What is Beacon Probing in vSphere Networking – Deep-Dive – Part1



Beacon probing is a network failover detection mechanism provided by VMware to detect upstream switch failures & also use to keep the track of health state of each uplink configured in a team.




Let’s Understand various Failure Scenarios for above mentioned topology:

  • What If link between vmnicX (uplink-X) and AS-X (Access Switch-X) fails.
  • What if vmnicX having hardware issue or network driver issue.
  • What if Access switch (AS-X) goes down due to power failure.

Below mentioned diagram, depicts these failure scenarios:


Do we need Beacon Probing to detect these failures?

Not necessarily. Default network failover detection method of vSphere Networking also known as “Link Status Only” should be able to detect these kinds of failures and VMkernel will take necessarily action in event of NIC failures. For ex. It will mark faulty NIC as bad/down or divert vm traffic to other active uplinks in a team.


Let’s analyze few more failure scenarios:

  1. What if link connectivity between AS-x (Access Switch) and CS-0 (Core Switch) goes down.
  2. What if user has configured wrong VLAN or MTU size on access switch.
  3. What if firmware of NIC adapter has issue and somehow hypervisor is not aware about that so hypervisor shows link status as UP but physical switch detects firmware hang issue in card and mark port state as DOWN.

Below mentioned diagram, depicts these failure scenarios:



Will “Link Status Only” failover detection method would be able to detect these failures?

NO. This mechanism is pretty straightforward – If Link is UP, use the adapter. If Link is Down, don’t use the adapter. It doesn’t check for end to end network health state, user configuration issues causing network. Link Status Only doesn’t have capability to detect k connectivity failures.

Due to Link Status Only Algorithm limitations, ESXi kernel still shows link state of NIC as UP and it continue to use this adapter and forward vm traffic to these adapters as if nothing has changed. Due to this behavior, you would see VM loosing networking connectivity or VM packets are getting dropped even when vSphere UI or “esxcfg-nics -l” CLI shows physical adapters state as UP.


Configuring Beacon Probing. If a downstream link beyond the immediate physical port fails, the adapter driver cannot detect it. This cause existing VMs using the same uplink which has lost network connectivity. If physical switch doesn’t support Link State Tracking (hardware based solution), beacon probing provides a software alternative to solve this problem.


ESXi periodically broadcast beacon packets out of all uplinks in a team. The physical switch is expected to forward all packets to other ports on the same broadcast domain. Hence, a team member is expected to see beacon packets from other team members. If an uplink fails to receive any beacon packets (actually 3 consecutive packets), VMkernel marks it bad. This failure can be due to immediate link or a downstream link failure.


In this scenario, VMkernel will be able to receive beacon probes for vmnic0 and vmnic1 adapter via below mentioned paths:
vmnic0 beacon will be received by vmnic1 via   AS-0 -> CS-0 -> AS-1 -> vmnic1
vmnic1 beacon will be received by vmnic0 via AS-1 -> CS-0 -> AS-0 -> vmnic0

But vmnic2 beacons will be dropped due to  link failures so it will be never be received by any of the other uplinks (vmnic0 and vmnic1) in a team.

Since ESXi Kernel didn’t receive beacon probes for vmnic2 so it will mark vmnic2 as DOWN and failover VM traffic to other surviving uplinks.

I hope you guys might have found this article useful. Please feel free to share your queries in comment box and i would be happy to answer your queries.

Keep sharing Keep Learning!!

In my next article, we will be discussing about VMware recommendations/Best practices to configure Beacon Probing as Failover Detection Method.





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VMware Solution Architect
I am VMware Solution Architect with 10+ Years of enriching experience in Datacenter Virtualization Technologies, Storage Area Networks and Software Defined Datacenter, Networking and Storage.
I hold Numerous certification including RHCE, CCNA, VCP4.0, VCP5.1, VCP5.5, vCloud and EMC certification.
While spending countless hours exploring the product inside and out and learning everything about it, Eventually I discovered my passion for teaching and helping others learn from my knowledge and experience so turned to Trainer cum Blogger to educate every single person keen to learn Virtualization.

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