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

[akileshc]

Description	This article describes that in OSPF, Type 5 LSAs (AS-external-LSA) are used to advertise external routes into the OSPF domain. When a route is redistributed into OSPF from BGP, the default behavior is to set the next-hop IP address of the route learned via BGP as the forwarder address in the Type 5 LSA. In some scenarios, it might be preferable to configure FortiGate to advertise its own interface IP address as the forwarder address. This article provides details and configuration steps to achieve this.
Scope	FortiGate.
Solution	Configuration Steps: 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 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 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: 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). After Configuration: The forwarder address is updated to the advertising FortiGate's interface IP address (172.16.30.1).