OSPF ECMP and Route Prio

[AlexW]

Hi,

In a static you have the priority option to favor traffic over a specific route in a ECMP situation. In OSPF you have the priority command, but this is used for calculation of the DR/BDR.

I was wondering if there is a way to favor traffic in OSPF (in ECMP situation) the same way as a static ?

Regards, Alex

[rwpatterson]

Have you looked into the cost? Standard OSPF allows you to weight routes by cost. There should be a similar mechanism in the 40Gate.

[AlexW]

With cost you don't have a situation that both routes are loaded/active. So if traffic is arriving on an interface which does not have an route back you could end up with asymmetric routing. This does not have to be a problem because the fortigate is statefull, and traffic is send back via the originated interface. But it would be nice if you could set this the same way you do with a static. From the kernel routing table; OSPF routes; (same cost and distance) tab=254 vf=3 scope=0 type=1 proto=11 prio=0 0.0.0.0/0.0.0.0/0->10.XX.YY.0/23 pref=0.0.0.0         gwy=10.10.20.1 flag=04 hops=0 oif=xx(GRE)         gwy=10.10.30.1 flag=04 hops=0 oif=yy(IPSEC) static routes; (same cost and distance, but with priority set) tab=254 vf=3 scope=0 type=1 proto=11 prio=1 0.0.0.0/0.0.0.0/0->10.XX.YY.0/23 pref=0.0.0.0 gwy=10.10.20.1 dev=xx(GRE) tab=254 vf=3 scope=0 type=1 proto=11 prio=2 0.0.0.0/0.0.0.0/0->10.XX.YY.0/23 pref=0.0.0.0 gwy=10.10.30.1 dev=yy(IPSEC) Static route; edit 1         set dst 10.XX.YY.0/23         set priority 1         set device "GRE"     next edit 1         set dst 10.XX.YY.0/23         set priority 2         set device "IPSEC"     next

Regards, Alex

[kallbrandt]

I don't know if this is possible to set in OSPF in a Fortigate, but I think not.

There is no real need for it, since the default setting in the Fortigate RPF (Reverse Path Forwarding) check is "loose" - So that the packet will find its way back even though it doesn't use the best route, it just need a route. Setting the RPF check to "strict" will make the Fortigate drop all traffic that doesn't use the BEST route back to the source.

But I understand your point, control is nice...

[rwpatterson]

By the simple definition, Equal Cost Multi Path, you use whatever is available, no difference in route value.

My two cents

[AlexW]

Thanks for the answers !

I've found that when i use the priority command on the OSPF interface traffic seems to favor the interface with lowest value given. I also found out that this is not always 100% accurate as sometimes traffic still seems to follow the other route. But most of the times it follows the correct route, so for now i'm happy.

regards, Alex