1194 Configuring OSPF and OSPFv3
switchport mode trunk
exit
interface loopback 0
ip address 172.21.254.2 255.255.255.255
exit
exit

Discussion

With no area range cost specified, the range uses auto cost:
(ABR-R0) #show ip ospf range 1
Prefix Subnet Mask Type Action Cost Active
172.21.0.0 255.255.0.0 S Advertise Auto Y
(ABR-R0) #show ip ospf database summary
Network Summary States (Area 0.0.0.0)
LS Age: 644
LS options: (E-Bit)
LS Type: Network Summary LSA
LS Id: 172.21.0.0 (network prefix)
Advertising Router: 10.10.10.10
LS Seq Number: 0x80000002
Checksum: 0x8ee1
Length: 28
Network Mask: 255.255.0.0
Metric: 2
Min—The cost can be set to 0, the minimum value. OSPF re-advertises the summary LSA with a metric of 0:
(ABR-R0) (config-router)#area 1 range 172.21.0.0 255.255.0.0 summarylink advertise cost ?
<0-16777215> Set area range cost
(ABR-R0) (config-router)#area 1 range 172.21.0.0 255.255.0.0 summarylink
advertise cost 0
(ABR-R0) #show ip ospf range 1
Prefix Subnet Mask Type Action Cost Active
172.21.0.0 255.255.0.0 S Advertise 0 Y
(ABR-R0) #show ip ospf 0 database summary
Network Summary States (Area 0.0.0.0)