ISIS redistribution from L2 to L1
Posted on April 12, 2010
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.
Got something to say?