Gate IT 2006 Full Duplex ethernet

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 wirespeed forwarding to be possible is (in micro­seconds)

  1. 0.01
  2. 3.36
  3. 6.72
  4. 8
Sumit Verma @sumitkgp
30 Jan 2017 11:40 pm

In order to process a packet in one interface maximum packet processing time will be T= 84*8/100 = 6.72 microsecond. 
But here we have two interfaces, so can use both of them. So maximum processing time = 6.72/2 = 3.36 microsecond. 

Shashank @ssinghmy
5 Apr 2017 11:44 am

A better explanation would be possible if we could first tell what wirespeed forwarding means. It means that the router should be able to forward data at the same speed as the connection bandwidth. So, if data is being sent at 100 Mbps from host 1, it should be received at 100 Mbps at host 2. Hence the time delay permitted at the router is only that much time in which the data (84 bytes) is loaded onto the outgoing link/interface, which is (84*8 b) / (100*10^6 bps) = (672*10^-8) = (6.72 micro-seconds)