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Routing

BGP Attribute Types and Flags

BGP attributes is an interesting subject of study.  BGP is a very flexible and extensible protocol and I like that, let’s see how flexible is that protocol when it comes to attributes handling. We all know that BGP has four types of attributes as listed: Well-known mandatory. Well-known discretionary. Optional transitive. Optional non-transitive. I am not going to explain them in this post, they are explained everywhere over the internet. However, in short well-known attributes must be recognized by all BGP implementations, some of them are mandatory and must be included in all update messages, others are discretionary and may …

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How to: use IS-IS overload bit

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 specific router to signal other routers not to use it as a transit hop in their SPF calculations. Typically this is done for a temporary situation like an overloaded router due to memory or processing shortage and released when the router recovers from the problematic …

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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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IS-IS DIS in Practice

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 the IIH PDU) win the election, if multiple routers have the same priority which is the case in most scenarios due to the default value “64” of most implementations, the router with the highest SNPA (MAC address discovered from the MAC header of the received …

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