Technical Tip: Configuring FortiGate to Use Its Interface IP Address as the Forwarder Address in OSPF Type 5 LSA

Configuration Steps:

1. Create a Prefix List to Match Specific Routes:

Define a prefix list to identify the specific routes for which the forwarder address will be modified:

config router prefix-list
    edit "FWD_ADDR_LIST"
        config rule
            edit 1
                set prefix 172.26.1.0 255.255.255.192
                unset ge
                unset le
            next
        end
    next
end

2. Configure a Route Map to Set the Forwarder Address:

Create a route map to modify the next-hop IP address for the matched prefixes:

config router route-map
    edit "FWD_ADDR_CHANGE"
        config rule
            edit 1
                set match-ip-address "FWD_ADDR_LIST"
                set set-ip-nexthop 172.16.30.1
            next
            edit 2
            next
        end
    next
end

3. Enable Redistribution of BGP into OSPF.

First, configure the FortiGate to redistribute BGP routes into OSPF:

config router ospf
    config redistribute "bgp"
        set status enable
        set routemap "FWD_ADDR_CHANGE"
    end
end

Verification:
After applying the changes, verify the Type 5 LSA database on a receiving router or FortiGate to confirm the forwarder address has been updated.

Before Applying the Configuration.
Run the following command on the receiving router:

get router info ospf database external lsa | grep 172.26.1.0 -A 10 -B 5

Example output:

AS External Link States

LS age: 39
Options: 0x2 (*|-|-|-|-|-|E|-)
LS Type: AS-external-LSA
Link State ID: 172.26.1.0 (External Network Number)
Advertising Router: 172.16.30.1
LS Seq Number: 8000002d
Checksum: 0x0f12
Length: 36
Network Mask: /26
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 10
Forward Address: 10.10.1.2
External Route Tag: 0

After Applying the Configuration:
Run the same command again to observe the updated forwarder address:

get router info ospf database external lsa | grep 172.26.1.0 -A 10 -B 5

Example output:

AS External Link States

LS age: 23
Options: 0x2 (*|-|-|-|-|-|E|-)
LS Type: AS-external-LSA
Link State ID: 172.26.1.0 (External Network Number)
Advertising Router: 172.16.30.1
LS Seq Number: 8000002e
Checksum: 0xbe9c
Length: 36
Network Mask: /26
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 10
Forward Address: 172.16.30.1
External Route Tag: 0

Explanation of Changes:

1. Before Configuration: The forwarder address in the Type 5 LSA is the next-hop IP address of the route learned via BGP (10.10.1.2).
2. After Configuration: The forwarder address is updated to the advertising FortiGate's interface IP address (172.16.30.1).