Introduction to IS-IS

In OSPF the SPF calculation is split into 3 phases (intra-area, inter-area and external).

I can’t find any information to see if IS-IS does a similar thing for Level1/2/external?

Hello Chris

I was unable to find a definitive reference in Cisco documentation or otherwise concerning the methodology of how the SPF algorithm is implemented as far as IS-IS level hierarchy is concerned. However, from the various piece of info I was able to glean from, it does seem to follow the same logic as OSPF. The SPF algorithm is run within each Level 1 area as a separate process from the SPF that is run between Level 2 devices.

I hope this has been helpful!


Hi Rene,
In this sentence “As you can see above, R4 has learned about the prefix from R1 and copies this prefix from the LSP in the level 1 database to its own LSP in the level 2 database.” , I think there is a error, R4 has learned about the prefix from R3 not R1.

Hello Eduardo

Yes, you are correct, thanks for catching that. I’ll let Rene know to correct it.


Thanks Eduardo, that’s a typo indeed. Just fixed it!

You’re welcome Rene.

Dear Rene,

Based on the lesson, “Once a level 1-2 router is connected to another area, it will set a special bit in its level 1 LSP called the attached bit. When a level 1 router sees this, it will generate a default route that is pointed to the level 1-2 router.”

What happens if more than one level 1-2 router is connected to different areas, how will level-1 routers know which path to take?


Hello Fadi

In Rene’s statement, what he is saying is that when a level 1-2 router connects to another area, it will send level 1 LSPs to all of its level 1 neighbors in the same area.

Now according to this Cisco Documentation, it says:

A Level-2 IS that indicates that it has one or more Level-2 neighbors in other areas may be used by Level-1 devices in the same area as the path of last resort, also called the default route. The Level-2 IS indicates its attachment to other areas by setting an attached bit (ATT) in its Level-1 LSP 0.

Also the RFC 1142 for IS-IS states that:

…the Level 2 Decision Process informs the Level 1 Update Process of the values of the default routeing metric to and ID of the partition designated level 2 Intermediate system in each other partition of this area.

In other words, each L2 router will provide a metric for the default route, which will be used to determine which one will become the default route.

I hope this has been helpful!


hi Guys
please anybody can tell me a scenario that explain every section in IS IS routing protocol
I mean All of the possible things can be happen in IS IS be on the scenario
sorry guys if my english is not good
i just need a scenario that explain everything in IS IS

Hello Alil

Take a look at this section of the site:

It contains all of the information concerning the IS-IS protocol. You can go through this material, and whatever questions you have, we’ll be here to help you out!

I hope this has been helpful!


hello Laz
thanks man but i have problem to understand DIS / PSN
i cant get that
can you explain it to me more simple
and please tell me what is TLV ? i cant get it too
and the last question is how i can show default ISIS timers interval
like lsp-refresh-interval or max life , ETC
thanks a lot dude

Hello Alil

IS-IS requires a full mesh neighbor adjacency to function. Each node must be able to communicate with every other node and create an adjacency to share routing information. Routers will flood their Link State Packets (LSPs) to a multicast destination, and all other routers will receive this LSP and add it to their databases. Once LSPs are received, there are no acknowledgements sent back to verify to the sender that the other routers have received the LSPs.

Another problem is the fact that the link state database grows exponentially with respect to the number of routers in the system.

To solve the acknowledgement and the exponential database size problems, IS-IS introduced a special mechanism.

When routers become neighbors, they perform an election to elect the Designated Intermediate System (DIS). Remember that for IS-IS, the term IS is just another name for a router. Now the DIS is responsible for creating a Pseudonode (PSN). This is something like a virtual device with which all other IS-IS routers create an adjacency with. This reduces the number of adjacencies and it solves the problem of acknowledgement.

More information and in more detail can be found at this lesson: IS-IS DIS and Pseudonode. If you have any specific questions after going through the lesson, let us know and we’ll help you through it.

The Type Length Value (TLV) is used to carry information in LSPs in order to make IS-IS extendable. TLVs can carry various kinds of information making IS-IS more versitile and flexible. More information about this can be found at this Cisco Documentation:

You can find the default values at the following Cisco command reference links:

I hope this has been helpful!


Hello laz
thank you very much i got that
but i will be more thankful if you explain me what is attach and over bit in isis

Hello Alil

The attached bit is a particular flag that exists within the LSP. This bit is set to 0 for LSPs that are sent from L1 routers.

In IS-IS, an L1 router will never learn about prefixes in other areas. They will only know about their own area. L1-2 routers however will learn about the prefixes in other areas. So, whenever L1-2 routers send LSPs to L1 router, they will set the attached bit in their level 1 LSPs to 1, thus informing L1 routers that they are “attached” to another area. As a response to the attached bit, an L1 will generate a default route towards the L1-2 router, since they represent “a way out” of the local area.

I hope this has been helpful!


Thank You
how i can read about it int detail?
i need to write a document about it
and can you tell about Overload bit? what is this now?
i am so sorry if i bother you and ask Q a lot
but i need help and coudnt find a good DOC

Hello Alil

No need to apologize for the many questions, that’s what we’re here for, so if you have more questions, please feel free to ask!

In the last post I forgot to write about the overload bit, I apologize!! The overload bit is a special bit in the LSP that is used to inform the network that the advertising router is not yet ready to forward transit traffic. A transit router is any router that transmits traffic between any two other routers. This bit was initially intended for times when there is too much network congestion, or when there is a resource shortage (CPU, memory) on a specific router. This is typically done temporarily, until congestion resides, or resources are no longer being taxed. It is possible to manually set the overload bit so that a router can signal to other routers not to use it as a transit hop in their SPF calculations. This is quite convenient if you want to isolate a specific router in the network before maintenance.

If you want to find out more about the in-depth understanding of the IS-IS protocol, the best place to go is the official description of the protocol. IS-IS was originally developed by ISO/IEC as standard 10589 and has been most recently updated to ISO/IEC 10589:2002. This document cannot be downloaded for free unfortunately, but the IETF have published RFC7142 as a republication of the ISO standard, and this is free. It has since been designated “historic” so it is not the official description (and it is a bit old) but it does have a description of the majority of the operations and mechanisms that you will require.

I hope this has been helpful!


Hello laz
Thank you A lot:heart::heart:

1 Like

There is an equal question in the forum but I still don’t understand how the attached bit works?
please correct me and answer the following statements

  • The attached bit is set to 0 for LSPs sent to and from level 1 routers?
  • The attached bit is set to 1 for LSPs sent to and from level 1-2 routers with level 1 routers?

Good afternoon, your help answering the following questions please

  1. Can you explain more about the NSEL selector?
  2. What use is given to TLV?

I appreciate your help

Hallo Alex

The attached bit is a bit found within a Level 1 LSP. When a Level 1-2 router is connected to another area, all of its level 1 LSPs will have set the attached bit to 1. For example, take a look at the following diagram from the lesson:

Once R2 connects to R4, it has created a connection to another area. This means that all level 1 LSPs sent from R2 to R1 will have the attached bit set to 1 indicating that the specific router has access to other areas. Does that make sense?

Concerning your other questions:

IS-IS was not based on IP as an addressing scheme like many other routing protocols, but on Network Service Access Point (NSAP) addressing. NSAP is roughly comparable to IP addresses but are used in OSI networking as opposed to TCP/IP networking. The Network Selector (NSEL) is a field in the NSAP address that identifies the network layer service to which a packet should be sent. Routers have a network layer service value of zero. IS-IS uses the NSEL as part of the Network Entity Title (NET) and since all IS-IS devices are routers, the NSEL value is always 0.

The type length value (TLV) is a parameter used to carry information in the Link State Packets (LSPs). The TLVs make IS-IS expandable so that it can carry various kinds of information in the LSPs. You can find out more about TLVs and their uses in IS-IS at the following Cisco document:

I hope this has been helpful!