Friday, June 30, 2017

Some of the coolest features/ enhancements in Hyper-V 2016

VM compute resiliency: This will help providing resiliency to transient issues like a temporary disconnection of a cluster node due to some network issues or if the cluster service itself on the node crashes etc. The VMs will still continue working "Unmonitored" even if the node falls out of cluster membership into an isolated state. Here the unmonitored state of the VM implies that it is no longer monitored by cluster service. The default resiliency period is 4 minutes. This means the Unmonitored VMs will be allowed to run on that isolated node for 4 minutes and after that VMs will be failed over to a suitable node/ nodes in the cluster. And that particular node which is isolated is moved to a down state. The cluster service itself is now not a necessary dependency for a VM to run. As long as connectivity exists the VM will continue working.

Node quarantine: If a cluster node is isolated certain number of times (default is 3) within an hour it will be moved to quarantine state and the VMs running on it (if any) will be failed over to another suitable node/ nodes in the cluster. 

Event 1 - cluster service stopped on node A - node A isolated (down) - cluster service restarted - node A online
Event 2 - cluster service stopped on node A - node A isolated (down) - cluster service restarted - node A online
Event 3 - cluster service stopped on node A - node A down - node A quarantined

The node will be quarantined for a period of 2 hours by default. But the administrator can manually start the cluster service on that node to join it back to the cluster.

VM storage resiliency: If there is a storage interruption, the VM identifies it and it will pause all the IO's for a certain duration and once the storage is available all IO operations will be resumed. This is very helpful in case of transient storage issues, saving the VM from blue screening or crashing. If the storage path is not back online after a certain period of time, it will pause the VM. Once storage comes back it auto resumes.

VM memory run time resize: You can now increase/ decrease RAM of a running VM.

Hot add/ remove VM network adapters: VM network adapters can also be added or removed on the fly.

Cluster OS rolling upgrades: With this feature you can upgrade your Hyper-V 2012 R2 cluster to Hyper-V 2016 cluster without shutting down the cluster. You can upgrade your existing cluster in 2 ways. Either you can add new 2016 nodes to the 2012 R2 cluster, migrate workload to new 2016 nodes and evict old nodes. Or you can evict one of the existing 2012 R2 node, do a clean installation of 2016, add it back to the cluster and do the same for rest of the nodes. Once all the nodes are 2016, you can update cluster functional level to 2016.

Thursday, May 11, 2017

Benchmarking Hyper-Converged Storage Spaces Direct (S2D) Cluster

Finally I managed to write some PowerShell code as I am completely inspired by my new PS geek friends. The scripts can be used to generate load and stress test your S2D as well as traditional Hyper-V 2016 cluster. These are functionally similar to VM Fleet. There are 7 scripts in total.
  1. create_clustered_testvms.ps1 : this script creates virtual machines on the cluster nodes which will be used for stress testing
  2. start_all_testvms.ps1 : start all those clustered VMs that you just created
  3. io_stress_trigger.ps1 : to trigger IO stress on all VMs using diskspd 
  4. rebalance_all_testvms.ps1 : this script is originally from winblog, I just made a small change so that it will use live migration while moving the clustered VMs back to their owner node
  5. watch_iops_live.ps1 : to view read, write and total iops of each CSV disk on the S2D cluster 
  6. stop_all_testvms.ps1 : to shutdown all the clustered VMs
  7. wipeoff_testvms.ps1 : to delete all VMs that you created using the first script
PS Version which I am using is given below.


Now I will explain briefly about how to use these scripts and a few prerequisites. Say, you have a 4 node hyper-converged S2D cluster.


As there are 4 nodes , you should have 4 cluster shared volumes (CSV). Assign one CSV to each cluster node as shown below. This means when you create VMs on NODE-01, it will be placed on CSV Volume AA, for NODE-02 VMs will be placed on Volume BB and so on. 


Volume AA is C:\ClusterStorage\Volume1


Similarly,
Volume BB is C:\ClusterStorage\Volume2
Volume CC is C:\ClusterStorage\Volume3
Volume DD is C:\ClusterStorage\Volume4


Your cluster shared volumes are ready now. Create 2 folders inside Volume1 as shown below.


Copy all the 7 PS scripts to scripts folder. Code of each script is given at the end.


You now need a template VHDX and that needs to be copied to template folder.
Note: It should be named as "template"


This template is nothing but a Windows Server 2016 VM created on a dynamically expanding disk. So you just have to create a VM with dynamically expanding VHDX, install Windows Server 2016 and set local administrator password to "Pass1234". Download diskspd from Microsoft, unzip it and just copy the diskspd.exe to C drive of the VM you just created.


Shutdown the VM. No need to sysprep it. Copy the VHDX disk of the VM to template folder and rename it to "template". The disk will be around 9.5 GB in size. Once the template is copied, you are all set to start. 

Step 1: 
Run create_clustered_testvms.ps1 
This will create clustered testvms on each of the nodes. It will be done in such a way that VMs on NODE-01 will be stored on Volume1, VMs on NODE-02 will be stored on Volume2 and so on 

Step 2:
Run start_all_testvms.ps1 
This will start all the testvms

Step 3:
Wait for a few seconds to ensure all the testvms are booted properly; then run io_stress_trigger.ps1 and provide necessary input parameters 



Step 4:
You can watch IOPS of the cluster using watch_iops_live.ps1


If you would like to live migrate some testvms while the stress test is running you can try it and observe the IO variations. But before running the io_stress_trigger.ps1 again you have to move/ migrate all those testvms back to their preferred owners. This can be done using rebalance_all_testvms.ps1 .  

If any testvms are not running on their preferred owner, then io_stress_trigger.ps1 will fail for those VMs. Here, testvms running on NODE-01 has preferred owner NODE-01, similarly for all other testvms. So you have to make sure all the testvms are running on their preferred owner before starting io stress script.

Use stop_all_testvms.ps1 to shutdown all the clustered testvms that you created on step 1. To delete all the testvms, you can use wipeoff_testvms.ps1 .

NOTE: While running scripts 2,3,6 and 7 please make sure all the testvms are running on their preferred owner. Use rebalance_all_testvms.ps1 to assign all testvms back to its preferred owner! Also please run all these scripts on PowerShell with elevated privileges after directly logging into any of the cluster nodes.

All codes given below. It might not be optimal but I am pretty sure it works! Cheers !
--------------------------------------------------------------------------------------------------------------------------

#BEGIN_create_clustered_testvms.ps1
#Get cluster info
$Cluster_name = (Get-Cluster).name
$Nodes_name = (Get-ClusterNode).name
$Node_count = (Get-ClusterNode).count

#Input VM config
$VM_count = Read-Host "Enter number of VMs/ node"
$Cluster_VM_count = $VM_count*$Node_count
[int64]$RAM = Read-Host "Enter memory for each VM in MB Eg: 4096"
$RAM = 1MB*$RAM
$CPU = Read-Host "Enter CPU for each VM"

#Creds to Enter-PSSession
$pass = convertto-securestring -asplaintext -force -string Pass1234
$cred = new-object -typename system.management.automation.pscredential -argumentlist "administrator", $pass

#Loop for each node in cluster
for($i=1; $i -le $Node_count; $i++){
    $VM_path = "C:\ClusterStorage\Volume$i"
    $Node = $Nodes_name[$i-1]

    #Remote session to each node
    $S1 = New-PSSession -ComputerName $Node -Credential $cred

    #Loop for creating new testvms on each node
    for($j=1; $j -le $VM_count; $j++){
        $VM_name = "testvm-$Node-$j"
        new-vm -name $VM_name -computername $Node -memorystartupbytes $RAM -generation 2 -Path $VM_path
        set-vm -name $VM_name -ProcessorCount $CPU -ComputerName $Node
        New-Item -path $VM_path\$VM_name -name "Virtual Hard Disks" -type directory

        #Copy template disk
        Copy-Item "C:\ClusterStorage\Volume1\template\template.vhdx" -Destination "$VM_path\$VM_name\Virtual Hard Disks" -Verbose
        Add-VMHardDiskDrive -VMName $VM_name -ComputerName $Node -path "$VM_path\$VM_name\Virtual Hard Disks\template.vhdx" -Verbose

        #Create new fixed test disk
        New-VHD -Path "$VM_path\$VM_name\Virtual Hard Disks\test_disk.vhdx" -Fixed -SizeBytes 40GB
        Add-VMHardDiskDrive -VMName $VM_name -ComputerName $Node -path "$VM_path\$VM_name\Virtual Hard Disks\test_disk.vhdx" -Verbose

        Get-VM -ComputerName $Node -VMName $VM_name | Start-VM
        Add-ClusterVirtualMachineRole -VirtualMachine $VM_name
        Set-ClusterOwnerNode -Group $VM_name -owner $Node
        Start-Sleep -S 10

        #Remote session to each testvm on the node to initialize and format test disk (drive D:)
        Invoke-Command -Session $S1 -ScriptBlock {param($VM_name2,$cred2) Invoke-Command -VMName $VM_name2 -Credential $cred2 -ScriptBlock {
            Initialize-Disk -Number 1 -PartitionStyle MBR
            New-Partition -DiskNumber 1 -UseMaximumSize -DriveLetter D
            Get-Volume | where DriveLetter -eq D | Format-Volume -FileSystem NTFS -NewFileSystemLabel Test_disk -confirm:$false
            }} -ArgumentList $VM_name,$cred

        Start-Sleep -S 5
        Get-VM -ComputerName $Node -VMName $VM_name | Stop-VM -Force
        }
    }
#END_create_clustered_testvms.ps1


--------------------------------------------------------------------------------------------------------------------------

#BEGIN_start_all_testvms.ps1
#Get cluster info
$Cluster_name = (Get-Cluster).name
$Nodes_name = (Get-ClusterNode).name
$Node_count = (Get-ClusterNode).count

for($i=1; $i -le $Node_count; $i++){
    $Node = $Nodes_name[$i-1]
    Get-VM -ComputerName $Node -VMName "testvm-$Node*" | Start-VM -AsJob

    }
#END_start_all_testvms.ps1


--------------------------------------------------------------------------------------------------------------------------

#BEGIN_io_stress_trigger.ps1
#Get cluster info
$Cluster_name = (Get-Cluster).name
$Nodes_name = (Get-ClusterNode).name
$Node_count = (Get-ClusterNode).count

#Creds to Enter-PSSession
$pass = convertto-securestring -asplaintext -force -string Pass1234
$cred = new-object -typename system.management.automation.pscredential -argumentlist "administrator", $pass

$time = Read-Host "Enter duration of stress in seconds (Eg: 300)"
$block_size = Read-Host "Enter block size (Eg: 4K)"
$writes = Read-Host "Enter write percentage (Eg: 20)"
$OIO = Read-Host "Enter number of outstanding IOs (Eg: 16)"
$threads = Read-Host "Enter number of threads (Eg: 2)"

#Loop for each node in cluster
for($i=1; $i -le $Node_count; $i++){
    $VM_path = "C:\ClusterStorage\Volume$i"
    $Node = $Nodes_name[$i-1]

    #Remote session to each node
    $S1 = New-PSSession -ComputerName $Node -Credential $cred

    $VM_count = (Get-VM -ComputerName $Node -VMName "testvm-$Node*").Count

    #Loop for creating new testvms on each node
    for($j=1; $j -le $VM_count; $j++){
        $VM_name = "testvm-$Node-$j"

        #Remote session to each testvm
        Invoke-Command -Session $S1 -ScriptBlock {param($VM_name2,$cred2,$time1,$block_size1,$writes1,$OIO1,$threads1) Invoke-Command -VMName $VM_name2 -Credential $cred2 -ScriptBlock {param($time2,$block_size2,$writes2,$OIO2,$threads2)
        C:\diskspd.exe -"b$block_size2" -"d$time2" -"t$threads2" -"o$OIO2" -h -r -"w$writes2" -L -Z500M -c38G D:\io_stress.dat
        } -AsJob -ArgumentList $time1,$block_size1,$writes1,$OIO1,$threads1 } -ArgumentList $VM_name,$cred,$time,$block_size,$writes,$OIO,$threads


        }
    }
#END_io_stress_trigger.ps1


--------------------------------------------------------------------------------------------------------------------------

#BEGIN_rebalance_all_testvms.ps1
$clustergroups = Get-ClusterGroup | Where-Object {$_.IsCoreGroup -eq $false}
 foreach ($cg in $clustergroups)
 {
     $CGName = $cg.Name
     Write-Host "`nWorking on $CGName"
     $CurrentOwner = $cg.OwnerNode.Name
     $POCount = (($cg | Get-ClusterOwnerNode).OwnerNodes).Count
     if ($POCount -eq 0)
     {
         Write-Host "Info: $CGName doesn't have a preferred owner!" -ForegroundColor Magenta
     }
     else
     {
         $PreferredOwner = ($cg | Get-ClusterOwnerNode).Ownernodes[0].Name
         if ($CurrentOwner -ne $PreferredOwner)
         {
             Write-Host "Moving resource to $PreferredOwner, please wait..."
             $cg | Move-ClusterVirtualMachineRole -MigrationType Live -Node $PreferredOwner
         }
         else
         {
             write-host "Resource is already on preferred owner! ($PreferredOwner)"
         }
     }
 }
 Write-Host "`n`nFinished. Current distribution: "

 Get-ClusterGroup | Where-Object {$_.IsCoreGroup -eq $false} 
#END_rebalance_all_testvms.ps1


--------------------------------------------------------------------------------------------------------------------------

#BEGIN_watch_iops_live.ps1
#Get cluster info
$Cluster_name = (Get-Cluster).name
$Nodes_name = (Get-ClusterNode).name

while($true)
{

    [int]$total_IO = 0
    [int]$total_readIO = 0
    [int]$total_writeIO = 0
 
    clear
     
    "{0,-15} {1,-15} {2,-15} {3,-15} {4, -15} {5, -15}" -f "Host", "Total IOPS", "Reads/Sec", "Writes/Sec", "Read Q Length", "Write Q Length"

    for($j=1; $j -le $Nodes_name.count; $j++){

        $Node = $Nodes_name[$j-1]

        $Data = Get-CimInstance -ClassName Win32_PerfFormattedData_CsvFsPerfProvider_ClusterCSVFS -ComputerName $Node | Where Name -like Volume$j

        [int]$T = $Data.ReadsPerSec+$Data.WritesPerSec
     
        "{0,-15} {1,-15} {2,-15} {3,-15} {4,-15} {5, -15}" -f "$Node", "$T", $Data.ReadsPerSec, $Data.WritesPerSec, $Data.CurrentReadQueueLength, $Data.CurrentWriteQueueLength

        $total_IO = $total_IO+$T
        $total_readIO = $total_readIO+$Data.ReadsPerSec
        $total_writeIO = $total_writeIO+$Data.WritesPerSec
        }

    echo `n
    "{0,-15} {1,-15} {2,-15} {3,-15} " -f "Cluster IOPS", "$total_IO", "$total_readIO", "$total_writeIO"

    Start-Sleep -Seconds 3

}
#END_watch_iops_live.ps1

--------------------------------------------------------------------------------------------------------------------------

#BEGIN_stop_all_testvms.ps1
#Get cluster info
$Cluster_name = (Get-Cluster).name
$Nodes_name = (Get-ClusterNode).name
$Node_count = (Get-ClusterNode).count

for($i=1; $i -le $Node_count; $i++){
    $Node = $Nodes_name[$i-1]
    Get-VM -ComputerName $Node -VMName "testvm-$Node*" | Stop-VM -Force -AsJob
    }
#END_stop_all_testvms.ps1

--------------------------------------------------------------------------------------------------------------------------

#BEGIN_wipeoff_testvms.ps1
#Get cluster info
$Cluster_name = (Get-Cluster).name
$Nodes_name = (Get-ClusterNode).name
$Node_count = (Get-ClusterNode).count

#Loop for each node in cluster
for($i=1; $i -le $Node_count; $i++){
    $VM_path = "C:\ClusterStorage\Volume$i"
    $Node = $Nodes_name[$i-1]

    $VM_count = (get-vm -ComputerName $Node -Name "testvm-$Node-*").Count

    #Loop to delete testvm on each node
    for($j=1; $j -le $VM_count; $j++){

        $VM_name = "testvm-$Node-$j"
        $full_path = "$VM_path\$VM_name"

        Get-VM -Computername $Node -VMname $VM_name | stop-vm -force
        Get-ClusterGroup $VM_name | Remove-ClusterGroup -Force -RemoveResources
        Get-VM -Computername $Node -VMname $VM_name | remove-vm -force
        Remove-Item $full_path -Force -Recurse -ErrorAction SilentlyContinue -Verbose
        }

    }
#END_wipeoff_testvms.ps1

--------------------------------------------------------------------------------------------------------------------------



Friday, April 28, 2017

Storage Spaces Direct - Volumes and Resiliency

Storage Spaces Direct (S2D) is the Microsoft implementation of software defined storage (SDS). This article briefly explains about the different types of volumes that can be created on a S2D cluster. Once you enable S2D using Enable-ClusterS2D cmdlet, it will automatically claim all physical disks in the cluster and forms a storage pool. On top of this pool you can create multiple volumes which is explained below.

Mirror
  • Recommended for workloads that have strict latency requirements or that need lots of mixed random IOPS
  • Eg: SQL Server databases or performance-sensitive Hyper-V VMs
  • If you have a 2 node cluster: Storage Spaces Direct will automatically use two-way mirroring for resiliency
  • If your cluster has 3 nodes: it will automatically use three-way mirroring
  • Three-way mirror can sustain two fault domain failures at same time
new-volume -friendlyname "Volume A" -filesystem CSVFS_ReFS -storagepoolfriendlyname S* -size 1TB
  • You can create two-way mirror by mentioning "PhysicalDiskRedundancy 1"
new-volume -friendlyname "Volume A" -filesystem CSVFS_ReFS -storagepoolfriendlyname S* -size 1TB -PhysicalDiskRedundancy 1

Parity
  • Recommended for workloads that write less frequently, such as data warehouses or "cold" storage, traditional file servers, VDI etc.
  • For creating dual parity volumes min 4 nodes are required and can sustain two fault domain failures at same time
new-volume -friendlyname "Volume B" -filesystem CSVFS_ReFS -storagepoolfriendlyname S* -size 1TB -resiliencysettingname Parity
  • You can create single parity volumes using the below
new-volume -friendlyname "Volume B" -filesystem CSVFS_ReFS -storagepoolfriendlyname S* -size 1TB -resiliencysettingname Parity -PhysicalDiskRedundancy 1

Mixed/ Tiered / Multi-Resilient (MRV)
  • In Windows Server 2012 R2 Storage Spaces, when you create storage tiers you dedicated physical media devices. That means SSD for performance tier and HDD for capacity tier
  • But in Windows Server 2016, tiers are differentiated not only by media types; it can include resiliency types too
  • MRV = Three-way mirror + dual-parity
  • In a MRV, three-way mirror portion is considered as performance tier and dual parity portion as capacity tier
  • Recommended for workloads that write in large, sequential, such as archival or backup targets
  • Writes land to mirror section of the volume and then it is gradually moved/ rotated in to parity portion later
  • Each MRV by default will have 32 MB Write-back cache 
  • ReFS starts rotating data into the parity portion at 60% utilization of the mirror portion and gradually as utilization increases the speed of data movement to parity portion also increases
  • You should have min 4 nodes to create a MRV
new-volume -friendlyname "Volume C" -filesystem CSVFS_ReFS -storagepoolfriendlyname S* -storagetierfriendlynames Performance, Capacity -storagetiersizes 1TB, 9TB

References:

Saturday, March 18, 2017

Real time disk IOPS and latency monitoring using PowerShell

The below code collects disk IOPS and average IO latency of a list of servers and output the real time values.

Note: Here we are monitoring only logical disk E of testvm-01, testvm-02 and testvm-03

while($true)
{

$Servers = "testvm-01","testvm-02","testvm-03"

$R_io = '\LogicalDisk(E:)\Disk Reads/sec'
$W_io = '\LogicalDisk(E:)\Disk Writes/sec'
$Lat = '\LogicalDisk(E:)\Avg. Disk sec/Transfer'

$Reads =  (Get-Counter -Counter $R_io -ComputerName $Servers).CounterSamples.CookedValue
$Writes = (Get-Counter -Counter $W_io -ComputerName $Servers).CounterSamples.CookedValue
$Latency = (Get-Counter -Counter $Lat -ComputerName $Servers).CounterSamples.CookedValue

clear

"{0,-15} {1,-15} {2,-15} {3,-15} {4, -15}" -f "Host", "Reads/Sec", "Writes/Sec", "Total IOPS", "Avg Latency (ms)"

echo `n

for($i=0; $i -le 2; $i+=1){

[int]$R = $Reads[$i]
[int]$W = $Writes[$i]
[int]$T = $R+$W
[int]$L = ($Latency[$i])*1000
[String]$N = $Servers[$i]

"{0,-15} {1,-15} {2,-15} {3,-15} {4,-15}" -f "$N", "$R", "$W", "$T", "$L"

}

}




Wednesday, March 15, 2017

Running a PowerShell script on multiple remote machines simultaneously

Below example shows how to trigger PowerShell scripts on a remote Windows machine and run as background jobs.

trigger.ps1

Invoke-Command -ComputerName testvm-03 -ScriptBlock {&C:\posh\io\iostress.ps1} -AsJob
Invoke-Command -ComputerName testvm-02 -ScriptBlock {&C:\posh\io\iostress.ps1} -AsJob
Invoke-Command -ComputerName testvm-01 -ScriptBlock {&C:\posh\io\iostress.ps1} -AsJob

Invoke-Command is used to execute scripts remotely. Above script (trigger.ps1) invokes a PS script (iostress.ps1) on 3 remote machines. Here the script being executed is saved on the corresponding remote machine itself. As each of the invoke-command is running as a background job on the local machine, the second invoke-command doesn't have to wait for the first invoke-command to complete and the third invoke-command doesn't have to wait for the first and second invoke-commands to complete. From the user perspective the command prompt returns immediately even if the jobs take longer time to complete. This case will be useful if you want to run a script on multiple remote machines at the same time.  

Tuesday, February 28, 2017

Stress test your storage system using iometer

In this article I will explain briefly about how to use iometer for simulating I/O load. Here in my test a 600GB LUN is provisioned from a Compellent storage array and connected to an ESXI 6.5 server as a data store. I've created a VM with 2 disks (C and E for OS and data) on the 600GB data store. We will be using drive E (40 GB) for the test with NTFS partition having 4KB allocation unit size.

Parameters:

Disk Workers - 2 (that means 2 worker threads will run simultaneously during the test)
Disk Targets - Select drive E on both disk workers
Maximum Disk Size - 0 (entire drive E will be used for the test)
Rest all values - default


Network Targets - leave the settings as it is (as we are not doing network stress)

Access Specifications - it specifies the read/ write block size, percentage of read/ write, random/ sequential access etc.

Max IOPS at - 512 bytes transfer request size, 100% sequential, 100% reads
Max throughput at - 64K transfer request size, 100% sequential, 100% reads

In a real world situation it totally depends on the type of workloads. Here we are considering 4KB blocks, with 90% write, 10% read and 100% random access. These values are close to and are applicable to most virtualization workloads. 


Note: Make sure you add access specifications to all the disk workers

Transfer Request Size - 4KB
Percent Random/ Sequential Distribution - 100% Random
Percent Read/ Write Distribution - 90% Write
Rest all values - default


Once all the above settings are configured, you can click the green flag on the top to start the test. Now when you start it, you can see a test file (iobw.tst) will be created in drive E which will grow to the entire size of the disk (approx. 40GB). This is shown in the screenshots below.

Note: For creating a 40GB test file it takes few minutes

On the results display tab, set update frequency to 1 second to view real time results. You can also use the performance monitor tools to view disk reads and writes to cross check with the values of iometer.

Total I/Os per second = disk reads/ sec + disk writes/ sec


You can also monitor read/ write operations of your data store as shown below. These values should also match the results obtained from iometer and perfmon (as there is only one VM on this data store). In the below graph you can see the data store was almost idle as there was no operations on it. The moment I started iometer, it is creating iobw.tst file which is basically a 40GB write operation on drive E.


4KB reads and writes will happen on this test file (iobw.tst).



There is one more way you can monitor the IOPS value. While the iometer is running open resource monitor and observe disk activity generated by dynamo.exe. Make a note of total bytes. Convert it to Kilobytes and divide it by 4. This gives you the total IOPS which also should be close to values generated by iometer which is shown below.


ESXI performance graph is also shown below.


Final results:

iometer - 3799 IOPS
Perfmon - 372 + 3344 = 3716 IOPS
Disk activity monitor - (15291932 Bytes) 14933KB/ 4KB = 3733 IOPS
ESXI performance monitor - 373 + 3365 = 3738 IOPS

Note: To simulate a complex real world scenario or to benchmark your storage system you can provision multiple LUNs from the storage array, host few virtual machines on those LUNs and run iometer with different access specifications.  

Hope this article was useful to you. Cheers.

Tuesday, January 31, 2017

Virtual Link Trunking - VLT

VLT is a proprietary layer-2 link aggregation protocol developed by Force10 which allows users to setup aggregated links between end devices (servers) to two different physical switches. The main property is that after enabling VLT, the two physical switches will act as a single logical switch. This offers redundancy for the connection from server and balanced traffic to the core network eliminating packet loops. 

In dynamic link aggregation protocol like LACP, the connections from a server can be only be terminated to a single logical switch (it can be a single physical switch or multiple switches in a single stacked switch setup). There wont be any redundancy if the aggregated links terminate to a single physical switch and the disadvantage of a switch stack is that incase of maintenance or firmware update the whole stack needs to be taken offline causing downtime which is not practical in a production environment.


The VLT peers exchange and synchronize layer-2 related table details (MAC tables, IGMP states etc) among the whole VLT domain. Devices connected to the VLT domain could be either servers or switches as long as they support port channel (LAG, LACP etc). It is recommended to enable RSTP and configure bridge priorities. Prefer static LAG between VLT peers and LACP towards hosts/ switches.

Configuration steps:

1.Enable RSTP and configure bridge priority on peer VLT switches
2.Configure VLT interconnect (VLTi), static LAG between VLT switches
3.Configure VLT domain
4.Configure LACP for the connected device
5.Verify status