Bird 1.6: OSPF reference bandwidth?

[Cybertinus]

Hello all,

I maintain a network with 4 Bird 1.6 routers in it. The 4 routers are 
connected in a square like this:
+-------+        +-------+
|       |        |       |
| dc1-1 +--------+ dc2-1 |
|       |        |       |
+---+---+        +---+---+
     |                |
     |                |
     |                |
+---+---+        +---+---+
|       |        |       |
| dc1-2 +--------+ dc2-2 |
|       |        |       |
+-------+        +-------+

Each link participates in OSPF Area 0. Until last weekend all links were 
1 gbit/sec, and all was fine and OSPF did it's thing. Last weekend we 
upgraded the top link (between dc1-1 and dcg2-1) to 10 gbit/sec, the 
other 3 links are still 1 gbit/sec. OSPF should take this increased 
bandwidth into account. But it can only do that if the reference 
bandwidth is configured correctly.
To what is the reference bandwidth set in Bird 1.6 and how can I tweak 
this? I've checked 
https://bird.network.cz/?get_doc&v=16&f=bird-6.html#ss6.8 but I couldn't 
find which option defines the reference bandwidth. Or should I define a 
cost for each interface now manually, so it gives the 10 gbit/sec link a 
10 times lower cost than the other 3?

Kind regards,
Cybertinus