Очевидно, что анализ трафика («сниффинг») ничего не «решает», но и ничего аномального не показал. Модель роутера указана в названии поста: hAP ac lite. Сейчас я пробовал точно такую же конфигурацию на HAP ac2 RBD52G (таком более мощном устройстве) — результат тот же. Также пытался переключиться на L2TP — результаты всё так же плачевные.
Случай довольно специфический: связь на той стороне очень плохая, но если переключиться на Windows VPN, связь становится работоспособной, а через туннель Mikrotik — никак нельзя пользоваться. Кстати, на WRT54G с DD-WRT тоже работает достаточно нормально.
Вот конфигурация, сгенерированная через веб-интерфейс (обычно я работаю с Cisco OS, с синтаксисом Mikrotik пока не знаком):
Локальная сеть: 192.168.88.0/24
Удалённая сеть: 192.168.1.0/24
На другой стороне Windows Server OS с встроенным VPN-сервером. Оба устройства находятся за NAT (работают с NAT-T).
[admin@MikroTik] > /export hide
# jun/03/2020 15:58:04 by RouterOS 6.43.10
# software id = M5C7-470P
#
# model = RBD52G-5HacD2HnD
# serial number = A6470ADB3269
/interface bridge
add admin-mac=C4:AD:34:06:DB:1E auto-mac=no comment=defconf name=bridge
/interface ethernet
set [ find default-name=ether1 ] advertise=10M-half,10M-full,100M-half,100M-full mac-address=C4:AD:34:06:DB:1D name=Internet speed=100Mbps
set [ find default-name=ether2 ] advertise=10M-half,10M-full,100M-half,100M-full mac-address=C4:AD:34:06:DB:1E speed=100Mbps
set [ find default-name=ether3 ] advertise=10M-half,10M-full,100M-half,100M-full mac-address=C4:AD:34:06:DB:1F speed=100Mbps
set [ find default-name=ether4 ] advertise=10M-half,10M-full,100M-half,100M-full mac-address=C4:AD:34:06:DB:20 speed=100Mbps
set [ find default-name=ether5 ] advertise=10M-half,10M-full,100M-half,100M-full mac-address=C4:AD:34:06:DB:21 speed=100Mbps
/interface l2tp-client
add connect-to=xxx.xxx.xxx.xxx dial-on-demand=yes disabled=no keepalive-timeout=disabled max-mru=1460 max-mtu=1460 name=l2tp-out1 use-ipsec=yes user=noone
/interface wireless
set [ find default-name=wlan1 ] name=wlan3 ssid=MikroTik
set [ find default-name=wlan2 ] name=wlan4 ssid=MikroTik
/interface pptp-client
add allow=mschap1,mschap2 connect-to=xxx.xxx.xxx.xxx keepalive-timeout=30 mrru=1500 name=pptp-out1 user=noone
/interface ethernet switch port
set 0 default-vlan-id=0
set 1 default-vlan-id=0
set 2 default-vlan-id=0
set 3 default-vlan-id=0
set 4 default-vlan-id=0
set 5 default-vlan-id=0
/interface list
add comment=defconf name=WAN
add comment=defconf name=LAN
/interface wireless security-profiles
set [ find default=yes ] supplicant-identity=MikroTik
add authentication-types=wpa2-psk management-protection=allowed mode=dynamic-keys name=WiFi supplicant-identity=MikroTik
/ip hotspot profile
set [ find default=yes ] html-directory=flash/hotspot
/ip ipsec peer profile
set [ find default=yes ] dh-group=modp1024 dpd-interval=disable-dpd enc-algorithm=aes-256,aes-128,3des
/ip ipsec proposal
set [ find default=yes ] auth-algorithms=sha256,sha1 enc-algorithms=aes-256-cbc,aes-192-cbc,aes-128-cbc,3des pfs-group=none
/ip pool
add name=default-dhcp ranges=192.168.88.10-192.168.88.254
/ip dhcp-server
add address-pool=default-dhcp disabled=no interface=bridge name=defconf
/ppp profile
set *0 change-tcp-mss=default
/interface bridge port
add bridge=bridge comment=defconf interface=ether2
add bridge=bridge comment=defconf interface=ether3
add bridge=bridge comment=defconf interface=ether4
add bridge=bridge comment=defconf interface=ether5
add bridge=bridge comment=defconf interface=*1
add bridge=bridge comment=defconf interface=*2
/ip neighbor discovery-settings
set discover-interface-list=LAN
/interface list member
add comment=defconf interface=bridge list=LAN
add comment=defconf interface=Internet list=WAN
/ip address
add address=192.168.88.1/24 comment=defconf interface=bridge network=192.168.88.0
/ip dhcp-client
add comment=defconf dhcp-options=hostname,clientid disabled=no interface=Internet
/ip dhcp-server network
add address=192.168.88.0/24 comment=defconf gateway=192.168.88.1
/ip dns
set allow-remote-requests=yes servers=8.8.8.8,8.8.4.4
/ip dns static
add address=192.168.88.1 comment=defconf name=router.lan
/ip firewall filter
add action=accept chain=input comment="defconf: accept established,related,untracked" connection-state=established,related,untracked
add action=drop chain=input comment="defconf: drop invalid" connection-state=invalid
add action=accept chain=input comment="defconf: accept ICMP" protocol=icmp
add action=accept chain=input comment="defconf: accept to local loopback (for CAPsMAN)" dst-address=127.0.0.1
add action=drop chain=input comment="defconf: drop all not coming from LAN" in-interface-list=WAN
add action=accept chain=forward comment="defconf: accept in ipsec policy" ipsec-policy=in,ipsec
add action=accept chain=forward comment="defconf: accept out ipsec policy" ipsec-policy=out,ipsec
add action=fasttrack-connection chain=forward comment="defconf: fasttrack" connection-state=established,related
add action=accept chain=forward comment="defconf: accept established,related,untracked" connection-state=established,related,untracked
add action=drop chain=forward comment="defconf: drop invalid" connection-state=invalid
add action=drop chain=forward comment="defconf: drop all from WAN not DSTNATed" connection-nat-state=!dstnat connection-state=new in-interface-list=WAN
/ip firewall mangle
add action=mark-routing chain=prerouting new-routing-mark=VPN passthrough=yes src-address=192.168.88.0/24
/ip firewall nat
add action=masquerade chain=srcnat out-interface=l2tp-out1
add action=masquerade chain=srcnat comment="defconf: masquerade" ipsec-policy=out,none out-interface-list=WAN
/ip route
add distance=1 dst-address=192.168.1.0/24 gateway=l2tp-out1 routing-mark=VPN
/ip traffic-flow
set cache-entries=16k
/system clock
set time-zone-name=Europe/Rome
/system leds
add interface=Internet leds="" type=interface-activity
add interface=ether2 leds="" type=interface-activity
add interface=ether3 leds="" type=interface-activity
add interface=ether4 leds="" type=interface-activity
add interface=ether5 leds="" type=interface-activity
/system logging
add topics=ipsec
/system ntp client
set enabled=yes primary-ntp=193.204.114.232 secondary-ntp=193.204.114.233
/system resource irq rps
set Internet disabled=no
set ether2 disabled=no
set ether3 disabled=no
set ether4 disabled=no
set ether5 disabled=no
/tool mac-server
set allowed-interface-list=LAN
/tool mac-server mac-winbox
set allowed-interface-list=LAN
/tool sniffer
set file-name=test filter-interface=Internet filter-ip-address=xxx.xxx.xxx.xxx/32
Есть какие идеи?