As you know there are 2 types of levels in ISIS Level 1 and level 2. A router can be in one of these 2 types of level or can be a L1/L2 router (kind of ABR).
A router L1/L2 set an ATT bit to indicate to routers in L1 a default gateway, whereas a L2 router will have all the more specific route.

In some case you’d like to have all (or a subset) of the L2 routes in L1. To do this you can redistribute from L2 to L1.

In this 3 router topology, R1 is the L1/L2, R2 the L1 and R3 the L2.
Here are our routing tables

R1(config-if)#do sh ip route
Gateway of last resort is not set

      1.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        1.1.1.0/24 is directly connected, Loopback0
L        1.1.1.1/32 is directly connected, Loopback0
      2.0.0.0/24 is subnetted, 1 subnets
i L1     2.2.2.0 [115/20] via 45.1.12.2, 00:04:24, GigabitEthernet1/0
      3.0.0.0/24 is subnetted, 1 subnets
i L2     3.3.3.0 [115/20] via 45.1.13.3, 00:03:47, GigabitEthernet2/0
      11.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        11.11.11.0/24 is directly connected, Loopback1
L        11.11.11.11/32 is directly connected, Loopback1
      22.0.0.0/24 is subnetted, 1 subnets
i L1     22.22.22.0 [115/20] via 45.1.12.2, 00:04:24, GigabitEthernet1/0
      33.0.0.0/24 is subnetted, 1 subnets
i L2     33.33.33.0 [115/20] via 45.1.13.3, 00:03:49, GigabitEthernet2/0
      45.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
C        45.1.12.0/24 is directly connected, GigabitEthernet1/0
L        45.1.12.1/32 is directly connected, GigabitEthernet1/0
C        45.1.13.0/24 is directly connected, GigabitEthernet2/0
L        45.1.13.1/32 is directly connected, GigabitEthernet2/0

R2-L1(config-router)#do sh ip route
Gateway of last resort is 45.1.12.1 to network 0.0.0.0

i*L1  0.0.0.0/0 [115/10] via 45.1.12.1, 00:04:02, GigabitEthernet1/0
      1.0.0.0/24 is subnetted, 1 subnets
i L1     1.1.1.0 [115/20] via 45.1.12.1, 00:03:59, GigabitEthernet1/0
      2.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        2.2.2.0/24 is directly connected, Loopback0
L        2.2.2.2/32 is directly connected, Loopback0
      11.0.0.0/24 is subnetted, 1 subnets
i L1     11.11.11.0 [115/20] via 45.1.12.1, 00:03:39, GigabitEthernet1/0
      22.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        22.22.22.0/24 is directly connected, Loopback1
L        22.22.22.22/32 is directly connected, Loopback1
      45.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        45.1.12.0/24 is directly connected, GigabitEthernet1/0
L        45.1.12.2/32 is directly connected, GigabitEthernet1/0

R3-L2(config-if)#do sh ip route
Gateway of last resort is not set

      1.0.0.0/24 is subnetted, 1 subnets
i L2     1.1.1.0 [115/20] via 45.1.13.1, 00:00:55, GigabitEthernet1/0
      2.0.0.0/24 is subnetted, 1 subnets
i L2     2.2.2.0 [115/30] via 45.1.13.1, 00:00:55, GigabitEthernet1/0
      3.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        3.3.3.0/24 is directly connected, Loopback0
L        3.3.3.3/32 is directly connected, Loopback0
      11.0.0.0/24 is subnetted, 1 subnets
i L2     11.11.11.0 [115/20] via 45.1.13.1, 00:00:35, GigabitEthernet1/0
      22.0.0.0/24 is subnetted, 1 subnets
i L2     22.22.22.0 [115/30] via 45.1.13.1, 00:00:55, GigabitEthernet1/0
      33.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        33.33.33.0/24 is directly connected, Loopback1
L        33.33.33.33/32 is directly connected, Loopback1
      45.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
i L2     45.1.12.0/24 [115/20] via 45.1.13.1, 00:00:56, GigabitEthernet1/0
C        45.1.13.0/24 is directly connected, GigabitEthernet1/0
L        45.1.13.3/32 is directly connected, GigabitEthernet1/0

You can see that R2 is missing the routes from R3 but it has a default gateway to reach them

Now lets redistribute, the configuration is pretty straightforward, you just need a command on the L1/L2 router

R1(config-router)#redistribute isis ip level-2 into level-1 route-map PERMIT_ALL

You may have noticed that I specified a route map even though I wanted to redistribute all routes, this is because it’s mandatory to insert a route-map or a prefix-list.

R1(config-router)#redistribute isis ip level-2 into level-1 ?
  distribute-list  select routes
  route-map        Route map for route matching

Now that’s done let’s check our R2 routing table.

R2-L1(config-router)#do sh ip route
Gateway of last resort is 45.1.12.1 to network 0.0.0.0

i*L1  0.0.0.0/0 [115/10] via 45.1.12.1, 00:03:46, GigabitEthernet1/0
      1.0.0.0/24 is subnetted, 1 subnets
i L1     1.1.1.0 [115/20] via 45.1.12.1, 00:15:25, GigabitEthernet1/0
      2.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        2.2.2.0/24 is directly connected, Loopback0
L        2.2.2.2/32 is directly connected, Loopback0
      3.0.0.0/24 is subnetted, 1 subnets
i ia     3.3.3.0 [115/158] via 45.1.12.1, 00:00:15, GigabitEthernet1/0
      11.0.0.0/24 is subnetted, 1 subnets
i L1     11.11.11.0 [115/20] via 45.1.12.1, 00:15:05, GigabitEthernet1/0
      22.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        22.22.22.0/24 is directly connected, Loopback1
L        22.22.22.22/32 is directly connected, Loopback1
      33.0.0.0/24 is subnetted, 1 subnets
i ia     33.33.33.0 [115/158] via 45.1.12.1, 00:00:18, GigabitEthernet1/0
      45.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
C        45.1.12.0/24 is directly connected, GigabitEthernet1/0
L        45.1.12.2/32 is directly connected, GigabitEthernet1/0
i ia     45.1.13.0/24 [115/148] via 45.1.12.1, 00:00:18, GigabitEthernet1/0

Here we are our routes from R3 show up as inter-area (ia) which is expected because of the net id I set.

R2-L1(config-router)#do sh run | i net 49.
 net 49.0002.0000.0000.2222.00

R3-L2(config-if)#do sh run | i net 49.
 net 49.0001.0000.0000.3333.00

It’s as simple as that.