Technical Note : FortiGate IP route selection, and how to change the administrative distance (preference) of a routing protocol

[rmetzger]

Description

This article describes how the FortiGate selects routes in the routing table from the different routing protocols and how to change the route preference.

Scope

FortiGate.

Solution

How does it work:

• The same IP destination prefix may be learned from different routing protocols (ex: multiple default routes 0.0.0.0/0.0.0.0).

• The routing table will, however, contain only one entry (or more if they have been learned from the same protocol supporting ECMP and have the same “distance”. ECMP is currently applicable to static and OSPF).

• The following  shows the default distance (preference) settings on a FortiGate (configurable for all types except direct interfaces) :
• Directly connected     0
•  Static routes             10
• EBGP routes             20
• OSPF routes           110
• RIP routes              120
• IBGP routes            200
  
  
• Note 1: The term 'distance' is sometimes also referred to as 'weight' by other manufacturers.
• Note 2: the 'distance' has got only local significance on the FortiGate, and has no impact on a route redistribution.

Case study

• Assume that a default route is learned via BGP, and another default route is configured as static route. By default, only the static default route will be in the routing table.
• It can however be desired that the preferred default route be the one received by BGP, and the static route be kept as backup.
• In this case, it will be necessary to adapt the route distances.

Route distance seen from the GUI:

The distance of the routes can be seen also after executing below command, as below:

     # 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
     V - BGP VPNv4
     * - candidate default

     Routing table for VRF=0
     S* 0.0.0.0/0 [10/0] via 10.191.47.254, port1, [1/0]   <- Distance is 10
     C 10.112.0.0/20 is directly connected, port2
     C 10.191.32.0/20 is directly connected, port1

Solution
How to change the route distance : some examples

config router rip

    config distance

       edit 1

           set distance 130

       next

    end

end

---

config router static

    edit 1

        set device "internal"

        set distance 20

        set gateway 172.31.225.254

    next

end

---

config router ospf

   set distance-external 105

   set distance-intra-area 100

end

---

config router bgp  

   set distance-external 30

   set distance-internal 210

end

Note :

• New values are only given as examples.

• Distance changes might need a reboot or an  “exec router restart” to be effective.

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