Save Digg Del. Building Scalable Cisco Networks. This supplemental chapter is not meant to replace any material in the original book, but it should be read between Chapters 4 and 5 of the book. The addendum to Appendix G, "Answers to the Review Questions," provides you with the answers to the review questions at the end of this chapter. The addendum to Appendix H, "Configuration Exercise Equipment Requirements and Backbone Configurations," provides the configuration commands for the backbone router for the configuration exercise at the end of this chapter.

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I ntroduction:. The Basis :. ISIS is different than other routing protocols. With LLC encap. With a SNAP header we add 5 bytes. The NET address is a variable data but it usually has a size of 10 bytes. The NET is made of 3 parts:. The NET address:. By default, the hostname configured on the router is included in the TLV For Link State routing protocols, the area groups several routers within the same flooding domain.

Those network designs allow better convergence times. Each router is usually configured with one Area ID but can belong to several Levels: Level 1 only, Level 2 only or Levels 1 and 2 note the default is Levels 1 and 2.

ISIS defines 2 hierarchical levels for Area. The Level 2 area must be contiguous. In other words a level 2 router cannot be isolated. The L1 areas are connected to L2 area. Each router keeps a separate LSDB per level. The Area ID has only a meaning for L1 adjacencies.

Indeed, to establish an L1 adjacency, 2 routers must have the same area ID. It is not the case for L2 adj. This ID has no meaning when it establishes an adj. Today, networks use only the new style of metrics for IS and IP information.

Notice: The passive mode is a per level configuration. The old style metric is a 6 bits field. So it offers at most 64 different metrics 0 to Moreover the aggregated metric max cost was limited to Notice: By default Junos sends both types of metrics : the old and new styles.

The default behaviour is the following :. This is not possible because ISIS includes a specific flag per IP reachability — aka per prefix which informs if a prefix has been leaked. ISIS adjacency allows to discover neighbors and checks several parameters.

The holdtime of the adjacency is conveyed in the IIH, too. The state machine is depicted below:. Note for holdtime the multiplier is by default 3 and it is not configurable. But you have the choice to set up either hello-interval or holdtime. Setting both has no sense. Juniper implements an Adjacency Holddown mechanism. So, sometimes, ISIS adjacency could take some time to become up. This is due to this mechanism. You cannot configure this holddown value but you can disable it enable by default :.

The following diagram shows this concept. The pseudonode is seen as a real router with its own LSDB. It should be unique by router. How the DIS is elected? The DIS election is done during the adjacency establishment phase and it is based on the priority field 0 to — default is 64 conveyed within the IIH.

The higher priority will win the DIS election. DIS pre-emption can be done at any time. If you configure an holdtime of 9 seconds — automatically the holdtime of the DIS will be 3 seconds. DIS election Diagram:. ISIS is a link state protocol. Those information are flooded through the area.

Each router then computes the Short Path First algorithm toward each destination. Each router generates at least one LSP. It is now used more for maintenance to remove traffic on a router. When this flag is set by a router : the router flushes all its LSP all its fragments and sends only one or more LSPs that will convey only its adjacencies UP and its passive interfaces. LSP flooding means that when a router sends its LSP, this one will be received by all the other routers of a given area excepted if you use mesh-group for example.

The flooding is performed like that:. Sometimes it might be useful to not wait for the LSP expired naturally. It is used, for example, when you manually set the Overload bit.

This network-wide-purge mechanism is performed by the router that owns the LSP. On large network, ISIS flooding could generate a lot of control plane messages. The ISIS protocol implements some kinds of rate-limiter. There are 2 types of flow control:.

The first limiter on Junos is called lsp-interval which is by default equal to ms. Indeed, Junos can only generate 3 rapid LSP and then wait a variable timer. The following diagram explains the concept:. We will see later but when an LSP is sent, an acknowledgement is needed. This value is not configurable on Junos. Database synchronisation — the concepts:.

Those 2 messages are also used to acknowledge LSPs in some cases. To better understand the synchronization concepts, we need to understand the internal ISIS state machine. Those 2 flags are allocated per LSP and per Interface. Hereafter an example of the internal view of the router A:. So for our example, a new adjacency between router A and router B came up, what does the router A do?

Based on the flags we can say that:. Database synchronisation on LAN segments:. No direct acknowledgement is need there. The following diagram summarizes this first case:. No direct ack are needed again on the LAN. No direct ack are needed on the LAN. This case is depicted below:. Database synchronisation on p2p segments:. If N is the number of Adj.

The 2 next diagrams describe in detail the synchronisation on a p2p link. Junos configuration:. ISIS implements its own fragmentation mechanism.

So no IIH fragmentation has been implemented. This one is called: fragment-ID. Like this :. The fragment-ID always begins at 0 and is incremented by 1 for the other fragments. The Fragment-ID is a 8 bits field.

This means around IPv4. When link states information change, the Juniper tries to change the less possible its fragmentation conservative algorithm. Hereafter an example:. Short Path First implementation:. Junos uses 2 runs of SPF. The first run will use only IS-reachability information to build the short path tree the tree between SystemID.

Then a second run will use the IS-reachability information. Those information will be seen as leaves graphed to the previous tree. Junos uses 3 parameters to configure SPF runs:.


Supported Standards for IS-IS

Intermediate System to Intermediate System IS-IS , also written ISIS is a routing protocol designed to move information efficiently within a computer network , a group of physically connected computers or similar devices. It accomplishes this by determining the best route for data through a Packet switching network. IS-IS has been called "the de facto standard for large service provider network backbones. IS-IS is an interior gateway protocol , designed for use within an administrative domain or network. IS-IS is a link-state routing protocol , operating by reliably flooding link state information throughout a network of routers. Each IS-IS router independently builds a database of the network's topology, aggregating the flooded network information. Packets datagrams are then forwarded, based on the computed ideal path, through the network to the destination.


Configuring IS-IS Protocol




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