Technical Tip: BGP recursive resolution not possible via another BGP route

sagha · ‎03-09-2021

Description

This article explains that recursive resolution is not possible for a BGP route via another BGP route.

These routes are being learned via BGP but are not getting installed into the active routing table.

FGT # get router info bgp network 10.10.10.0
Paths: (2 available, no best path)
Local
10.20.1.4 (inaccessible) from 10.21.16.15 (10.21.116.20)
Local
10.20.1.4 (inaccessible) from 10.21.16.16 (10.21.116.20)

The underlying route for the next hop which 10.20.1.4, have another BGP route.

FGT # get router info routing-table details 10.20.1.4
Routing entry for 10.20.1.0/28
Known via "bgp", distance 200, metric 0, best
Last update 07w2d18h ago
* 10.21.116.28 (recursive via 10.21.161.21)

The issue faced is because the route to reach the next hop 10.20.1.4 is also a BGP route.

FortiGate considers the next hop as invalid and does not install the route in the routing table.

FGT # get router info routing-table details 10.10.10.0
% Network not in table

Solution

The alternatives to fix this issue are as follows:

1) Add a static route to reach the next hop that overrides the BGP route.

2) Configure the 'set next-hop-self enable' under the BGP neighbour.

# config router bgp
    # config neighbor
        edit "x.x.x.x"
            set next-hop-self enable
        next
    end
end

If there is already a static route present, the next-hop-self command would not take precedence over it.

3) When a BGP next hop requires recursive resolution, the default behavior is to consider all other routes except BGP routes. In FortiOS v7.0.0 the following option has been introduced, when enabled, allows the recursive next hop resolution to use BGP routes as well.

# config router bgp
set recursive-next-hop [ enable | disable* ]
end

By default, this option is disabled.