Question

A router has two full-duplex Ethernet interfaces each operating at 100 Mb/s. Ethernet frames are at least 84 bytes long (including the Preamble and the Inter-Packet-Gap).
The maximum packet processing time at the router for wire speed forwarding to be possible is (in micro-seconds)

• A. 3.36
• B. 0.01
• C. 6.72
• D. 8

Answer 1

The correct option is A 3.36

It is given that 2 full duplex ethernet interfaces, each operating at 100 Mbps. So, we are needed that packets are comes at speeds of 200 Mbps. Since, transmission of 84 byte long packet takes 84 B/100 Mbps = 6.72 $\mu$sec on single interface.
Now if we get 2 packets of processing times 6.72 $\mu$sec, we cannot manage both packets in given time, which results overflow of incoming queue.

So, 6.72/2=3.36$\mu$sec processing time possible to forward packet from router interface and till 6.72 $\mu$sec we send 2 packets one at 3.36 $\mu$sec and second at 6.72 $\mu$sec.