Orhan Ergun No Comments

IS-IS level 1, IS-IS Routing Protocol Levels

What is IS-IS Level 1 ? Why IS-IS level 1 is used ? What are the IS-IS levels ? What is the corresponding Area type in OSPF ?

IS-IS Level 1 is also called as IS-IS Level 1 sub domain. IS-IS is a link state routing protocol, similar to OSPF. You can read detail comparison of OSPF vs. IS-IS from here.

There is an Area concept in IS-IS as well but the purpose of IS-IS Areas are different than OSPF Areas.

In IS-IS Areas are used to create  IS-IS Levels and IS-IS Levels are the similar logical concepts with OSPF Areas.


Two create IS-IS Level 1 adjacency between two routers, same IS-IS area number should be used. Otherwise only IS-IS Level 2 adjacency can be created. This is the first rule of IS-IS adjacency relationship.


Which OSPF Area type is similar to IS-IS Level 1 Sub domain ?

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IS-IS Design: Avoiding Traffic Blackholing

IS-IS, a link state routing protocol, requires careful attention during network design in order to avoid traffic blackholing. In the topology below, IS-IS routing protocol is used.

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IS-IS Suboptimal Routing Design

If you design multi level IS-IS network and if you have more than one exit (L1-L2 routers) from the Level 1 domain, you will likely create a suboptimal routing. Multi-level IS-IS design is for large-scale network. What’s more, most of the real life networks use only flat Level 2 IS-IS as their interior gateway protocol (IGP).


is-is routing protocol


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Orhan Ergun 4 Comments

Routing design

Understanding everything about routing design is no brainer, especially if you have the chart below on your wall.

The table below highlights the pros and cons of each routing protocol. Of course, you need to consider the design attributes shown in Figure A before embarking on routing design.

Should you like the comparison of the routing protocols illustrated in the table below or should you want to see similar comparison for other technologies, feel free to add your comment in the comment section.

Another boon for all my readers!

If you are interested in network design or considering CCDE, CCDP, or CCDA certification, you can subscribe for membership here so that you can peruse all the design resources (Videos , Tests , Case Studies , and E-Books).

Figure A: Comparison of Routing Protocols

Orhan Ergun 9 Comments

IS-IS Routing Protocol FAQ

IS-IS is a link state routing protocol. Commonly used in Service Provider networks.

Back in old days, IS-IS routing protocol software was more stable and robust compare to OSPF,  thus many service provider choose IS-IS as their interior routing protocol.

I collected the questions which I received from my students and readers related with IS-IS routing protocol and my answers in this post. Read more

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OSPF and IS-IS for MPLS Traffic Engineering

You need OSPF or IS-IS to distribute link information such as reserved, unreserved and used bandwidth, metric, link colouring information.These informations are used by CSPT ( Constraint based shortest path first ) algorithm.

For those who are familiar with MPLS-Traffic Engineering, path is calculated either at each and every device or with the offline computation tools such as NMS from the central place.

For the distributed computation, CSPF which is one of the flavour of Shortest Path First (SPF) algorithm is used.

CSPF computes a dynamic unidirectional MPLS TE LSP ( Label Switch Path ) by reaching the Traffic Engineering Database (TED).
TED database has different attributes than regular link state database which is created such as reserved , used , unreserved bandwidth on the interfaces, link colouring attributes and so on.Link colouring information is used to avoid SRLG ( Shared Risk Link Group ) path at the transport network.
These information can only be provided by the link state protocols. Thus if you want to calculate the MPLS TE LSP without helping the NMS ( Network Management System ) but on each and every LSR as distributed, you need to use link state routing protocols which are OSPF and IS-IS currently.