Troubleshooting Tip: To prefer the specific OSPF route over EBGP

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Description
This article describes how to prefer specific OSPF route over BGP.

Useful links:

Fortinet Documentation
Fortinet KB – https://kb.fortinet.com/kb/documentLink.do?externalID=FD30086
https://help.fortinet.com/cli/fos60hlp/60/index.htm#FortiOS/fortiOS-cli-ref/config/router/bgp.htm

- FORTIGATE2 is advertising 0.0.0.0/0, 10.174.0.0/20 and 10.175.0.0/20 routes to FORTIGATE1 via EBGP.

- Also, FORTIGATE3 is advertising the same routes 0.0.0.0/0, 10.174.0.0/20 and 10.175.0.0/20 to FORTIGATE1 via OSPF. 
- FORTIGATE1 will install all 3 routes received from EBGP as EBGP has administrative distance of 20 which is less than administrative distance of OSPF which is 110.
- However, the requirement is that only default route should be preferred via OSPF and rest two routes 10.174.0.0/20 & 10.175.0.0/20 should be preferred via EBGP.

- All BGP routes are active in the routing table.
FORTIGATE1 # get router info routing-table all

Routing table for VRF=0

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

B*      0.0.0.0/0 [20/0] via 10.171.12.5, port1, 01:06:39
B       10.174.0.0/20 [20/0] via 10.171.12.5, port1, 01:13:00
B       10.175.0.0/20 [20/0] via 10.171.12.5, port1, 01:12:31

-As per routing database, all OSPF routes are inactive

FORTIGATE1 # get router info routing-table database
Routing table for VRF=0

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

B    *> 0.0.0.0/0 [20/0] via 10.171.12.5, port1, 01:06:49
O E2    0.0.0.0/0 [110/10] via 10.172.12.49, port3, 01:08:13
O E2    10.174.0.0/20 [110/6] via 10.172.12.49, port3, 01:11:33
B    *> 10.174.0.0/20 [20/0] via 10.171.12.5, port1, 01:13:10
O E2    10.175.0.0/20 [110/6] via 10.172.12.49, port3, 01:11:33
B    *> 10.175.0.0/20 [20/0] via 10.171.12.5, port1, 01:12:41

Solution
To achieve the requirement that only default route should be preferred via OSPF and rest two routes 10.174.0.0/20 & 10.175.0.0/20 should be preferred over EBGP, on FORTIGATE1, increase EBGP Administrative Distance more than 110 only for 0.0.0.0/0 using below steps.

-Create the Access-list only for Default-Route with exact-match enable

#config router access-list
 edit "DEFAULT_ROUTE"
        set comments "MATCH ONLY DEFAULT ROUTE"
        config rule
            edit 1
                set prefix 0.0.0.0 0.0.0.0
                set exact-match enable
            next
        end
    next
end

-Increase Distance for default route to 120 under admin-distance for EBGP neighbor 10.171.12.5

#config router bgp
    config admin-distance
        edit 1
            set neighbour-prefix 10.171.12.5 255.255.255.255
            set route-list "DEFAULT_ROUTE"
            set distance 120
        next
    end
end

-Soft Clear the BGP neighbor

#execute router clear bgp ip 10.171.12.5 soft

RESULTS:
-As per routing table only default route is preferred via OSPF and rest two routes 10.174.0.0/20 & 10.175.0.0/20 are preferred via EBGP.
 
FORTIGATE1 # get router info routing-table all
Routing table for VRF=0

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

O*E2    0.0.0.0/0 [110/10] via 10.172.12.49, port3, 01:45:25
B       10.174.0.0/20 [20/0] via 10.171.12.5, port1, 01:50:22
B       10.175.0.0/20 [20/0] via 10.171.12.5, port1, 01:49:53

- Routing database is showing the default route via EBGP with modified Distance 120 and via OSPF with default distance 110.
FORTIGATE1 # get router info routing-table database
Routing table for VRF=0

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

B       0.0.0.0/0 [120/0] via 10.171.12.5, 00:03:36
O E2 *> 0.0.0.0/0 [110/10] via 10.172.12.49, port3, 01:49:00
O E2    10.174.0.0/20 [110/6] via 10.172.12.49, port3, 01:52:20
B    *> 10.174.0.0/20 [20/0] via 10.171.12.5, port1, 01:53:57
O E2    10.175.0.0/20 [110/6] via 10.172.12.49, port3, 01:52:20
B    *> 10.175.0.0/20 [20/0] via 10.171.12.5, port1, 01:53:28