Integrated IS-IS Configuration on Cisco IOS

In my introduction to IS-IS, I explained the basics of IS-IS and how it works. In this lesson, I’ll show you how to configure Integrated IS-IS on a small network with four routers and two areas.

Configuration

Here is the topology we will use:

Above we have four routers. R1 and R2 are in area 12, R3 and R4 in area 34. R1 and R3 are intra-area routers so they will be configured as level 1 routers. R2 and R4 form the backbone, so these routers will be configured as level 1-2 routers.

Area 12

Let’s start with area 12. Instead of just showing you the configuration commands, we will also take a look at the different databases so you can see what is going on.

Level-1 Routers

Let’s start with R1. First, we have to start the IS-IS process and set a NET (Network Entity Title). We will keep it simple, the AFI will be 49, and the system ID will be 0000.0000.000X where X is the router number. Here’s R1:

R1(config)#router isis
R1(config-router)#net 49.0012.0000.0000.0001.00

R1 is an intra-area router, so we will configure it as a level-1 router. The default is level 1-2 on Cisco IOS routers, so this is something we have to change:

R1(config-router)#is-type level-1

By default, IS-IS will not show when a neighbor adjacency goes up or down on the console. I like to see this so let’s enable it:

R1(config-router)#log-adjacency-changes

The only thing left to do is to enable IS-IS on the interfaces:

R1(config)#interface GigabitEthernet 0/1
R1(config-if)#ip router isis

R1(config)#interface Loopback 0
R1(config-if)#ip router isis

Before we continue with R2, let’s take a look at the database of R1:

R1#show isis database 

IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R1.00-00            * 0x00000004   0x4240        907               0/0/0

Above we see a single LSP (Link State Packet). This is the LSP that R1 has generated when we enabled IS-IS. You can see a sequence number, checksum, and holdtime. Let’s take a look at the contents of this LSP:

R1#show isis database verbose 

IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R1.00-00            * 0x00000004   0x4240        870               0/0/0
  Area Address: 49.0012
  NLPID:        0xCC 
  Hostname: R1
  IP Address:   1.1.1.1
  Metric: 10         IP 192.168.12.0 255.255.255.0
  Metric: 10         IP 1.1.1.1 255.255.255.255

Above we see the contents of the LSP. There are two prefixes with the metrics and subnet masks.

Let’s continue with R2. This router will be a level 1-2 router since it’s connected to a different area. I’d like to show you the differences between a level 1 and level 1-2 router, so before we use R2 as a level 1-2 router, I’m going to configure it as a level-1 router first:

R2(config)#router isis
R2(config-router)#net 49.0012.0000.0000.0002.00
R2(config-router)#is-type level-1
R2(config-router)#log-adjacency-changes 

Don’t forget to enable the interfaces:

R2(config)#interface GigabitEthernet 0/1
R2(config-if)#ip router isis

R2(config)#interface Loopback 0
R2(config-if)#ip router isis

A few seconds later, you will see the neighbor adjacency appearing. This only shows up on the console because of the log-adjacency-changes command:

R1#
%CLNS-5-ADJCHANGE: ISIS: Adjacency to 0000.0000.0002 (GigabitEthernet0/1) Up, new adjacency

R2#
%CLNS-5-ADJCHANGE: ISIS: Adjacency to 0000.0000.0001 (GigabitEthernet0/1) Up, new adjacency

Excellent, we now have two neighbors. You can also verify this with the show isis neighbors command:

R1#show isis neighbors 

System Id       Type Interface     IP Address      State Holdtime Circuit Id
R2              L1   Gi0/1         192.168.12.2    UP    9        R2.01

R2#show isis neighbors 

System Id       Type Interface     IP Address      State Holdtime Circuit Id
R1              L1   Gi0/1         192.168.12.1    UP    22       R2.01

Let’s take another look at the database of R1:

R1#show isis database verbose 

IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R1.00-00            * 0x00000002   0x6DFB        1115              0/0/0
  Area Address: 49.0012
  NLPID:        0xCC 
  Hostname: R1
  Metric: 10         IS R2.01
  IP Address:   1.1.1.1
  Metric: 10         IP 1.1.1.1 255.255.255.255
  Metric: 10         IP 192.168.12.0 255.255.255.0
R2.00-00              0x00000002   0xA0BE        1113              0/0/0
  Area Address: 49.0012
  NLPID:        0xCC 
  Hostname: R2
  Metric: 10         IS R2.01
  IP Address:   2.2.2.2
  Metric: 10         IP 2.2.2.2 255.255.255.255
  Metric: 10         IP 192.168.12.0 255.255.255.0
R2.01-00              0x00000001   0x7CD6        1113              0/0/0
  Metric: 0          IS R2.00
  Metric: 0          IS R1.00

Above we now also see the LSP from R2 in the database of R1. We can see two prefixes with the metrics and subnet masks.

Network 2.2.2.2/32 was unknown to R1, so this will be installed in the routing table:

R1#show ip route isis 

Gateway of last resort is not set

      2.0.0.0/32 is subnetted, 1 subnets
i L1     2.2.2.2 [115/20] via 192.168.12.2, 00:03:18, GigabitEthernet0/1

Above we see 2.2.2.2/32 in the routing table of R1. The administrative distance of IS-IS is 115, and the total metric is 20. You can see that the level (L1) also shows up in the routing table. Let’s take a look at the database of R2:

R2#show isis database verbose 

IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R1.00-00              0x00000002   0x6DFB        1014              0/0/0
  Area Address: 49.0012
  NLPID:        0xCC 
  Hostname: R1
  Metric: 10         IS R2.01
  IP Address:   1.1.1.1
  Metric: 10         IP 1.1.1.1 255.255.255.255
  Metric: 10         IP 192.168.12.0 255.255.255.0
R2.00-00            * 0x00000002   0xA0BE        1017              0/0/0
  Area Address: 49.0012
  NLPID:        0xCC 
  Hostname: R2
  Metric: 10         IS R2.01
  IP Address:   2.2.2.2
  Metric: 10         IP 2.2.2.2 255.255.255.255
  Metric: 10         IP 192.168.12.0 255.255.255.0
R2.01-00            * 0x00000001   0x7CD6        1017              0/0/0
  Metric: 0          IS R2.00
  Metric: 0          IS R1.00

Above we see the LSP of R1 with its two prefixes: 1.1.1.1/32 and 192.168.12.0/24. 1.1.1.1/32 was unknown to R2, so this will be installed in the routing table:

R2#show ip route isis

      1.0.0.0/32 is subnetted, 1 subnets
i L1     1.1.1.1 [115/20] via 192.168.12.1, 00:03:16, GigabitEthernet0/1

There it is.

hostname R1
!
ip cef
!
interface Loopback0
 ip address 1.1.1.1 255.255.255.255
 ip router isis 
!
interface GigabitEthernet0/1
 ip address 192.168.12.1 255.255.255.0
 ip router isis 
!
router isis
 net 49.0012.0000.0000.0001.00
 is-type level-1
 log-adjacency-changes
!
end

hostname R2
!
ip cef
!
interface Loopback0
 ip address 2.2.2.2 255.255.255.255
 ip router isis 
!
interface GigabitEthernet0/1
 ip address 192.168.12.2 255.255.255.0
 ip router isis 
!
interface GigabitEthernet0/2
 ip address 192.168.24.2 255.255.255.0
!
router isis
 net 49.0012.0000.0000.0002.00
 is-type level-1
 log-adjacency-changes
!
end

Level 1-2 Router

R2 is supposed to connect to area 34 so it has to become a level 1-2 router. I’d like to show you the differences in the database when we change R2 from level 1 to level 1-2. To do this, I will shut the interface that connects to R1 and we will clear the IS-IS neighbor adjacency:

R2(config)#interface GigabitEthernet 0/1
R2(config-if)#shutdown

Clearing the process manually will speed things up. Otherwise, you have to wait until the hold time has expired:

R2#clear isis *

Let’s change the level:

R2(config)#router isis
config-router)#is-type ?
  level-1       Act as a station router only
  level-1-2     Act as both a station router and an area router
  level-2-only  Act as an area router only

Let’s go for level 1-2:

R2(config-router)#is-type level-1-2 

Let’s take a look at the databases:

R2#show isis database 

IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R2.00-00            * 0x00000002   0x96EE        1187              0/0/0
IS-IS Level-2 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R2.00-00            * 0x00000001   0x98ED        1187              0/0/0

The first thing we see is that R2 now creates two databases; one for level 1 and another for level 2. Let’s take a look at the level 1 database:

R2#show isis database level-1 verbose 

IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R2.00-00            * 0x00000002   0x96EE        1170              0/0/0
  Area Address: 49.0012
  NLPID:        0xCC 
  Hostname: R2
  IP Address:   2.2.2.2
  Metric: 10         IP 2.2.2.2 255.255.255.255

Above we see the LSP that R2 created for level 1. There is only one prefix (2.2.2.2/32) because I shut the GigabitEthernet 0/1 interface. Let’s check the level 2 database:

R2#show isis database level-2 verbose 

IS-IS Level-2 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R2.00-00            * 0x00000001   0x98ED        1167              0/0/0
  Area Address: 49.0012
  NLPID:        0xCC 
  Hostname: R2
  IP Address:   2.2.2.2
  Metric: 10         IP 2.2.2.2 255.255.255.255

The level 2 database is the same, R2 generated an LSP with the 2.2.2.2/32 prefix in it. Let’s enable the GigabitEthernet 0/1 interface so that R1 and R2 form a level 1 neighbor adjacency:

R2(config)#interface GigabitEthernet 0/1
R2(config-if)#no shutdown

Wait for a few seconds until the neighbor adjacency is established, then check the level 2 database:

R2#show isis database level-2 verbose 

IS-IS Level-2 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R2.00-00            * 0x00000005   0xAB24        1122              0/0/0
  Area Address: 49.0012
  NLPID:        0xCC 
  Hostname: R2
  IP Address:   2.2.2.2
  Metric: 10         IP 2.2.2.2 255.255.255.255
  Metric: 10         IP 192.168.12.0 255.255.255.0
  Metric: 20         IP 1.1.1.1 255.255.255.255

Above we now see that R2 has added 1.1.1.1/32 in its own LSP in the level 2 database. R2 learns about 1.1.1.1/32 from R1’s level 1 LSP and adds this prefix in its own level 2 database.

When we now look at the routing table of R1, you will only see the 2.2.2.2/32 prefix:

R1#show ip route isis

      2.0.0.0/32 is subnetted, 1 subnets
i L1     2.2.2.2 [115/20] via 192.168.12.2, 00:23:02, GigabitEthernet0/1

Once R2 is connected to another area, you will find a default route here.

Area 34

Let’s continue with our configuration. First, we will configure R3 and R4 so that they form a level 1 neighbor adjacency. Let’s start with R3:

R3(config)#router isis
R3(config-router)#net 49.0034.0000.0000.0003.00
R3(config-router)#is-type level-1
R3(config-router)#log-adjacency-changes  

Don’t forget to enable IS-IS on the interfaces:

R3(config)#interface GigabitEthernet 0/1
R3(config-if)#ip router isis

R3(config)#interface Loopback 0
R3(config-if)#ip router isis   

And here’s R4:

R4(config)#router isis
R4(config-router)#net 49.0034.0000.0000.0004.00    
R4(config-router)#log-adjacency-changes 

Add the interfaces:

R4(config)#interface GigabitEthernet 0/1
R4(config-if)#ip router isis

R4(config)#interface Loopback 0
R4(config-if)#ip router isis

Let’s verify that R3 and R4 are neighbors:

R3#show isis neighbors 

System Id       Type Interface     IP Address      State Holdtime Circuit Id
R4              L1   Gi0/1         192.168.34.4    UP    8        R4.01 

This is looking good.

Area 12-34 connectivity

Now it’s time to connect area 12 and area 34 to each other. Before we do, let’s take a quick look at the level 2 databases of R2 and R4 so that you can see the difference later: