Review Problems Lecture 3: Switching

Solutions to Review Problems (Lecture 3: Switching)

Explain the difference between datagram and virtual-circuit networks.

1.     Suppose there are three routers between a source host and a destination host.

Determine the number of interfaces an IP datagram will travel over when it sent from the source host to the destination host.

Determine the number of forwarding tables will be indexed to move the datagram from the source to the destination.

Solution:

8 interfaces; 3 forwarding tables

2.     Consider the network below.

a.     Suppose that this network is a datagram network. Show the forwarding table in router A, such that all traffic destined to host H3 is forwarded through interface 3.

Solution:

Data destined to host H3 is forwarded through interface 3

                                                             

Destination Address	Link Interface
H3	3

b.     Suppose that this network is a datagram network. Can you write down a forwarding table in router A, such that all traffic from H1 destined to host H3 is forwarded through interface 3, while all traffic from H2 destined to host H3 is forwarded through interface 4? (Hint: this is a trick question.)

Solution:

No, because forwarding rule is only based on destination address.

c.      Now suppose that this network is a virtual circuit network as shown in the below figure and that there is one ongoing call between H1 and H3, and another ongoing call between H2 and H3. Write down a forwarding table in router A, such that all traffic from H1 destined to host H3 is forwarded through interface 3, while all traffic from H2 destined to host H3 is forwarded through interface 4.

Forwarding table in Router A:

Incoming interface	Incoming VC#	Outgoing Interface	Outgoing VC#
1	12	3	22
2	63	4	18

Note, those two flows (from H1 and H2) must have different VC#s, true for both incoming and outgoing VC#s.

d.     Assuming the same scenario as (c), write down the forwarding tables in nodes B, C, and D.

Forwarding table in Router B:

Incoming interface	Incoming VC#	Outgoing Interface	Outgoing VC#
1	22	2	24

Forwarding table in Router C:

Incoming interface	Incoming VC#	Outgoing Interface	Outgoing VC#
1	18	2	50

Forwarding table in Router D:

Incoming interface	Incoming VC#	Outgoing Interface	Outgoing VC#
1	24	3	70
2	50	3	76