Starting IS-IS routing protocol without CCNP training

The routing protocol Intermediate System to Intermediate System (IS-IS) is an advanced and robust link-state protocol used in many service provider networks. Most of the other enterprises I know of prefer to use protocols like OSPF or EIGRP instead of IS-IS. Therefore, enterprise administrators might not be familiar to this protocol at all.

Here I want to share my short note to start IS-IS quickly without digging into protocol details. In case you must configure and maintain an IS-IS network, this note might save you some time.

You can follow these five steps to start IS-IS routing protocol in short time.

Step 1: Prepare a pool of IPv4 addresses as non-overlapping Router-IDs

Router ID is a unique identifier of any single router. Identifiers are just unique numbers. We know one citizen should have one and only one Citizen Identifier. No two citizens share the same identifier. This is the same for router IDs.

No matter we are starting a production network or we just want to practice in lab, my recommendation is to always start from this step. The earlier we do this, the lesser time we might waste afterwards.

In addition, I also recommend reserving a pool of IPv4 addresses just for router IDs. This pool should not overlap with any other network addresses. Any host route (/32) in this pool is for a single router. For easier discussion, I assume we reserve 10.0.0.0/16 for router IDs. Router 1 (R1) is assigned router ID: 10.0.0.1/32, and Router 2 (R2) is assigned router ID: 10.0.0.2/32, and so on.

This unique host address is not just for identifying a single router. We can use this address for management protocols such as SSH, SNMP, and SSL. We can even add DNS mapping so we don’t have to remember the IPv4 address. For example, R1.MyDomain.COM can be mapped to 10.0.0.1. When I am about to manage a router on command line, all I do is starting a SSH to R1.MyDomain.COM, like “ssh admin@R1.MyDomain.COM”.

Step 2: Pick a unique Area ID for Level 1

Here I want to emphasis starting from Level 1 (First Floor, Ground Floor) area. My recommendation is always start from Level 1. Expands to Level 2 only when necessary (most of the time we never need Level 2).

Area ID is a unique number within 0000 to FFFF in hexadecimal (or 0~65,535 in decimal).

If you really want, Area ID Zero (0000) is also a legal IS-IS area number. Area 0000 in
IS-IS is just a normal Level 1 area. This is quite different from OSPF. Because we might easily confuse this area with the special OSPF Area Zero (Backbone Area), I recommend avoid using this area number at all.

Many connected routers are grouped into a single area. All routers in the same area should be assigned with the same Area ID.

For easier discussion, I assume we use Area 7 for Level 1.

Step 3: Compose Network Entity Title (NET) for every router

Network Entity Title (NET) is really an awkward name for many network administrators. It’s just a name used in ISO documents that define IS-IS protocol. It is the format IS-IS protocol recognizes as Router-ID.

You are correct, NET must also be unique, and we must convert the router IDs in Step 1 into this NET format. The question now is “How”.

I learned on Cisco Live an easy trick to convert unique IPv4 address into a unique NET. Here you are.

First, expand the four decimal numbers of IPv4 address to 3 digits. For example,

10.0.0.1 -> 010.000.000.001.

Now we have a 12-digit string. Then, we just see this number as hexadecimal in digits, and reposition the “dots” to separate into 3 parts instead of 4. For example,

010.000.000.001 -> 0100.0000.0001

In case you really want to know, this converted number “0100.0000.0001” is called System ID in IS-IS protocol. We will need this number again when we are expanding the IS-IS network to Level 2 connected topology.

Finally, we can create NET now.

NET is in a format of 49.[Area ID].[System ID].00.
The NET for R1 is now “49.0007.0100.0000.0001.00”.

Step 4: Start IS-IS on every router

We can start IS-IS protocol on every router with the following partial commands.

router isis
 net 49.0007.0100.0000.0001.00
 is-type level-1
 metric-style wide
Interface loopback 999999
! This interface is for easier management only. IS-IS doesn’t need it.
 ip address 10.0.0.1 255.255.255.255
 ipv6 address fd00::1/128
 ip router isis
 ipv6 router isis

I purposely neglect the explanation of every components of NET.

Step 5: Enable IS-IS on interfaces.

It is simpler than you might expect. We look at the network map and every connected interface of every connected router should be enabled with IS-IS protocol, like this example.

interface Ethernet0/0
 no shutdown
 ip router isis
 ipv6 router isis

That’s all. Folks!


One more thing…

In the partial configuration example above, “interface loopback 999999” is only for easier management. IS-IS doesn’t need this interface at all. We can safely skip it for plain IS-IS practice.

We can even assign illegal IPv4 address such as 0.0.0.1 or 0.0.0.2 as router IDs for easier typing IS-IS lab practices.

In ISO documents, they don’t call routers as “routers”. They call routers the “Intermediate Systems” instead. Therefore, IS-IS protocol is exactly a protocol for “routers to routers”. Straightforward, isn’t it!

Sunset at Gongguan Waterfront Plaza (公館水岸廣場)
Taipei City, Taiwan