What will the receiving router do when it receives a packet that is too large to forward, and the DF bit is not set in the IP header?
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A. B. C. D.C.
When a router receives a packet that is too large to forward, it has two options: drop the packet or fragment it into smaller segments. The router determines the Maximum Transmission Unit (MTU) of the outbound interface and compares it with the packet size. If the packet size is larger than the MTU, the router must take action.
In IPv4, the Don't Fragment (DF) bit is a flag in the IP header that indicates whether or not the packet can be fragmented. If the DF bit is set to 1, it means the packet cannot be fragmented, and if it is too large to forward, the router must drop it and send an ICMP Packet Too Big message back to the source. If the DF bit is set to 0, it means the packet can be fragmented.
So, in this scenario, we have a packet that is too large to forward, and the DF bit is not set to 1. The router can fragment the packet into smaller segments to fit the MTU of the outbound interface.
Now, let's go through each answer choice:
A. Drop the packet, and send the source an ICMP packet, indicating that the packet was too big to transmit. This option would be applicable if the DF bit was set to 1. However, since the DF bit is not set to 1, the router can fragment the packet.
B. Fragment the packet into segments, with all segments having the MF bit set. This option means that all the fragments will have the More Fragments (MF) bit set to 1 except for the last fragment, which will have it set to 0. This is a correct option when the DF bit is set to 0.
C. Fragment the packet into segments, with all except the last segment having the MF bit set. This option means that all the fragments will have the MF bit set to 1 except for the last fragment, which will have it set to 0. This is the same as option B and is also a correct option.
D. Fragment the packet into segments, with all except the first segment having the MF bit set. This option means that the first fragment will have the MF bit set to 0, indicating that it is the last fragment, while all the other fragments will have the MF bit set to 1. This is not a correct option because the first fragment should have the MF bit set to 1 since it is not the last fragment.
Therefore, the correct answers are B and C, which both involve fragmenting the packet into segments with all segments except the last one having the MF bit set to 1.