How Vlan works in plain language

To understand how VLANs work, let’s consider VLAN applications in a traditional IT environment, where physical hosts are attached to a physical switch, and no virtualization is involved. Imagine a scenario where you want three isolated networks but you only have a single physical switch. The network administrator would choose three VLAN IDs, for example, 10, 11, and 12, and would configure the switch to associate switchports with VLAN IDs. For example, switchport 2 might be associated with VLAN 10, switchport 3 might be associated with VLAN 11, and so forth. When a switchport is configured for a specific VLAN, it is called an access port. The switch is responsible for ensuring that the network traffic is isolated across the VLANs.

Now consider the scenario that all of the switchports in the first switch become occupied, and so the organization buys a second switch and connects it to the first switch to expand the available number of switchports. The second switch is also configured to support VLAN IDs 10, 11, and 12. Now imagine host A connected to switch 1 on a port configured for VLAN ID 10 sends an Ethernet frame intended for host B connected to switch 2 on a port configured for VLAN ID 10. When switch 1 forwards the Ethernet frame to switch 2, it must communicate that the frame is associated with VLAN ID 10.

If two switches are to be connected together, and the switches are configured for VLANs, then the switchports used for cross-connecting the switches must be configured to allow Ethernet frames from any VLAN to be forwarded to the other switch. In addition, the sending switch must tag each Ethernet frame with the VLAN ID so that the receiving switch can ensure that only hosts on the matching VLAN are eligible to receive the frame.

A switchport that is configured to pass frames from all VLANs and tag them with the VLAN IDs is called a trunk port. IEEE 802.1Q is the network standard that describes how VLAN tags are encoded in Ethernet frames when trunking is being used.

 

The meanings of SR、LRM、LR、ER and ZR

SR、LRM、LR、ER、ZR are terms used in fiber optic communications, stand for the transmission distance of the 10G modules.

SR stands for Short Range, these transceivers support link length of 300m over multi-mode fiber and use 850nm lasers.

LRM means Long Reach Multimode, these transceivers support distance up to 220m over multi-mode fiber and use 1310nm lasers.

LR means Long Reach, these transceivers support distance up to 10km over single mode fiber and use 1310nm lasers.

ER means Extended Reach, the data rate of these transceivers support distance up to 40km over single mode fiber and use 1550nm lasers.

ZR also stands for Extended Reach which can transmit 10G data rate and 80km distance over single mode fiber and use 1550nm lasers.

Problem with SSD corruption on power failure .. flactuation

only enterprise-class SSDs can be relied upon (at all) for safe behavior on power fail.

The enterprise-class SSDs have super-capacitors that store enough power to write all data stored in the RAM within the SSD on power fail, and vendors charge three times as much as they do for consumer class SSDs.

Some vendors do certify their SSDs, but you should check them under real power-fail conditions, like pulling the plug.

It is better to use an Arduino with a relay for experimenting with cutting off power.

New open source medical image repository debuts

A team of medical researchers from four European institutions has created the Image Data Resource, a repository “that is capable of storing and integrating data from multiple laboratories, whilst also significantly enhancing the potential for sharing and reusing imaging data.”

The repository “offers potential to identify new therapies and targets, and broadens the scope of research by allowing scientists around the world to access each other’s imaging datasets,” the team writes in a recently published paper. The software underpinning the repository is open source, which “opens the possibility of the tech being deployed in other image data systems.”