Technical Tip : Routes to the same destination known by different protocols

[emmanouilg]

Description	This article describes the behavior of FortiGate when a route is known by two different protocols.
Scope	FortiGate.
Solution	When a route is known by different protocol routes (dynamic, static), only one protocol's route will be installed in the routing table even if Equal Cost Multi-Path (ECMP) is enabled. If a route is learned from two protocols with the same distance and metric, the route that will be installed is the latest one updated in the kernel. In the example below, a static route to 2.2.2.2/32 was configured with an Administrative Distance of 200, the same as the iBGP distance of 200. Only the static route was installed in the routing table. universe-esx55 # 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 10.109.31.254, port1 S 2.2.2.2/32 [200/0] is directly connected, port2 C 10.109.16.0/20 is directly connected, port1 C 10.119.0.0/20 is directly connected, port2 universe-esx55 # get router info routing-table database 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 > - selected route, * - FIB route, p - stale info Routing table for VRF=0 S *> 0.0.0.0/0 [10/0] via 10.109.31.254, port1 S *> 2.2.2.2/32 [200/0] is directly connected, port2 B 2.2.2.2/32 [200/0] via 10.119.1.24, port2, 00:01:44 C *> 10.109.16.0/20 is directly connected, port1 C *> 10.119.0.0/20 is directly connected, port2 universe-esx55 # show router static 2 config router static edit 2 set dst 2.2.2.2 255.255.255.255 set distance 200 set device "port2" next end By default, ECMP is enabled. If the route 2.2.2.2/32 is learned by two different static route entries with the same distance, then both will be installed in the routing table. A maximum of 255 paths can be installed for a particular destination. universe-esx55 # 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 10.109.31.254, port1 S 2.2.2.2/32 [10/0] is directly connected, port2                        [10/0] is directly connected, port3 C 10.109.16.0/20 is directly connected, port1 C 10.119.0.0/20 is directly connected, port2 To disable ECMP and enforce a maximum of one route per prefix, set ecmp-max-paths to 1.  config system setting set ecmp-max-paths 1 end It is recommended to leave ECMP enabled in most environments. See Administration Guide: Equal Cost Muti Path (v7.6.2).