| When the link monitor is inactive, it can remove the failed link from the routing table. This article shows the background when there is no default route on the routing table and how probing still occurs. The following examples are provided below.
Config done for PORT3 and set the server to 8.8.8.8.
FGT # show sys link-monitor
config system link-monitor
edit "1"
set srcintf "port3"
set server "8.8.8.8"
set gateway-ip 10.47.15.254
set update-policy-route disable
next
end By default, update-static-route is enabled to remove the static route or default route on the routing table once the set server is not reachable. FGT# show full-configuration system link-monitor config system link-monitor edit "1" set srcintf "port3" set server "8.8.8.8" set protocol ping set gateway-ip 10.47.15.254 set update-static-route enable <------ set status enable next end The routing table shows the default route via port3. FGT # get router info routing-table all
Codes: K - kernel, C - connected, S - static, R - RIP, B - BGP
O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default Routing table for VRF=0
S* 0.0.0.0/0 [10/0] via 10.47.31.254, port3, [3/0] <----- Default route.
C 10.47.0.0/20 is directly connected, port3
C 10.47.16.0/20 is directly connected, port1
C 10.232.0.0/22 is directly connected, port2
FGT # diagnose sniffer packet port3 'host 8.8.8.8' 4 0 l <----- Testing probing for the interface, it is possible to see the request and reply packet as the link monitor is active.
Using Original Sniffing Mode
interfaces=[port3]
filters=[host 8.8.8.8]
2022-09-12 15:11:05.111151 port3 -- 10.47.2.106 -> 8.8.8.8: icmp: echo request
2022-09-12 15:11:05.113606 port3 -- 8.8.8.8 -> 10.47.2.106: icmp: echo reply
2022-09-12 15:11:05.614420 port3 -- 10.47.2.106 -> 8.8.8.8: icmp: echo request
2022-09-12 15:11:05.617194 port3 -- 8.8.8.8 -> 10.47.2.106: icmp: echo reply
2022-09-12 15:11:06.117835 port3 -- 10.47.2.106 -> 8.8.8.8: icmp: echo request
2022-09-12 15:11:06.119121 port3 -- 8.8.8.8 -> 10.47.2.106: icmp: echo reply
2022-09-12 15:11:06.612113 port3 -- 10.47.2.106 -> 8.8.8.8: icmp: echo request
2022-09-12 15:11:06.614613 port3 -- 8.8.8.8 -> 10.47.2.106: icmp: echo reply
2022-09-12 15:11:07.115256 port3 -- 10.47.2.106 -> 8.8.8.8: icmp: echo request
2022-09-12 15:11:07.117122 port3 -- 8.8.8.8 -> 10.47.2.106: icmp: echo reply
2022-09-12 15:11:07.617935 port3 -- 10.47.2.106 -> 8.8.8.8: icmp: echo request
2022-09-12 15:11:07.620604 port3 -- 8.8.8.8 -> 10.47.2.106: icmp: echo reply
^C
14 packets received by filter
0 packets dropped by kernel
Changing the health-check to a non-working server. FGT# config sys link-monitor
FGT (link-monitor) # edit 1
FGT (1) # set server 4.4.4.4 <----- Non-working server.
FGT (1) # end
FGT # get router info routing-table all
Codes: K - kernel, C - connected, S - static, R - RIP, B - BGP
O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default
Routing table for VRF=0<----- Route associated with port3 is removed.
C 10.47.0.0/20 is directly connected, port3
C 10.47.16.0/20 is directly connected, port1
C 10.232.0.0/22 is directly connected, port2 To check the link-monitor status: FGT# diagnose sys link-monitor status The below justifies that even when the static route is associated with the port, the FortiGate interface will still send probe to the server. FGT # diagnose sniffer packet port3 'icmp' 4 0 l <-----Testing probing for the port3 interface, it is only possible to see request packets.
Using Original Sniffing Mode
interfaces=[port3]
filters=[icmp]
2022-09-12 15:12:12.846581 port3 -- 10.47.2.106 -> 4.4.4.4: icmp: echo request
2022-09-12 15:12:13.347184 port3 -- 10.47.2.106 -> 4.4.4.4: icmp: echo request
2022-09-12 15:12:13.847876 port3 -- 10.47.2.106 -> 4.4.4.4: icmp: echo request
2022-09-12 15:12:14.348557 port3 -- 10.47.2.106 -> 4.4.4.4: icmp: echo request
2022-09-12 15:12:14.839245 port3 -- 10.47.2.106 -> 4.4.4.4: icmp: echo request
2022-09-12 15:12:15.339899 port3 -- 10.47.2.106 -> 4.4.4.4: icmp: echo request
2022-09-12 15:12:15.840569 port3 -- 10.47.2.106 -> 4.4.4.4: icmp: echo request
2022-09-12 15:12:16.341178 port3 -- 10.47.2.106 -> 4.4.4.4: icmp: echo request
2022-09-12 15:12:16.841901 port3 -- 10.47.2.106 -> 4.4.4.4: icmp: echo request
2022-09-12 15:12:17.342536 port3 -- 10.47.2.106 -> 4.4.4.4: icmp: echo request
^C
10 packets received by filter
0 packets dropped by kernel Correcting the server to 8.8.8.8. FGT # config sys link-monitor
FGT (link-monitor) # edit 1
FGT (1) # set server 8.8.8.8
FGT (1) # end
The default route associated with Port3 is added back to the routing table.
FGT # get router info routing-table all
Codes: K - kernel, C - connected, S - static, R - RIP, B - BGP
O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default Routing table for VRF=0
S* 0.0.0.0/0 [10/0] via 10.47.31.254, port3, [3/0] <----- Route is added back.
C 10.47.0.0/20 is directly connected, port3
C 10.47.16.0/20 is directly connected, port1
C 10.232.0.0/22 is directly connected, port2 | 
