GATE CSE 2020 | Question: 15

Question

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?

• A. I and II only
• B. I only
• C. II and III only
• D. II only

Comments

A router can operate without implementing a routing protocol if it uses static routes. While dynamic routing protocols (e.g., OSPF, BGP) are common, they are not strictly required for a router to function.

Answer 1

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.

Comments

👍

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more explanation of Option III :

Reassembly done at destination not on router because , the fragmented packet may follow different paths and different routers.
There is a condition in ICMP redirection where the first packet go through one path where the next packet follow the shortest path. 

Answer 2

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)

Answer 3

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

 

Comments

Nice

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Nice explanation !!

Answer 4

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.

Answer 5

For knowledge purpose:

Ways by which router can modify IP packets:

• Modifying the TTL after every hop (Since RIP, max TTL is 16)
• Fragmenting the IP packets
• During fragmentation, each header checksum is recomputed by the router.

Answer 6

Statement I:

"A router does not modify the IP packets during forwarding."

❌ False

A router does modify certain fields in the IP header while forwarding a packet.

The router updates:

• TTL (Time To Live): Decreases by 1 at every router.
• Header Checksum: Recomputed because TTL has changed.
• If fragmentation occurs, it also modifies:
  • Fragment Offset
  • More Fragments (MF) flag
  • Total Length

Example

Suppose an IP packet has:

TTL = 64

After passing through one router:

TTL = 63

Since the header changed, the checksum must also be updated.

Hence, the router does modify the IP packet.

✅ Statement I is FALSE.

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Statement II:

"It is not necessary for a router to implement any routing protocol."

✅ True

A router can forward packets without running a routing protocol.

There are two possibilities:

1. Static Routing

The routing table is manually configured.

Example:

Destination        Next Hop
10.0.0.0/24        Router A
20.0.0.0/24        Router B

No routing protocol is running.

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2. Dynamic Routing

The router learns routes using protocols like:

• RIP
• OSPF
• BGP
• EIGRP

These protocols are optional, not mandatory.

Therefore,

✅ Statement II is TRUE.

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Statement III:

"A router should reassemble IP fragments if the MTU of the outgoing link is larger than the size of the incoming IP packet."

❌ False

This is a very common GATE concept.

Who reassembles IP fragments?

Only the destination host.