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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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 …

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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 …

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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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MPLS Label Operations

In my previous post I explained what MPLS is and how it works from a high level perspective. In this post I will explain MPLS label operations and how labeled packets are processed in MPLS networks. When a labeled packet is received the label value at the top of the stack is examined to determine two things: The next hop and the exit interface to which the packet is to be forwarded. The operation to be performed by the LSR on the label stack before forwarding the packet. Listed below are the operations performed by the LSR on the MPLS …

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What is MPLS?

What is MPLS? MPLS stands for Multi-protocol Label Switching. MPLS is a packet forwarding technology that is capable of carrying any L3 protocol and here comes the word multi-protocol. MPLS is capable of tunneling L3 packets inside the MPLS network using  MPLS labels. The MPLS label is pushed into the packet between the layer two header and the layer three header of the packet at the ingress router and is used to switch the packets across the network to its destination. What is the MPLS Label and How is it used? The MPLS label is a fixed 4 byte identifier …

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