DELL PC2748 User Manual

Head of Line (HOL) blocking results in traffic delays and frame loss caused by traffic competing 

for the same egress port resources. HOL blocking queues packets, and the packets at the head of 

the queue are forwarded before packets at the end of the queue. By default, the device is configured 

so that the HOL blocking prevention mechanism is active at all times, except when QoS (Quality 

of Service), Flow Control or Back Pressure is active on a port where the HOL blocking prevention 

mechanism is disabled on the whole system.

• Unmanaged Mode — Operates independent of user-configuration. The switch does not have 

an IP address, nor is there a web management interface and thus cannot be managed. This is 

the system default. From Unmanaged Mode, when the Managed Mode button is pressed, the 

switch enters Managed Mode with the default IP address of 192.168.2.1.

•

Managed Mode — Provides switch management through a web interface, and maintains the 

device configuration through power cycles. From Managed Mode, when the Managed Mode 

button is pressed, the switch enters Unmanaged Mode.

•

Secure Mode (PowerConnect 2748 only) — Secure Mode works by the user configuring 

the switch in Managed Mode and then enabling Secure Mode. Once enabled, it prevents 

users from making any further configuration changes to the switch. This is done by removing 

the IP address to the switch so that it becomes inaccessible. In Secure Mode the switch 

retains configuration through power cycles just like Managed Mode. To use Secure Mode, the 

user puts the switch in Managed Mode, configures the switch as desired, and then switches to 

Secure Mode via the web interface. To exit Secure Mode, the user presses the Managed Mode 

button. From Secure Mode when the Managed Mode button is pressed, the switch enters 

Managed Mode default configuration with the default IP address of 192.168.2.1.

On half-duplex links, the receiving port prevents buffer overflows by occupying the link so that it is 

unavailable for additional incoming traffic. The user may enable or disable this feature on a 

per-port basis. The default status on all ports is set to 

OFF. However, this applies to links operating 

at Half Duplex only.