diff --git a/tests/sys/netpfil/pf/fragmentation.sh b/tests/sys/netpfil/pf/fragmentation.sh
index 943c76e478f3..1be6a3bc17cd 100755
--- a/tests/sys/netpfil/pf/fragmentation.sh
+++ b/tests/sys/netpfil/pf/fragmentation.sh
@@ -1,123 +1,172 @@
 # $FreeBSD$
 
 . $(atf_get_srcdir)/utils.subr
 
 atf_test_case "too_many_fragments" "cleanup"
 
 too_many_fragments_head()
 {
 	atf_set descr 'IPv4 fragment limitation test'
 	atf_set require.user root
 }
 
 too_many_fragments_body()
 {
 	pft_init
 
 	epair=$(vnet_mkepair)
 	vnet_mkjail alcatraz ${epair}a
 
 	ifconfig ${epair}b inet 192.0.2.1/24 up
 	jexec alcatraz ifconfig ${epair}a 192.0.2.2/24 up
 
 	ifconfig ${epair}b mtu 200
 	jexec alcatraz ifconfig ${epair}a mtu 200
 
 	jexec alcatraz pfctl -e
 	pft_set_rules alcatraz \
 		"scrub all fragment reassemble"
 
 	# So we know pf is limiting things
 	jexec alcatraz sysctl net.inet.ip.maxfragsperpacket=1024
 
 	# Sanity check
 	atf_check -s exit:0 -o ignore ping -c 1 192.0.2.2
 
 	# We can ping with < 64 fragments
 	atf_check -s exit:0 -o ignore ping -c 1 -s 800 192.0.2.2
 
 	# Too many fragments should fail
 	atf_check -s exit:2 -o ignore ping -c 1 -s 20000 192.0.2.2
 }
 
 too_many_fragments_cleanup()
 {
 	pft_cleanup
 }
 
 atf_test_case "v6" "cleanup"
 v6_head()
 {
 	atf_set descr 'IPv6 fragmentation test'
 	atf_set require.user root
 	atf_set require.progs scapy
 }
 
 v6_body()
 {
 	pft_init
 
 	epair_send=$(vnet_mkepair)
 	epair_link=$(vnet_mkepair)
 
 	vnet_mkjail alcatraz ${epair_send}b ${epair_link}a
 	vnet_mkjail singsing ${epair_link}b
 
 	ifconfig ${epair_send}a inet6 2001:db8:42::1/64 no_dad up
 
 	jexec alcatraz ifconfig ${epair_send}b inet6 2001:db8:42::2/64 no_dad up
 	jexec alcatraz ifconfig ${epair_link}a inet6 2001:db8:43::2/64 no_dad up
 	jexec alcatraz sysctl net.inet6.ip6.forwarding=1
 
 	jexec singsing ifconfig ${epair_link}b inet6 2001:db8:43::3/64 no_dad up
 	jexec singsing route add -6 2001:db8:42::/64 2001:db8:43::2
 	route add -6 2001:db8:43::/64 2001:db8:42::2
 
 	jexec alcatraz ifconfig ${epair_send}b inet6 -ifdisabled
 	jexec alcatraz ifconfig ${epair_link}a inet6 -ifdisabled
 	jexec singsing ifconfig ${epair_link}b inet6 -ifdisabled
 	ifconfig ${epair_send}a inet6 -ifdisabled
 
 	jexec alcatraz pfctl -e
 	pft_set_rules alcatraz \
 		"scrub fragment reassemble" \
 		"block in" \
 		"pass in inet6 proto icmp6 icmp6-type { neighbrsol, neighbradv }" \
 		"pass in inet6 proto icmp6 icmp6-type { echoreq, echorep }"
 
 	# Host test
 	atf_check -s exit:0 -o ignore \
 		ping6 -c 1 2001:db8:42::2
 
 	atf_check -s exit:0 -o ignore \
 		ping6 -c 1 -s 4500 2001:db8:42::2
 
 	atf_check -s exit:0 -o ignore\
 		ping6 -c 1 -b 70000 -s 65000 2001:db8:42::2
 
 	# Forwarding test
 	atf_check -s exit:0 -o ignore \
 		ping6 -c 1 2001:db8:43::3
 
 	atf_check -s exit:0 -o ignore \
 		ping6 -c 1 -s 4500 2001:db8:43::3
 
 	atf_check -s exit:0 -o ignore\
 		ping6 -c 1 -b 70000 -s 65000 2001:db8:43::3
 
 	$(atf_get_srcdir)/CVE-2019-5597.py \
 		${epair_send}a \
 		2001:db8:42::1 \
 		2001:db8:43::3
 }
 
 v6_cleanup()
 {
 	pft_cleanup
 }
 
+atf_test_case "mtu_diff" "cleanup"
+mtu_diff_head()
+{
+	atf_set descr 'Test reassembly across different MTUs, PR #255432'
+	atf_set require.user root
+}
+
+mtu_diff_body()
+{
+	pft_init
+
+	epair_small=$(vnet_mkepair)
+	epair_large=$(vnet_mkepair)
+
+	vnet_mkjail first ${epair_small}b ${epair_large}a
+	vnet_mkjail second ${epair_large}b
+
+	ifconfig ${epair_small}a 192.0.2.1/25 up
+	jexec first ifconfig ${epair_small}b 192.0.2.2/25 up
+
+	jexec first sysctl net.inet.ip.forwarding=1
+	jexec first ifconfig ${epair_large}a 192.0.2.130/25 up
+	jexec first ifconfig ${epair_large}a mtu 9000
+	jexec second ifconfig ${epair_large}b 192.0.2.131/25 up
+	jexec second ifconfig ${epair_large}b mtu 9000
+	jexec second route add default 192.0.2.130
+
+	route add 192.0.2.128/25 192.0.2.2
+
+	jexec first pfctl -e
+	pft_set_rules first \
+		"scrub all fragment reassemble"
+
+	# Sanity checks
+	atf_check -s exit:0 -o ignore ping -c 1 192.0.2.2
+	atf_check -s exit:0 -o ignore ping -c 1 192.0.2.130
+	atf_check -s exit:0 -o ignore ping -c 1 192.0.2.131
+
+	# Large packet that'll get reassembled and sent out in one on the large
+	# epair
+	atf_check -s exit:0 -o ignore ping -c 1 -s 8000 192.0.2.131
+}
+
+mtu_diff_cleanup()
+{
+	pft_cleanup
+}
+
 atf_init_test_cases()
 {
 	atf_add_test_case "too_many_fragments"
 	atf_add_test_case "v6"
+	atf_add_test_case "mtu_diff"
 }