Practice Question answer clarification

RouterB is receiving routing updates from RouterC. However, RouterC is unable to access the
192.168.1.32/28 network on RouterB. Which of the following is the most likely reason that RouterC cannot access this
network? (Select the best answer.)

A. RouterC is not configured to support RIPv2.
B. RouterB is missing a network statement for the 192.168.1.32/28 network.
C. RouterC is ignoring the routing updates from RouterB.
D. The Serial 0/0 interface on RouterB is not in an up/up state.

I don't underStand the explanation:

Correct Answer: C
Explanation:
Of the choices available, the most likely reason that RouterC cannot access the
192.168.1.32/28 network is because RouterC is ignoring the routing updates from RouterB. By
default, Cisco devices that are configured to use Routing Information Protocol (RIP) transmit
RIP version 1 (RIPv1) routing updates and receive both RIPv1 and RIPv2 routing updates. You
can issue the version command from router configuration mode to specify the appropriate
version of RIP. For example, if you issue the version 2 command from RIP router
configuration mode, the router will send and receive only RIPv2 routing information. RIPv2 is
an enhancement to RIPv1 that adds support for variable-length subnet masks (VLSMs),
Classless Inter-Domain Routing (CIDR), authentication, route summarization, and key
management.

How can it ignore routing updates?

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Comments

NetworkVeteran

binaryhat wrote: »

RouterB is receiving routing updates from RouterC. However, RouterC is unable to access the
192.168.1.32/28 network on RouterB. Which of the following is the most likely reason that RouterC cannot access this
network? (Select the best answer.)

A. RouterC is not configured to support RIPv2.
B. RouterB is missing a network statement for the 192.168.1.32/28 network.
C. RouterC is ignoring the routing updates from RouterB.
D. The Serial 0/0 interface on RouterB is not in an up/up state.

A - LESS LIKELY. I would discount this because (i) it's unlikely the router is running RIP and (ii) if it were running RIP, the default is for them to be capable of understanding both version 1 and 2 updates.

B. - MORE LIKELY. Most routing protocols use "network" statements, and if RouterB is missing such a statement then it typically won't advertise that network to routerC and thus routerC can't see/access it.

C. - LESS LIKELY. The text states RouterB is receiving updates from RouterC. This typically means RouterC has a network statement for that interface and that an adjacency is established. Still, sometimes sending routes doesn't require an adjacency and in that case all sorts of parameter mismatches could cause ignoring routes.

D. LESS LIKELY. We have no reason to believe the target network is on interface Serial0/0.

Of the choices available, the most likely reason that RouterC cannot access the
192.168.1.32/28 network is because RouterC is ignoring the routing updates from RouterB. By
default, Cisco devices that are configured to use Routing Information Protocol (RIP) transmit
RIP version 1 (RIPv1) routing updates and receive both RIPv1 and RIPv2 routing updates.

Either the question is really whacked--why should we assume RIP?!--or more likely you've excluded information such as a configuration snippet or topology diagram, which would lend context and suggest one of the above answers.

binaryhat

It is a sim. I caN't post a sim. The answer is C btw.

NetworkVeteran

The correct answer, based on the info you provided above, is B.

A device can ignore updates for all sorts of reasons--not speaking the same protocol, mismatched routing protocols parameters, unicast vs. multicast, corrupt frame/packet, bad PDU size due to mismatched MTU, hold down timers, etc. Basic show and debug commands can often make quick work of narrowing such issues down.

Of course, as I noted before, information you've excluded may impact what is the most likely answer. There's not much more to say without knowing what that additional information might tell us about what's going wrong.

binaryhat

See Q18

Q18.pdf

NetworkVeteran

With the addition of the "show ip protocol" output, answer C becomes the correct answer.

RouterB wrote:

Routing Protocol is "rip"
Sending updates every 30 seconds, next due in 17 seconds
Invalid after 180 seconds, hold down 180, flushed after 240
Outgoing update filter list for all interfaces is
Incoming update filter list for all interfaces is
Redistributing: rip
Default version control: send version 1, receive any version
Interface Send Recv Key-chain
Serial0/1 1 1 2
FastEthernet0 1 1 2

RouterC wrote:

Routing Protocol is "rip"
Sending updates every 30 seconds, next due in 23 seconds
Invalid after 180 seconds, hold down 180, flushed after 240
Outgoing update filter list for all interfaces is
Incoming update filter list for all interfaces is
Redistributing: rip
Default version control: send version 2, receive version 2
Interface Send Recv Key-chain
Serial0/0 2 2
Serial0/1 2 2
FastEthernet0 2 2
FastEthernet0 2 2

Imagine RIPv1 is Spanish and RIPv2 is English. RouterB is speaking Spanish but understands English+Spanish. RouterC is speaking English and only understands English. It follows that RouterB can understand RouterC, but RouterC cannot understand RouterB.

This corresponds to, "not speaking the same protocol" or "mismatched routing protocols parameters".

boobobobobob

You can ignore specific networks in a routing update by using distribute-lists, ie for the example above:

On Router B:
router rip
distribute-list 1 in

access-list 1 deny 192.168.1.31 0.0.0.240

Here's my take on the above options:

A. Obviously wrong. the version of RIP running doesn't matter here.
B. The question states router B is receiving updates which means router B and Router C have a neighbor relationship. If the network command was not applied router B would not be exchanging routing updates. Also there are other ways to create neighbor relationships in RIP besides the network command for example the "neighbor ip-address" command.
C. I think this answer is actually correct
D. Just wrong.

Please correct me if i'm wrong with anything!

NetworkVeteran

boobobobobob wrote: »

You can ignore specific networks in a routing update by using distribute-lists

That is certainly another way to ignore routing updates.

MickQ

boobobobobob wrote: »

You can ignore specific networks in a routing update by using distribute-lists

That might be a little too much for the CCENT

Xyro

Hmmm, I thought RIPv1 was supposed to receive RIPv2 updates though. Is my material incorrect?

NetworkVeteran

Xyro wrote: »

Hmmm, I thought RIPv1 was supposed to receive RIPv2 updates though. Is my material incorrect?

RIPv1 is a protocol. It's almost like a language. It doesn't make much sense to say, "English is supposed to receive Spanish", or similarly "RIPv1 is supposed to receive RIPv2." Cisco IOS devices only learn routes from RIPv2 updates if they're configured to do so, and the fruits of that configuration can be seen in the "show ip protocol" output, per earlier messages in this thread.?

I will add RIPv2 updates are typically multicast while RIPv1 updates are typically broadcast. This alone means an old device that speaks only RIPv1 would be unlikely to hear RIPv2 updates by default.

theodoxa

RIPv1 and RIPv2 send and receive to different addresses. RIPv1 sends to the broadcast address, whereas RIPv2 uses Multicast (224.0.0.9 if I remember correctly).