V-Stack Split Brain


We have two X670 stacks, A and B using V-Stack ports 47 and 48 on the switches. Both these stacks A and B are connected to each via LAG ports 1:38,1:39,2:38,:2:39. We are also using MLAG from these stacks to the our top of rack switches.
Last night we have a small issue were we lost both cables between our stack A,splitting the stack up "Split Brain" When this happened the LAG between stack A and stack B broke.

What I would like to understand in this case when there is a split brain and the lag goes down will the switch forward traffic on ports 1:38,1:39,2:38,:2:39 like normal ports. Yes I configure all my vlans on are these ports, good thing or not...

Also what happens in a "Split Brain" will the two switches that are now discounted from each other work as normal standalone switches and will stack B think its connected to two switches via the LAG ports 1:38,1:39,2:38,:2:39.


3 replies

Userlevel 2

About the Split-Brain question. I understand you have a stack made of 2 switches.

In a stack, only Master-Capable switches can become Master. This is configurable, and using easy-setup the default is to have 2 Master-Capable (Master & Backup). Which, in a stack made of 2, means that every switch is Master-Capable.

In such a situation, when a split-brain situation occurs, both switch will act as the Master. You have, then, twice the same config, with same IP, and that can be a problem for your network. If, for some reason, the second switch is not Master-Capable, it will reboot and act as a standalone switch.

Regarding the LAG question, are you using LACP or not? Without it, the LAG would still work imho, as only a link Up is enough. That would be bad. That's why LACP is far better to use. With LACP, the Control Plane can detect something is wrong and prevent the LAG to come Up. This is what happened, so I assume you have LACP. In that case, no traffic flow through the ports.

Please read the procedure when you rebuild the stack, to be sure of what can be the impact.
Hi Stephane,

We have four X670-V switches two in a stack, each stack is connected to each other via LAG LACP.

Re the stacking if both switches are enabled for master and the stacking for some reason breaks both switches t become active and they DONT reboot, I understand this.

But once you connect or fix the stacking cables again switch 2 in the stack reboots, is this normal?

Yes re the LACP this is enabled.

Userlevel 2

yes, this is the expected behavior.