Technical Tip: Use BGP Weight attribute to prefer default route received from neighbor over 'capability-default-originate' route

[ppatel]

Description
This article describes how to use BGP Weight attribute to prefer default route received from BGP neighbor over the default route originated by 'capability-default-originate' command in BGP.

When a FortiGate is receiving a default route from BGP neighbor 11.11.11.143, enabling 'capability-default-originate' for neighbor 100.1.1.141 will cause the FortiGate to stop displaying default route (learnt from neighbor 11.11.11.143) in the routing table.

Solution
- Post enabling 'capability-default-originate' for neighbor 100.1.1.141, the default route is locally originated in the BGP routing table.
- By default, BGP Weight attribute is set to 32768 for FortiGate locally originated prefixes.
- Weight is only locally significant in the FortiGate where it is configured, so for the routes received from the BGP neighbors, the weight attribute value is always 0.
- In the BGP best route selection criteria, weight is the first attribute to be checked.
- BGP prefers the routes with the highest Weight, hence default route originated using 'capability-  default-originate' command is preferred as its weight is 32768 which is higher than default route received from neighbor 11.11.11.143 which is 0.
- To prefer the default route from the BGP neighbor 11.11.11.143, apply a route-map matching default route with weight more than 32768.

1) Create a Prefix set for default route.

# config router prefix-list
    edit "acl-143"
            # config rule
                edit 1
                    set prefix 0.0.0.0 0.0.0.0
                    unset ge
                    unset le
                next
            end
    next
end

2) Create route-map matching default prefix-list and set the weight value to 40000.

# config router route-map
    edit "143-in"
            # config rule
                edit 1
                    set match-ip-address "acl-143"
                    set set-weight 40000
                next
            end
    next
end

3) Apply the route-map against neighbor 11.11.11.143.

# config router bgp
    set as 142
        # config neighbor
            edit "100.1.1.141"
                set capability-default-originate enable
                set next-hop-self enable
                set soft-reconfiguration enable
                set remote-as 141
            next
            edit "11.11.11.143"
                set next-hop-self enable
                set soft-reconfiguration enable
                set remote-as 143
                set route-map-in "143-in"
            next

4) Check the weight value.

vm142 # get router info bgp network 0.0.0.0
BGP routing table entry for 0.0.0.0/0
Paths: (2 available, best 0000002, table Default-IP-Routing-Table)
  Advertised to non peer-group peers:
   100.1.1.141
  143
    11.11.11.143 from 11.11.11.143 (172.18.18.143)
      Origin incomplete metric 0, localpref 100, valid, external
      Last update: Mon Jun 16 14:59:32 2014

  Local
    0.0.0.0 from 0.0.0.0 (172.18.18.142)
      Origin IGP, localpref 100, weight 32768, valid, sourced, local, best
      Last update: Mon Jun 16 14:59:31 2014

5) Still, Weight 40000 is not set against default route received from neighbor 11.11.11.143 though route-map is applied. For route-map to come into effect soft clear the BGP neighbor.

# execute router clear bgp ip 11.11.11.143 soft

6) Now, weight 40000 applied for the default route received from neighbor 11.11.11.143. Hence displaying as valid & best.

vm142 # get router info bgp network 0.0.0.0
BGP routing table entry for 0.0.0.0/0
Paths: (2 available, best 1, table Default-IP-Routing-Table)
  Advertised to non peer-group peers:
   100.1.1.141
  143
    11.11.11.143 from 11.11.11.143 (172.18.18.143)
      Origin incomplete metric 0, localpref 100, weight 40000, valid, external, best
      Last update: Mon Jun 16 15:09:31 2014

  Local
    0.0.0.0 from 0.0.0.0 (172.18.18.142)
      Origin IGP, localpref 100, weight 32768, valid, sourced, local
      Last update: Mon Jun 16 15:09:30 2014

7) Routing table is displaying the default route from neighbor 11.11.11.143.

vm142 # 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

B* 0.0.0.0/0 [20/0] via 11.11.11.143, port1, 00:02:17
C 11.11.11.0/24 is directly connected, port1
C 100.1.1.0/24 is directly connected, port10
C 172.18.18.0/24 is directly connected, port6

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