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Overload bit is special bit in the IS-IS LSP used to inform the network that the advertising router is not yet ready to forward transit traffic.  The overload bit was first intended for signaling overload or resource shortage on specific router for the rest of the network. You can use the command set-overload-bit intentionally on [...]

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.

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 [...]

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.

In the previous post IS-IS Neighbor Discovery we have discussed how IS-IS automatically discovers neighbors, in this post we will discuss the DIS role in broadcast networks. After the adjacency state reached the UP state the DIS election process take place, the router with the highest priority value (0-127 specified in the Priority field of [...]

Like other routing and signaling protocols IS-IS has built-in automatic neighbor discovery mechanism which is known by IIHs (IS-IS Hello PDUs), because IS-IS is not IP based protocol the IIH PDUs and all other IS-IS PDUs are directly encapsulated on the data-link layer. IS-IS has two hierarchical levels (L1 and L2) and two network types [...]

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 [...]

IS-IS uses the concept of distributed map or database, each router originates LSPs for his own links and relays its adjacent  LSPs to build up the distributed map, a local computation is performed on the database to extract IP reachability information to formalize the forwarding plane. What is the problem? IS-IS is an extended protocol [...]

Been a while since my last post, I was extremely busy doing a lot of things, anyway I am glade to be back. This post I am going to cover a nice tool for enhancing IS-IS convergence, I am really amazed by the ideas that the guys out there pop up. Inventing such wonderful tools [...]

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 [...]