FreeBSD 10.2 , ospf vs. aggregated static routes, performance issue

[Evgeny Khorokhorin]

Hi,

I have 10.2-STABLE, 2 CPU Intel E5-2643v3, network Intel XL710 with 
1.4.0 driver from Intel
I know that going through routing table is very fast (rn_match). But I 
decided to optimize routing table.
I'm using 2 interfaces - ixl0 and ixl1.
Behind ixl0 I have 304 networks 172.16.. from /28 to /24 all via the 
same gw 1.1.1.1 (because ip on ixl0 with /30 mask). And behind ixl1 I 
have default route via 2.2.2.2.
That 304 172.16 networks I receive via OSPF (quagga). Now all is OK - on 
every interface I have up to 500kpps/395kpps, 4.5Gbps/1.57Gbps (rx/tx on 
ixl1 and tx/rx on ixl0).
If I disable OSPF and in zebra add static route 172.16.0.0/12 via 
1.1.1.1, the system works good until traffic grow up to 251kpps/181kpps 
, 2.27Gbps/637Mbps. After that the system is degrading: ixl's queue 
threads utilizes 100% CPU and I see many many traffic drops (netstat -i)
If I turn on ospfd and receive 304 more specific routes the problem 
disappears.

Where is the problem? Or I have misunderstanding about how FreeBSD uses 
routing table..
P.S. I use this machine as NAT. I checked this on ipfw and pf, all the same.

-- Cheers,
Evgeny