Layer 2 of the OSI Model – Data Link Layer

The 2nd layer of the OSI layer is called the Data Link Layer.  This is where the method of networking is determined.  (wired or wireless or token ring etc)
Data Link Layer (Layer 2)

The second-lowest layer (layer 2) in the OSI Reference Model stack is the data link layer, often abbreviated “DLL” (though that abbreviation has other meanings as well in the computer world). The data link layer, also sometimes just called the link layer, is where many wired and wireless local area networking (LAN) technologies primarily function. For example, Ethernet, Token Ring, FDDI and 802.11 (“wireless Ethernet” or “Wi-Fi’) are all sometimes called “data link layer technologies”. The set of devices connected at the data link layer is what is commonly considered a simple “network as opposed to Internetwork

Data Link Layer Sublayers: Logical Link Control (LLC) and Media Access Control (MAC)The data link layer is often conceptually divided into two sublayers: logical link control (LLC) and media access control (MAC). This split is based on the architecture used in the IEEE 802 Project, which is the IEEE working group responsible for creating the standards that define many networking technologies (including all of the ones I mentioned above except FDDI). By separating LLC and MAC functions, interoperability of different network technologies is made easier, as explained in our earlier discussion of networking model concepts.

Data Link Layer Functions

The following are the key tasks performed at the data link layer:

Logical Link Control (LLC): Logical link control refers to the functions required for the establishment and control of logical links between local devices on a network. As mentioned above, this is usually considered a DLL sublayer; it provides services to the network layer above it and hides the rest of the details of the data link layer to allow different technologies to work seamlessly with the higher layers. Most local area networking technologies use the IEEE 802.2 LLC protocol.

Media Access Control (MAC): This refers to the procedures used by devices to control access to the network medium. Since many networks use a shared medium (such as a single network cable, or a series of cables that are electrically connected into a single virtual medium) it is necessary to have rules for managing the medium to avoid conflicts. For example. Ethernet uses the CSMA/CD method of media access control, while Token Ring uses token passing.

Data Framing: The data link layer is responsible for the final encapsulation of higher-level messages into frames that are sent over the network at the physical layer.

Addressing: The data link layer is the lowest layer in the OSI model that is concerned with addressing: labeling information with a particular destination location. Each device on a network has a unique number, usually called a hardware address or MAC address, that is used by the data link layer protocol to ensure that data intended for a specific machine gets to it properly.

Error Detection and Handling: The data link layer handles errors that occur at the lower levels of the network stack. For example, a cyclic redundancy check (CRC) field is often employed to allow the station receiving data to detect if it was received correctly.

Networking OSI Layers

The part of networking that I always have problems with is the OSI model.  Because of this, I am documenting my study of those layers here.  To help with this, I have copied the Dummies guide description of the explanation.  They always say that typing things out helps in memorization so over the next week or so I am going to translate the “Dummies'” definition to the complete Idiot’s definition that I need to finally understand this stuff.

Wish me luck

The layers of the OSI model

Under its official name, the Open Systems Interconnection Reference Model, or the OSI model, was developed by the International Organization for Standardization, which uses the abbreviation of ISO. And, yes, the full acronym of the OSI is ISO OSI.
The OSI model is a layered model that describes how information moves from an application program running on one networked computer to an application program running on another networked computer. In essence, the OSI model prescribes the steps to be used to transfer data over a transmission medium from one networked device to another. The OSI model is a seven-layer model developed around five specific design principles:
Whenever a discrete level of abstraction is required, a new layer should be created.
Each layer of the model should carry out a well-defined function.
The function of each layer should define internationally standardized protocols.
The boundaries of the layers should be placed to minimize the flow of information across interfaces.
There should be a sufficient number of layers defined to prevent unnecessary grouping of functions and the number of layers should also be small enough so that the model remains manageable.

Moving down through the layers

The OSI model breaks the network communications process into seven separate layers. From the top, or the layer closest to the user, down, these layers are:
Layer 7, Application: The Application layer provides services to the software through which the user requests network services. Your computer application software is not on the Application layer. This layer isn’t about applications and doesn’t contain any applications. In other words, programs such as Microsoft Word or Corel are not at this layer, but browsers, FTP clients, and mail clients are.
Layer 6, Presentation: This layer is concerned with data representation and code formatting.
Layer 5, Session: The Session layer establishes, maintains, and manages the communication session between computers.
Layer 4, Transport: The functions defined in this layer provide for the reliable transmission of data segments, as well as the disassembly and assembly of the data before and after transmission.
Layer 3, Network: This is the layer on which routing takes place, and, as a result, is perhaps the most important OSI layer to study for the CCNA test. The Network layer defines the processes used to route data across the network and the structure and use of logical addressing.
Layer 2, Data Link: As its name suggests, this layer is concerned with the linkages and mechanisms used to move data about the network, including the topology, such as Ethernet or Token Ring, and deals with the ways in which data is reliably transmitted.
Layer 1, Physical: The Physical layer’s name says it all. This layer defines the electrical and physical specifications for the networking media that carry the data bits across a network.

Other interesting OSI layer stuff

Layers 5 through 7 are generally referred to as the upper layers. Conversely, Layers 1 through 4 are collectively called the lower layers. Seems obvious, but you’ll see these references on the test.
You need to know the seven layers in sequence, either top-to-bottom or bottom-to-top. Here are some mnemonic phrases to help you remember the layers of the OSI model:
“Please Do Not Throw Salami Pizza Away” — this works for bottom-to-top. If you don’t like salami pizza, then how about seafood or spinach pizza instead?
“All People Seem To Need Data Processing” — a top-to-bottom reminder.
“APS Transports Network Data Physically” — APS refers to Application, Presentation, and Session. This one separates the upper and lower layer groups.
“Please Do Not Tell Secret Passwords Anytime” — Shh! Another bottom-to-top phrase.

Packaging the data

Each layer of the OSI model formats the data it receives to suit the functions to be performed on that layer. In general, the package of data that moves through the layers is called a Protocol Data Unit (PDU). However, as the data is reformatted and repackaged, it takes on unique names on certain layers. Table 1 lists the name each layer uses to refer to a message.

Vmotion without shared storage

I have been running into issues on my home lab when it comes to load balancing. Apparently with the release of the VSphere 5.1 there is a new feature that allows you to migrate running images between hosts without shutting them down first.

The requirements are as follows

Requirements and Limitations for vMotion Without Shared Storage

A virtual machine and its host must meet resource and configuration requirements for the virtual machine files and disks to be migrated with vMotion in the absence of shared storage.

vMotion in an environment without shared storage is subject to the following requirements and limitations:

The hosts must be licensed for vMotion.

The hosts must be running ESXi 5.1 or later.

The hosts must meet the networking requirement for vMotion. See vSphere vMotion Networking Requirements.

The virtual machines must be properly configured for vMotion. See Virtual Machine Conditions and Limitations for vMotion in the vSphere Web Client

Virtual machine disks must be in persistent mode or be raw device mappings (RDMs). See Storage vMotion Requirements and Limitations.

The destination host must have access to the destination storage.

When you move a virtual machine with RDMs and do not convert those RDMs to VMDKs, the destination host must have access to the RDM LUNs.

Consider the limits for simultaneous migrations when you perform a vMotion migration without shared storage. This type of vMotion counts against the limits for both vMotion and Storage vMotion, so it consumes both a network resource and 16 datastore resources. See Limits on Simultaneous Migrations in the vSphere Web Client.


Migration with vMotion in Environments Without Shared Storage

You can use vMotion to migrate virtual machines to a different host and datastore simultaneously. In addition, unlike Storage vMotion, which requires a single host to have access to both the source and destination datastore, you can migrate virtual machines across storage accessibility boundaries.

In vSphere 5.1 and later, vMotion does not require environments with shared storage. This is useful for performing cross-cluster migrations, when the target cluster machines might not have access to the source cluster’s storage. Processes that are working on the virtual machine continue to run during the migration with vMotion.

You can place the virtual machine and all of its disks in a single location or select separate locations for the virtual machine configuration file and each virtual disk. In addition, you can change virtual disks from thick-provisioned to thin-provisioned or from thin-provisioned to thick-provisioned. For virtual compatibility mode RDMs, you can migrate the mapping file or convert from RDM to VMDK.

vMotion without shared storage is useful for virtual infrastructure administration tasks similar to vMotion with shared storage or Storage vMotion tasks.

Host maintenance. You can move virtual machines off of a host to allow maintenance of the host.

Storage maintenance and reconfiguration. You can move virtual machines off of a storage device to allow maintenance or reconfiguration of the storage device without virtual machine downtime.

Storage load redistribution. You can manually redistribute virtual machines or virtual disks to different storage volumes to balance capacity or improve performance.



Migrate a Virtual Machine to a New Host and Datastore by Using vMotion in the vSphere Web Client

You can move a virtual machine to another host and move its disks or virtual machine folder to another datastore. With vMotion, you can migrate a virtual machine and its disks and files while the virtual machine is powered on.

You can perform vMotion in environments without shared storage. Virtual machine disks or contents of the virtual machine folder are transferred over the vMotion network to reach the destination host and datastores.

To make disk format changes and preserve them, you must select a different datastore for the virtual machine files and disks. You cannot preserve disk format changes if you select the same datastore on which the virtual machine currently resides.


Verify that your hosts and virtual machines meet the necessary requirements. See Requirements and Limitations for vMotion Without Shared Storage.

Required privilege: Resource.HotMigrate



Right-click the virtual machine and select Migrate.


To locate a virtual machine, select a datacenter, folder, cluster, resource pool, host, or vApp.


Click the Related Objects tab and click Virtual Machines.


Select Change both host and datastore and click Next.


Select the destination resource for the virtual machine migration.


Select a destination host or cluster for the virtual machine, and click Next.

Any compatibility problems appear in the Compatibility panel. Fix the problem, or select another host or cluster.

Possible targets include hosts and fully automated DRS clusters. You can select a non-automated cluster as a target. You are prompted to select a host within the non-automated cluster.


Select the format for the virtual machine’s disks.



Same format as source

Use the same format as the source virtual machine.

Thick Provision Lazy Zeroed

Create a virtual disk in a default thick format. Space required for the virtual disk is allocated during creation. Any data remaining on the physical device is not erased during creation, but is zeroed out on demand at a later time on first write from the virtual machine.

Thick Provision Eager Zeroed

Create a thick disk that supports clustering features such as Fault Tolerance. Space required for the virtual disk is allocated at creation time. In contrast to the thick provision lazy zeroed format, the data remaining on the physical device is zeroed out during creation. It might take longer to create disks in this format than to create other types of disks.

Thin Provision

Use the thin provisioned format. At first, a thin provisioned disk uses only as much datastore space as the disk initially needs. If the thin disk needs more space later, it can grow to the maximum capacity allocated to it.


Assign a storage profile from the VM Storage Profile drop-down menu.

Storage profiles define the storage capabilities that are required by the applications running on the virtual machine.


Select the datastore location where you want to store the virtual machine files.



Store all virtual machine files in the same location on a datastore.

Select a datastore and click Next.

Store all virtual machine files in the same Storage DRS cluster.


Select a Storage DRS cluster.


(Optional) To not use Storage DRS with this virtual machine, select Disable Storage DRS for this virtual machine and select a datastore within the Storage DRS cluster.


Click Next.

Store virtual machine configuration files and disks in separate locations.


Click Advanced.


For the virtual machine configuration file and for each virtual disk, select Browse, and select a datastore or Storage DRS cluster.


(Optional) If you selected a Storage DRS cluster and do not want to use Storage DRS with this virtual machine, select Disable Storage DRS for this virtual machine and select a datastore within the Storage DRS cluster.


Click Next.


Select the migration priority level and click Next.



Reserve CPU for optimal VMotion performance

vCenter Server attempts to reserve resources on both the source and destination hosts to be shared among all concurrent migrations with vMotion. vCenter Server grants a larger share of host CPU resources. if sufficient CPU resources are not immediately available, vMotion is not initiated.

Perform with available CPU resources

vCenter Server reserves resources on both the source and destination hosts to be shared among all concurrent migration with vMotion. vCenter Server grants a smaller share of host CPU resources. If there is a lack of CPU resources, the duration of vMotion can be extended.


Review the information on the Review Selections page and click Finish.

vCenter Server moves the virtual machine to the new host and storage location. Event messages appear in the Events tab. The data that appears in the Summary tab shows the status and state throughout the migration. If errors occur during migration, the virtual machines revert to their original states and locations.

VCA-DCV class completed

I just finished the class portion of the VCA-DCV  (VMWare Certified Associate-Data Center Virtualization).  The class is better than others I have attended online but it is still painfully dull.  

I did in fact learn about a cool piece of tech that is part of the VMWare suite.  VSA, or Virtual Storage appliance.  This is a set of tools that allows you to share non networked storage as if it were network attached.  This allow you to set up things like VMotion and HA.  I may have to try this in my lab.  (Or I could just get an actual NAS switch for my filer)  Anyways, I think one or 2 more times through the class lessons will be enough to make me ready for the test.  I am also going through the VCA-Cloud lessons.  I figure since the tests are half price right now I should take advantage and sit them both.


A beginning.

This is not The beginning but it is A beginning.

This blog will be my personal notepad of resources and thoughts on my various training endeavors.  My current thoughts are to work to getting my VCP/VCAP.  MS hyper-v is a nice product but after playing with it and using it for a few years, it still seems young and brittle.  it does not take a lot to break it but it does take a lot to fix it once it is broken.

VMWare on the other hand.  Offerings seems solid and “just work”  Price is a lot more but I think it is worth it.  also things like usb and being able to nest vm’s is worth it.