Some strangeness with CARP

From: Andrea Venturoli <>
Date: Sat, 12 Feb 2022 11:53:43 UTC

I've set up a network with CARP and I think I'm seeing something strange.

What follows is a simplified setup (the real one involves lagg and vlan, 
but this should not matter).

I have a Zyxel managed switch,
two "servers":
- A
- B
and two "clients"
- C
- D

Now let's add the "shared" CARP IP (vhid 1) to server A and 
server B and start sniffing on C and D.

If C or D talks with A or B using their own IP ( 
the other client does not see that traffic (as is to be expected on a 
switched network).
However if any client talks with the CARP IP ( every node on 
the LAN can sniff that traffic!

I tracked this down to the switch not learning the MAC address 
00:00:5e:00:01:01 (which is what CARP vhid 1 uses), so every outgoing 
packet is broadcast to the whole network.
Is this normal???

Changing to any other VHID (I tried 2, 4 and 10) does not show the same 
problem, as 00:00:5e:00:01:xx will show up in the switch MAC database.

I'm scrapping my head trying to find an explanation, but so far I could 
only think the switch is misbehaving.
Or am I missing some info and there's a reason for this?

  bye & Thanks