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Why do many ISPs still use the IS-IS protocol instead of the OSPF protocol Highlighted

Latest reply: Sep 26, 2019 10:06:35 1565 6 9 4

The full name of ISIS is Intermediate System to Intermediate System, which was first developed by ISO 10589. Based on ISO CLNS (ConnectionLess-mode Network Services), IP was added in RFC1195.

BOTH ISIS and OSPF are link-state routing protocols that support SPF fast convergence. Why do carriers prefer ISIS?


Typical networking topologies of OSPF and IS-IS

app


I think there are several reasons:

 

1. Stability

ISIS is based on CLNS, that is, it runs at the link layer. In this way, even if the IP layer is faulty, the Layer 2 communication is not interrupted.  OSPF runs at the IP layer with the protocol number of 89. Encapsulation in IP means that OSPF packets can be attacked by IP packets spoofing and denial of service (DoS) attacks.

 

2.  Scalability:

In ISIS, all routing information is transmitted using TLVs (TYPE/LENGTH/VALUE), ensuring simple structure and providing easy scalability. For example, to support IPv6, only two TLVs are added to ISIS. Additionally, ISIS also supports protocols such as IPX. OSPF is developed to support IP and provides two independent versions OSPFv2 and OPPFv3 to support IPv4 and IPv6.

 

3.    Complexity

a. All non-backbone areas of OSPF must be interconnected with the backbone AREA 0. ISIS areas are classified into level1 and level2, the backbone area of ISIS is relatively flexible.

This means you don't have to worry about achieving the perfect physical layout, and you can more freely implement physical connections.

b. There are many types of OSPF LSAs, and the database structure is complex. Therefore, fault locating is difficult. ISIS has few types of LSPs, the database structure is simple, resource usage is low, and faults can be easily located.

c. When ISIS is configured, Hello interval" and "router dead interval" do not need to be matched between routers, and OSPF must be set to the same value.

d. The ISIS router can belong to only one area, and the OSPF router can connect to more than two areas. In management, this increases the complexity of OSPF.


isis


4. convergence

In ISIS, when changes occur on the network segment where a node in an area resides, PRC algorithm is triggered, ensuring fast route convergence and low route calculation cost, In OSPF, network address is used for building the network topology, and ISPF algorithm is triggered when the network segment address in an is changed, which is complicated.

When the network scale is large, the convergence speed of ISIS is much faster than that of OSPF.

 

5. Scale

ISIS supports a larger network architecture and a more stable network. The number of ISIS single-domain nodes can be greater.

As I know, the world's top running ISIS has more than 500 devices in a single domain, and OSPF can be configured to 350.

 

This is why ISPs choose to use ISIS. OSPF also has many advantages. Compared with ISIS, OSPF is more flexible and adapts to the complex network environment of enterprise networks. In fact, many carriers' non-backbone networks, such as the bearer network and MAN, use OSPF. The selection of different protocols depends on the actual requirements.


 OSPF


This just a personal understanding. You are welcome to express your views.

 

Thanks.



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Popeye_Wang
Admin Created Sep 2, 2019 03:15:42 Helpful(0) Helpful(0)

Good summary.
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Leah21
Created Sep 19, 2019 08:57:53 Helpful(0) Helpful(0)

Thanks
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armanx
Created Sep 22, 2019 03:21:06 Helpful(0) Helpful(0)

Thanks for the info. But do you have any data or statistics to backup that indeed most ISPs are using IS-IS as interior gateway protocol within their Autonomous System (AS)? I knew one country that all of their Telcos are using OSPF within their AS. Thanks and God bless!
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Philippians 3:8
umaryaqub
MVE Created Sep 22, 2019 04:31:50 Helpful(0) Helpful(0)

Thanks for sharing.
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Leah21
Created Sep 23, 2019 01:23:24 Helpful(0) Helpful(0)

Posted by armanx at 2019-09-22 03:21 Thanks for info. But do you have any data or statistics to backup that indeed most ISPs are using IS ...
Thank you for reading my article. I have worked in an ISP for some time. Most of the backbone networks of the ISPs I have contacted use ISIS. The use of IS-IS may also be related to the development history of the ISP network. ISIS has an earlier large-scale stable use case than OSPF. No one is willing to take risks. So in most cases, IS-IS is selected when ISPs establish backbone networks. Since the overall network change cost is too high, it is difficult to change the protocol.As long as there is no failure, the initial protocol will usually be used all the time.
However, with the improvement of device performance and bandwidth, the advantages of IS-IS such as occupying fewer resources and faster convergence are becoming less, the frequency of OSPF on the new network is indeed increasing.
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armanx
Created Sep 26, 2019 10:06:35 Helpful(0) Helpful(0)

Posted by Leah21 at 2019-09-23 01:23 Thank you for reading my article. I have worked in an ISP for some time. Most of the backbone netw ...
Thank you very much for your reply Leah21. I am currently preparing for my HCIP-RS exam and really fell in love with this routing protocols. A friend of mind who is an IP Engineer told me that all Telcos in our country are currently using OSPF. We do have one incoming Telco though that is planning to use IS-IS as their IGP within their AS. You know, I love facts based on data the reason I did ask you that question. I'm glad to hear that most of the ISPs you contacted are using IS-IS. Thank you once again Leah21 and God bless!
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Philippians 3:8

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