Hi Herve,
The uplink ports on the switch would be configured as trunk ports and would be carrying the traffic from all vlans. The stack master on a stack would be responsible for managing the stack and relay the information to all the members of the stack.
The following details explain the role of the stack master switch which will help us
The stack master controls the configuration and is the central point for management. All Layer 2 protocol traffic (for example, VLAN Trunking Protocol [VTP], Dynamic Trunking Protocol [DTP] , Cisco Discovery Protocol, Link Layer Discovery Protocol [LLDP]) is forwarded to the master regardless of where the protocol packet ingresses. The master will also transmit all Layer2 protocol packets. If the egress interface is on another member, then the protocol packet is passed from the master along the stack to the destination member.
The same behavior is seen for management traffic. Simple Network Management Protocol (SNMP), HTTP, Telnet, and Secure Shell (SSH) Protocol type of management traffic is all forwarded to the master regardless of the ingress interface. The response from the master is sent through the stack to the destination interface.
On configuration changes, the stack master pushes a copy of the configuration to every member. This way all members have a copy of the saved configuration.
Stack upgrades occur on the master. The master pushes new Cisco IOS Software images to all members. Each member stores a copy of the Cisco IOS Software image on its local Flash.
Hence, if we configure the switch which has uplinks configured on it as the master in the stack, then it would cause an overhead on the master and may affect the performance of the stack or the uplink ports.
Hence it is recommended to make the switch which does not contain the uplink ports as teh master in the stack.
Hope this would be helpful.
