How to block specific subnet not to use SD-WAN Rule rule if the BGP link is down.

[KKM_forti]

Hi all, 

This is a DR site so the internet traffic goes via DC not directly via WAN.  This has sd-wan rule for internet traffic

Let's say... 

My FGT  is learning 192.168.1.x/28 (in real its public IP range ) subnet from external BGP. Whenever this link goes down. Fortigate starts using the default route and tries to reach 192.168.1.x even though it's a Public IP but it's not reachable via the internet. it should use the same BGP link to reach.  When the BGP link is back up the sessions are attached with the WAN link and each time I have to manually clear the session then only it starts working. 

1. Blackhole route for 192.168.1.x -- to drop the traffic when BGP is down but SD-WAN rule will have a preference so it will never hit that blackhole route I guess?

Any suggestion...

Thank you!

[ap]

Hi @KKM_forti,

With blackhole route configured for 192.168.1.x/28 subnet with AD 254 : When BGP neighbroship goes down, routing lookup will point towards blackhole route (due to longest prefix match ). So, It will not use default route to reach the destination in this subnet and traffic will be blackholed.

Regards,
AP

[KKM_forti]

Hi @ap 

Thanks for your reply. The only thing to consider is that it has an SD-WAN rule for internet traffic / 0.0.0.0/0, so I am a little confused about it.  Will it check the routing table or SD-WAN rule first?   This makes sense " routing lookup will point towards blackhole route (due to longest prefix match )"

Thank you!