UPDATE the FreeBSD Handbook has been updated.
Here is a quick post regarding my struggle with CARP while upgrading a server from FreeBSD-9.2 to FreeBSD-10.
The first problem I encountered was loading the CARP kernel module. It turns out it has been renamed carp.ko from if_carp.ko.
So enable carp by adding the following line to /boot/loader.conf
carp_load="YES"
The next hangup I ran into was the carp syntax change.
You no longer define carp interfaces with ifconfig_carp. Instead you create a interface alias.
Here is an example of a working rc.conf
ifconfig_em1="inet 38.111.159.78/32"
ifconfig_em2="inet 38.111.159.78/32"
ifconfig_em1_alias0="vhid 12 advskew 210 pass RD0B4OBZ 192.168.1.11/32"
ifconfig_em1_alias1="vhid 12 advskew 210 pass RD0B4OBZ 192.168.1.12/32"
ifconfig_em2_alias0="vhid 12 advskew 210 pass RD0B4OBZ 192.168.2.11/32"
ifconfig_em2_alias1="vhid 12 advskew 210 pass RD0B4OBZ 192.168.2.12/32"
Note.
The alias must be numbered starting with 0 for each interface. The alias number must be sequential, skipping numbers will result in the interface not being created.
Alternatively we could setup the interfaces using ifconfig_<interface>_aliases property.
ifconfig_em1_aliases="\
vhid 12 advskew 210 pass RD0B4OBZ 192.168.1.11/32 \
vhid 12 advskew 210 pass RD0B4OBZ 192.168.1.12/32 \
\