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:

is-is area 12 34 topology

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.

If you look closely, you can see a third entry in the output above (R2.01-00). This is about the DIS that creates a pseudonode. It is a similar mechanism to OSPF’s DR/BDR. We will cover this in another lesson.

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.

Configurations

Want to take a look for yourself? Here you will find the configuration of R1 and R2 as level 1 routers.

R1

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

R2

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:

We're Sorry, Full Content Access is for Members Only...

If you like to keep on reading, Become a Member Now! Here is why:

  • Learn any CCNA, CCNP and CCIE R&S Topic. Explained As Simple As Possible.
  • Try for Just $1. The Best Dollar You've Ever Spent on Your Cisco Career!
  • Full Access to our 651 Lessons. More Lessons Added Every Week!
  • Content created by Rene Molenaar (CCIE #41726)

568 Sign Ups in the last 30 days

satisfaction-guaranteed
100% Satisfaction Guaranteed!
You may cancel your monthly membership at any time.
No Questions Asked!

Forum Replies

  1. Hi Rene,
    Thx for your very nice article as always :slight_smile: If there is multiple Level1-2 Router on a area then due to default Route asymmetric Routing will be occurred. I saw your topic about Route leaking but I think its not scalable more . So is there any other scaleble option here ??

  2. Hello René,
    Thank you a lot for this lesson. But I don’t see this command circuit-type level anywhere in the course. Normal?

  3. Hello Staut

    Yes, this is correct.

    Laz

  4. Hello Stuat

    Yes, if two routers are set to be L1-L2, and they are in the same area, they will only establish an L1 neighborship because they detect that they are in the same area.

    I hope this has been helpful!

    Laz

  5. Thank you! Just wanted to make sure that it’s not necessarily to set is-type level1 command if routers are in the same area.

7 more replies! Ask a question or join the discussion by visiting our Community Forum