Switching and Routing

Correct Answer: Implementing virtual LANs (VLANs)

The best way to segment a network, reducing the size of the broadcast domains, and increasing security is by implementing virtual LANs (VLANs). VLANs allow a network administrator to divide a network into separate isolated networks. This could significantly reduce the size of broadcast domains as each VLAN is a separate broadcast domain, improving network performance and security. Enabling Spanning Tree Protocol (STP) can prevent network loops but does not segment the network into smaller broadcast domains. Disabling DHCP and using static IP addresses does not inherently segment the network or reduce the size of broadcast domains. While upgrading to Layer 3 switches could provide more control over data traffic, it doesn't exactly segment a network or reduce broadcast domains size without additional configurations like creating VLANs.

Correct Answer: Incorrect VLAN assignments on the new switch

The issue is most probably due to 'Incorrect VLAN assignments on the new switch'. VLAN assignments are typically stored in the configuration file of a switch. When replacing the switch, if the VLAN assignments were not transferred correctly, devices could be placed on the wrong VLAN, thus preventing them from accessing the network. 'A misconfigured Spanning Tree Protocol (STP)' is unlikely the cause. Although STP misconfigurations could disrupt network connections, there's no reason to suspect an STP issue in this case since the identical replacement switch was used 'A duplicate IP address on the new switch' is also less probable as the problem since the devices themselves are suffering from access, not the new switch. Lastly, if 'The new switch is using an outdated firmware', it would typically still be able to connect devices to the network unless the outdated firmware specifically had an issue with network connections.

Correct Answer: A loop created by Spanning Tree Protocol (STP) misconfiguration

The most likely cause of the flooding traffic through a managed switch leading to network congestion is a loop created by a Spanning Tree Protocol (STP) misconfiguration. STP is a network protocol designed to prevent loops by logically disabling some physical network paths. If STP is misconfigured on a switch, it could inadvertently allow a loop to form. In this loop, the same frames are constantly forwarded leading to traffic flooding. The other choices are less likely because a malfunctioning router on a stick configuration, a VLAN misconfiguration, or an inter-VLAN routing misconfiguration wouldn't typically result in the flooding of traffic from multiple networks causing network congestion in a manner described in the question.