Getting CARP to broadcast on a different interface

[David DeSimone]

One of the purposes of the CARP announcements is to announce the location of the virtual mac address to the upstream switch fabric.  Since CARP uses a virtual mac that floats between multiple ports, you need to have the CARP master continually assert that its particular port is the target that should be used for delivery of packets to the virtual MAC address.  Without this function, switches might still mistakenly deliver their frames to the standby node.

The CARP announcements are also helpful in detecting and routing around some odd failure scenarios, such as a failure within the upstream fabric, where the master sees link on its port, but can't actually send frames that reach the rest of the network.  If the standby can't hear the master's announcements any more, it can promote itself to master and hopefully keep your cluster online.  This would not happen without the announcement feature.

I would hope you could explain this to your provider and get them to white-list CARP announcements because they are defeating important safety features you wish to use.


-----Original Message-----
From: owner-freebsd-net at freebsd.org [mailto:owner-freebsd-net at freebsd.org] On Behalf Of Niklaas Baudet von Gersdorff
Sent: Wednesday, June 08, 2016 7:43 AM
To: freebsd-questions at freebsd.org; freebsd-net at freebsd.org
Subject: Getting CARP to broadcast on a different interface

Hello,

is it possible to configure CARP in such a way that it sends its
broadcasts on an interface different from the one that gets the shared
IP address assigned? Unfortunately, my provider blocks broadcast and
multicast on public interfaces of virtual machines.

However, they offer to set up an additional virtual NIC that directly
connects multiple virtual machines on which broadcast and multicast are
not blocked. So, while I assign a shared IP to the public interface
vtnet0, I would like to configure CARP to broadcast on the private
interface vtnet1.

Is that possible? Or are there alternatives for CARP that support this
function?

    Niklaas

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