Home » Routing (page 2)

Routing

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 …

Read More »

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 …

Read More »

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. …

Read More »

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 …

Read More »

IS-IS Neighbor Discovery

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 (Point-to-Point and Broadcast) so it’s essential to have different types of hello messages or PDUs for serve these networks since the data-link layer for broadcast and P2P links have different style of addressing so they need different treatment, due to this differences  there are three …

Read More »

The Role of BGP in MPLS networks

In almost every book you will read about MPLS, the author will state that one of the MPLS benefits is having a BGP free core network; sometimes they explain it sometimes not. However, to really understand this statement I encourage you to imagine removing MPLS from your core network and see what adjustments you need to do in order to get the transit traffic to its destination. Do this  just in your mind’s eye please If you are not willing to do this I will try in this post to bring the picture closer to you. Lets first examine the …

Read More »