In the diagram shown below, L1 is an Ethernet LAN and L2 is a Token-Ring LAN.…
2014
In the diagram shown below, L1 is an Ethernet LAN and L2 is a Token-Ring LAN. An IP packet originates from sender S and traverses to R, as shown. The links within each ISP and across the two ISPs, are all point-to-point optical links. The initial value of the TTL field is 32. The maximum possible value of the TTL field when R receives the datagram is _______.
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Correct answer: 26
Key idea:
Each time a packet visits network layer it decrements it's TTL field. Source initializes it and others decreaments it. Inside LAN it never goes to network layer, it is forwarded from data link layer itself.. in routers it goes upto network layer to make a routing decision.. and the router decrements it because the packet has visited the network layer.. and at the receiver too, the packet has visited the network layer and network layer will do it's job and decrements the TTL value.
Router at the boundary of LAN L1 and the ISP1 network (first IP hop).
First internal router inside ISP1 (second IP hop).
Router at the end boundary of ISP1 (third IP hop).
Router at the start boundary of ISP2 (fourth IP hop).
Router at the boundary of ISP2 and LAN2 (Fifth IP hop)
The receiver will also decrement the value of TTL when the packet goes to network layer.
There are 5 routers, So Network Layer will be visited 5 times and 1 time on the destination
So, TTL =26
