Bigip : connection mirroring

Scenario
There are two loadbalancers which are setup as a redudant pair which provides a default simple virtual server (with pool).
Let’s say you’d setup a connection towards this virtual server, and afterwards there’dd be a failover.

What happens to the connection that was setup to the load balancer that was setup?

  • The connection is being migrated to the other load balancer.
  • The connection remains as it was, directed to the “failed” load balancer.
  • The connection is terminated/reset (by the BigIP).

It might be a surprise to some, that the correct answer is the second one. The connection remains “as it was”. Off course it won’t be functional; Yet there will be no fail over of this connection by default, or will the connection be terminated/reset by the BigIP
When (and how) the connection will be reset depends solely on the client!

So one might ask: “Why doesn’t the BigIP send a fin/rst to the client?”
Another question might answer this: “How would the BigIP be able to send the fin/rst packet as it failed?” A failover occurs when the unit isn’t accessible anymore (If it got disconnected from the network, crashed… etc). It wouldn’t be able to send this packet.

There is however a mechanism that does a fail over to the other BigIP. But there is a (performance) trade off involved. This mechanism is called “connection mirroring”. So the state of all the connections made to the active BigIP are also kept on the standby BigIP.

HOWTO
You can enable the “connection mirroring” thru :
– the command line (“man virtual”) : “b virtual *name* mirror conn enable”
– the GUI (virtual server -> advanced) : GUI Screenshot

So if you REALLY need it, you can use the option, yet be aware of the performance degradation that’ll cause.

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