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OSPF

The endless story of OSPF vs IS-IS – Part 4 “The Inside Out”

In this post we’ll be covering a couple of topics from the Inside Out of the link-state protocols that have always been ambiguous and full of details, we’ll try to make them as crystal clear as we can. MTU: Both link-state routing protocols consider MTU in order to prevent any related problems, mainly loss of routing information due to large routing messages being dropped (consider an OSPF LSU or an IS-IS LSP that is over sized and thus dropped), however each protocol tests the MTU in a different way as we’ll see in the upcoming section. OSPF requires routers to …

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The endless story of OSPF vs IS-IS – Part 3 “Packets and Database”

In this post we are going to cover the protocol packets and database structure for both routing protocols. To start let’s first highlight a couple of facts. OSPF runs on top of IP, that is it uses IP packets to exchange its messages (and thus it is vulnerable to spoofing and DoS attacks, and accordingly the use of authentication is strongly recommended), while on the other hand IS-IS runs directly over layer 2, it creates its own packet (or PDU (Protocol Data Unit) to be more specific) and then encapsulates it directly inside the layer 2 frame, this leverages IS-IS …

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The endless story of OSPF vs IS-IS – Part 2 “The history”

In our previous post we started consolidating the endless story of OSPF vs IS-IS, in this post we will cover the historical part of the story, it might not be interesting for some people, but I do believe that the history is what makes the future, so please bare with me through this post. The IS-IS protocol was developed in 1987 by Digital Equipment Corporation (DEC) as part of DECnet Phase V. and was standardized later in 1992 by the International Standards Organization (ISO) in ISO/IEC 10589:1992, the second and current edition ISO/IEC 10589:2002 cancels and replaces the first edition. …

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The endless story of OSPF vs IS-IS

Whenever you have a little IGP chit chat you’ll hit this endless story. I’ve tried to reach a final solid conclusion my self but IMHO its all about personal preference and taste. It is something like a Ferrari vs Lamborghini story, they offer comparable performance, but totally different feeling. It is all about a good design, that contains a balanced mixture of scalability, convergence, flexibility, extensibility, resources consumption, configuration, troubleshooting, etc. In this series of posts I’ll try to contrast their likes and differences (not the Ferrari vs Lamborghini of course!), however I am not going to try to influence …

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OSPF & IS-IS Router ID

When you think about Router ID in any link state protocol¬† two requirements come to mind: Each router must be identified by a Router ID and an Area to exist in. This Router ID must be unique inside a single IGP domain. IS-IS refers to the this ID as a System ID (SysID) and OSPF refers to this ID as a Router ID (RID). There are some best practices that can be used to ensure the uniqueness of the router ID within the IGP domain. In a previous post Mohammed discussed the OSPF RID, so I will focus on the …

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Link state protocols and Areas concept

Link state protocols have introduced the concept of multiple routing areas withing the same routing domain. Link state protocols depend on the fact that all routers must have an identical link state database and then each router will start calculating its very own routing table from this information. However, this rule sometimes introduce scalability limitations to network designers. In very large networks all routers must maintain the same link state database; this induces some scalability limitations in these networks.

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