VLT Domain Failure Scenarios

I did some investigation tonight in my Lab running OS 9.5.0.1 with VLT and taking down the VLTI and Heartbeat Links.  Here is what I found out in case your customers ask you "What happens when…?"

Scenario 1:  All link up

Results : All links from the end-devices to the 2 VLT switches are up.

Scenario 2: VLTI link between switches goes down but Heartbeat Link is up

Results: The End-devices link(s) to the Secondary switch will go down to prevent a Loop but continue to pass traffic through the Primary switch.  If there are any Non-VLT configured interfaces they will not be effected by the VLTI link going down and continue to pass traffic normally. The Heartbeat connection is still passing the hello packet.

Scenario 3: VLTI Link up but Heartbeat link goes down

Results: Everything continues to pass traffic normally without taking any interfaces down.  The reason is that the only thing that the Heartbeat link does is pass a 56 Byte hello Packet stating "I'm here".

Scenario 4: VLTI and Heartbeat Link go down but the switches are still up

Results: Each switch within the VLT Domain will become the Primary with the End-devices connection still up passing traffic as normal.  This is call a Split Brain scenario where there is a loop within the Network as the End-Devices are passing traffic down each interface to the switches within the VLT Domain.  This is a reason to have RSTP or PVST+ (future Klamath release) configured in conjunction to VLT so in case a Split Brain Scenario happens we can have some type of backup to help prevent loops within the Network.

So the bottom line is that we want the Customers to use the OOB interface to support the Heartbeat due to the fact that if they used the VLTI and Heartbeat as the same interconnection and it went down they would cause a Split Brain scenario and cause more issues within their Network.

JOHN BECK, Enterprise Technologist | Networking

Dell | Office of Technology & Architecture | CTO

Office #: (847) 515-5910

Cell / SMS #: (847) 207-6348

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E-mail: j_beck@Dell.com