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Consider the following statements about the functionality of an $\textsf{IP}$ based router.

  1. A router does not modify the $\textsf{IP}$ packets during forwarding.
  2. It is not necessary for a router to implement any routing protocol.
  3. A router should reassemble $\textsf{IP}$ fragments if the $\textsf{MTU}$ of the outgoing link is larger than the size of the incoming $\text{IP}$ packet.

  Which of the above statements is/are TRUE?

  1. I and II only
  2. I only
  3. II and III only
  4. II only
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Best answer

Answer: D. II Only

Explanation

Taking the given statements one by one:

I. (FALSE) Router needs to fragment the incoming IP packets if the connecting line has smaller MTU (Maximum Transmission Unit) than size of incoming packets.

II. (TRUE) That's the case of static routing where forward paths are pre-loaded/downloaded to routers.

III. (FALSE) This is not possible in case of cut-through-switching where IP packet is forwarded as it arrives.

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25 votes
25 votes
only (II) is correct. A router need not implement any routing protocol. It can just forward packets in all the directions without doing any routing.

Router won't assemble the packets. Assembling is done at destination system

Router does modify the IP packets during forwarding. Simple example.. TTL ( time to live) field is decremented at routers)
4 votes
4 votes
1. Router modifies ip packet (TTL)during forwarding.

2.yes,flooding can be used.

3.reassembling of packets is done at destination as they can follow differennt path.

so b is true.
1 vote
1 vote

Answer : Option D) II only.

Option I : IP packets need to be fragmented at routers if the outgoing link has a smaller MTU. Even TTL has to be decreased by devices having Network Layer (eg. Router). As it is mentioned whole IP Packet  and not only IP data, so router definitely modifies packets when forwarding. So FALSE.

Option II : Yes, it is not necessary for a router to implement any routing algorithm. We can definitely use flooding and forward packets in all directions. So, TRUE

Option III: Reassembly procedure is done only at the destination and not at the routers. This is because, packets may follow different routes and all the packets may not arrive at the specific router. This might lead to perform fragmentation again and again Hence, a router should not reassemble IP fragments if the MTU of the outgoing link is larger than the size of the incoming IP packet. So, FALSE. 

Resources: RFC: 815 | IP DATAGRAM REASSEMBLY ALGORITHMS

 

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