Consider N switches supporting ‘k’ VLAN groups that are connected via a trunking protocol.

• Each switch has two trunking ports. One is input port and another one is output port.

• The first switch does not use input port. It uses only output port. Therefore, the number of ports required by the first switch is 1.

• The last switch does not use output port. It uses only input port. Therefore, the number of ports used by last switch is 1.

• The remaining N-2 switches other than the last switches use 2 ports each. Therefore, the number of ports all these switches uses is** 2 x (N - 2)** ports.

• The total number of ports that are used by all these N switches is

**2 + 2 x (N - 2) = (2 x N) - 2 **ports.

Therefore, **(2 x N) - 2** ports are required to connect the switches.

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