Cisco CCNP and CCIE Study Notes

Consider two routers connected back-to-back to each other via their serial interfaces. Here is the config from the two routers

Router1

interface Loopback0
 ip address 1.1.1.1 255.255.255.0
!        
interface Serial2
 ip address 10.10.10.1 255.255.255.0
 ip router isis
!
router isis 1
 net 49.0100.1111.1111.1111.00
 passive-interface Loopback0

Router2

interface Loopback0
 ip address 2.2.2.2 255.255.255.255
!        
interface Serial2
 ip address 10.10.10.2 255.255.255.0
 ip router isis
!
router isis 1
 net 49.0100.2222.2222.2222.00
 passive-interface Loopback0

The two routers fail to form an IS-IS neighbor adjacency. What is the problem with the configuration?