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OSPF vs IS-IS

Created: Apr 3, 2020 18:19:32Latest reply: Apr 4, 2020 05:07:08 133 5 0 0
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Hello everyone,

What is the difference between ospf and is-is? and where should I use is-is instead of ospf?

thanks in advance.

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MVE Created Apr 4, 2020 05:07:08 Helpful(1) Helpful(1)

Is-Is is used on the ISP side because it is scalable due to the TLV field.
OSPF is mostly used in enterprise customer networks.
The difference between OSPF & ISIS
- OSPF works in Layer 3 only.
- ISIS works in both layer 2 & 3.

- OSPF works in multarea
- Router works in level with single area

- OSPF adjecency is full
- ISIS adjecency is only full to the DIS (pseudo node) router.

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lubna Created Apr 3, 2020 18:29:14 Helpful(0) Helpful(0)

hello dear

Diff between OSPF and ISIS

OSPF can support virtual links, IS-IS cannot (because it rides L2 directly). On broadcast networks, OSPF elects a DR and BDR which cannot be preempted, whereas IS-IS elects only a single DIS which may be preempted. ... OSPF routers can belong to multiple areas, IS-IS routers belong to exactly one area.
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sohaib.ansar MVE Created Apr 3, 2020 18:33:46 Helpful(2) Helpful(2)

Hi



I have came accross some questions on differences and similarities of OSPF and IS-IS. Here are some of the similarities and differnces between both. I hope these are helpful.


Similarities between OSPF and IS-IS Routing Protocols

There are a lot of similarities between the OSPF and IS-IS routing protocols. The following are some of the major similarities that one should keep in mind while implementing the OSPF or IS-IS routing protocol.

  1. Both the routing protocols are Interior Gateway Protocols (IGP).

  2. Both the routing protocols can only be used to distribute routing information between routers belonging to the same Autonomous System (AS).

  3. Both the routing protocols are link-state protocols.

  4. Both the routing protocols maintain a link-state database and run the Dijkstra SPF algorithm to compute the shortest path.

  5. Both the routing protocols use Hello packets to create and maintain adjacencies between the neighboring routers.

  6. Both the routing protocols use areas to create a hierarchical structure.

  7. Both the routing protocols are classless routing protocols and support CIDR, VLSM, and discontinuous network.

  8. Both the routing protocols elect a designated router in the broadcast networks.

  9. Both the routing protocols support authentication mechanisms.

  10. Both the routing protocols support the unlimited number of hops count.

Differences between OSPF and IS-IS Routing Protocols

Apart from these similarities, both the routing protocols differ from each other with many features. Both the routing protocols have some key differences that one should keep in mind because many people think (better to say confused) that both are the same protocols and the implementation of any protocol will not affect network performance a lot. But the following list will help you to distinguish both the routing protocols.

  1. OSPF supports NBMA and point-to-multipoint links, whereas IS-IS does not support.

  2. IS-IS runs on the data link layer, whereas OSPF runs on the network layer.

  3. OSPF supports virtual link, whereas IS-IS does not support.

  4. OSPF elects a DR and BDR, whereas IS-IS elects only a single DR called DIS.

  5. OSPF defines a backbone area called area 0 for inter-area advertisements, whereas IS-IS categorizes the domain into two layers.

  6. An OSPF router can belong to multiple areas whereas an IS-IS router can belong to only one area.

  7. OSPF uses Router ID, whereas IS-IS uses System ID to identify each router on the network.

OSPF and IS-IS Terminology

Apart from these similarities and differences between the OSPF and IS-IS routing protocols, one should also be familiar with the terminology used to refer the basic terms for both the routing protocols.

The following table list some of the basic terms used refer the components of the OSPF an IS-IS routing protocols.

table

In this post, we have learned the similarities and differences between the OSPF and IS-IS routing protocols. Let us know, if you have something that can help us to improve the article.Please let me know if there are any further confusions, and I'll try to answer then one-by-one.


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lubna Created Apr 3, 2020 18:34:59 Helpful(1) Helpful(1)

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

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?

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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.

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. 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.


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Popeye_Wang Admin Created Apr 4, 2020 03:44:13 Helpful(1) Helpful(1)

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tesfama MVE Created Apr 4, 2020 05:07:08 Helpful(1) Helpful(1)

Is-Is is used on the ISP side because it is scalable due to the TLV field.
OSPF is mostly used in enterprise customer networks.
The difference between OSPF & ISIS
- OSPF works in Layer 3 only.
- ISIS works in both layer 2 & 3.

- OSPF works in multarea
- Router works in level with single area

- OSPF adjecency is full
- ISIS adjecency is only full to the DIS (pseudo node) router.

View more
  • x
  • convention:

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