common: Added route prefer order (cf. bird doc)

etc/local/bird/common/local.conf

@@ -10,3 +10,13 @@ router id 80.67.168.5;
 
 # AS number
 define my_as = 20766;
+
+# Prefer route with the highest Local Preference attribute.
+# Prefer route with the shortest AS path.
+# Prefer IGP origin over EGP and EGP origin over incomplete.
+# Prefer the lowest value of the Multiple Exit Discriminator.
+# Prefer routes received via eBGP over ones received via iBGP.
+# Prefer routes with lower internal distance to a boundary router.
+# Prefer the route with the lowest value of router ID of the advertising router.
+#
+#prefer older on;