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dangelov
dangelov
Visitor III
February 26, 2025
Question

ECMP with OSPF

  • February 26, 2025
  • 1 reply
  • 1538 views

Hi all,

Do I need to configure/enable ECMP for OSPF or does it work by default? I know that it needs to be enabled for BGP.

I have the below scenario with OSPF between all 4 firewalls.

Currently using statics for 3 to get to 4 and 4 to 3, as there are 2 equal cost paths.

If I remove the statics will the Fortigates be load balancing over the 2 paths or do I need to enable something on ECMP or OSPF? I was worried about asymmetric routing.  

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1 reply

Toshi_Esumi
SuperUser
Toshi_Esumi
SuperUser
February 26, 2025

If you're concerning about routes between FGT3 and FGT4, at least path1: 3-2-4 and path2:3-1-2-4 have different total costs. So both sides use path1 to take the shortest path. And it's automatic by OSFP protocol.

Toshi

dangelov
dangelovAuthor
Visitor III
February 26, 2025

Toshi_Esumi thank you for your reply.

For router 3 path 3-1-4 equals path 3-2-4 and

for router 4, 4-1-3 and 4-2-3 are equal, so they will appear in the routing table. Currently using statics with link monitor, but I do not like statics and would be nice to load balance.

Thanks for answering my question.

Is there a documentation that explains the usage of the different ECMP modes?

Toshi_Esumi
SuperUser
Toshi_Esumi
SuperUser
February 26, 2025

There is no "different ECMP modes". But need to enable RFC1583 compatibility.
https://community.fortinet.com/t5/FortiGate/Technical-Tip-OSPFv2-ECMP-for-External-Routes-Expected-Behavior/ta-p/216048

Toshi

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