like to go a little deeper because it's very important to me.

1/ We are on the same page for the first part, but why "that route won’t be accepted in any other routers rib table shown in the diagram as it would mean the hop has incremented by 1..."?

Why would the hop increase by one? I understand that packets will come across 15 hop before delivery which is under 16.

2/ Are you referring to that answer? May you please elaborate "...any

/32 address

is classed as a

host

route, so that why the pc

(host)

with that ip address is referred to as

10.10.13.10/32

What is a host route?

/32

indicates that four bytes are allocated to network portion but I am not sure to follow..

3/ Do you mean a router or many routers could be there but are not portrayed in the picture?

4/ That one is clear for me.

5/ Something crucial metric, so the cost, rather than AD. In this case it makes sense. Is not

ip route

supposed to display AD and not metric? How do you see it's static route? I am confused with router

ospf 1.. Are 1, 5, 6, 9, 10

metric in the picture? 

When I see 

ip route 10.10.130.0 255.255.255.128 10.10.10.10 XXX

I know I deal with metric while something like 

10.11.2.0 XXX/XX via 10.11.1.1, 00:01:37, FastEthernet0/1

will always indicate AD value?

Best Regards,

Chris

[Giuseppe Larosa]

Hello @LetMePass ,

>>

1/ We are on the same page for the first part, but why "that route won’t be accepted in any other routers rib table shown in the diagram as it would mean the hop has incremented by 1..."?

Why would the hop increase by one? I understand that packets will come across 15 hop before delivery which is under 16.

The question asks what

prefix

cannot be propagated to downstream routers using RIP, because

prefix 10.0.0.0/8

has already metric 15 and because the downstream routers should add one to that value the total value would reach 16 that means not usable in

RIP ( both RIPv1 and RIPv2)

The local router can still route to

10.0.0.0/8

but this is not possible from routers downstream that are away from the destination.

As it has been correctly explained by @paul driver .

2)  host

10.10.13.10/32

is reached via

OSPF route 10.10.13.0/25 . The /32

is just a way to indicate a single IP address but the routing table most specific route is the

O 10.10.13.0/25

Here the idea is that longest

prefix

match is used first.

Hope to help

Giuseppe

[Giuseppe Larosa]

Hello @paul driver  and @LetMePass ,

5) The answer is based on the static route entry of

rtr1 
ip route 10.10.130.0 255.255.255.128 10.10.10.10 108

and it assigned lowest AD value 108, so the router will choose that route to forward traffic towards

host 10.10.13.10/32

because it is lower then OSPF AD 110 the other static routes are ignored as they use higher AD values.

Hope to help

Giuseppe

[LetMePass]

Hi Giuseppe,

1/ Alright, thank you.

2/

host 10.10.13.10/32 is reached via OSPF route 10.10.13.0/25

 The

/32

is just a way to indicate a single IP address but the routing table most specific route is the

O 10.10.13.0/25

Here the idea is that longest

prefix

match is used first. 

Is my demonstration accurate? It is important that I understand how to arrive at the answer logically. I find strange to change CIDR notation, but I assume I will have to refer to the table during the exam.

10.10.13.10 = 00001010.00001010.00001101.00001010 (destination IP address) <-

******************************************************************************************

10.10.10.0/30 = 00001010.00001010.0000101.0

10.10.13.0/25 = 00001010.00001010.00001101.0 <-

10.10.10.16/30 = 00001010.00001010.00001010.0001

10.10.13.144/28 = 00001010.00001010.00001101.1001

5/ So it has to be a floating route and not a static route as the AD was reassigned? Static route is supposed to have an AD of 1.

Best Regards,

Chris

[Giuseppe Larosa · Accepted solution]

Hello Chris @LetMePass ,

2)  yes you need to examine the routing table provided  to see if there is a specific route to the destination address.

in your case you just need to compare

O 10.10.13.0/25

O 10.10.13.144/28

host 10.10.13.10

is an address in the first half so the

prefix

used to route is

O 10.10.13.0/25

The question asks what kind of route is used to reach the

host 10.10.13.10/32

and the correct answer is B a network route . There is no host route in the routing table and the

default route

would be used only if no specific route exists for the destination address.

5)  Yes all the static routes in the config provided are floating static routes with different IP

next-hops

and different AD values also OSPF process is configured. So here the key point is to choice the only floating

static route

with

AD 108 < 110 OSPF AD

all the other

static routes

are not used for their higher then

OSPF AD

Hope to help

Giuseppe

[LetMePass]

Hi Giuseppe,

Thank you, it's all clear

Best Regards,

Chris