Integrated Versus Ships-in-the-Night Routing
Integrated IS-IS represents one of the two ways to support multiple network layer protocols in a network, the other being the ships-in-the-night approach. In other words, using integrated routing with IS-IS will enable you to support the use of multiple protocols only through the use of IS-IS. The alternative ships-in-the-night routing technique requires configuring multiple protocols on your network to achieve the same results.
Integrated routing has the capability to route multiple network layer protocols through tables calculated by a single routing protocol, thus saving some router resources. Integrated IS-IS uses this approach.
Ships-in-the-night routing advocates the use of a completely separate and distinct routing protocol for each network protocol so that the multiple routing protocols essentially exist independently (with different types of routing information passing like ships in the night).
The very idea of integrated routing appears on the surface to be very seductive. Imagine the possibilities and ease of managing or designing a network that only had one routing protocol because that was all it ever neededa very seductive thought for those of us who prefer working smarter instead of working harder.
The fact though is that integrated routing is not as seductive as it might first appear. The truth of the matter is that corporate networks often carry multiple protocols. This is done because different services are expected from the different protocols and, as is expected, each of these protocols has a slightly different routing algorithm. The drawback is that network engineers will have to find the one common denominator in order for the network to operate smoothly.
There is no real evidence that proves that integrated routing leads to a more efficient network management or design than Ships In The Night, in which each protocol is routed based upon its own capabilities. Because there is more than one, the ships end up passing in the night like ships on the same ocean without ever interacting, each left to its own devices.
The steps for IS-IS routing are as follows:
IS-IS has an impressive number of timers that can be configured when compared to other protocols. It is beyond the scope of this book to detail in depth each and every one. Basically, they are very extensive and not relevant.
Building the Topology Database
Link state update messages help ISs learn about the network topology through the following steps:
IS-IS uses a single required default metric with a maximum path value of 1,023, with 1,024 being infinity. The metric number assigned is arbitrary and is typically assigned by a network administrator based upon the network architecture. Maximum metric values are set at this level to provide the granularity needed to support various link types, while at the same time ensuring that the shortest path algorithm used for route computation will be reasonably efficient.
Any single link can have a maximum value of 63. IS-IS also calculates the metric along the path to a destination by summing the single link values.
IS-IS also defines three optional metrics (costs) to further assist in defining the optimal routes:
IS-IS maintains a mapping of the default metric and three optional metrics to the quality of service (QoS) option in the CLNP packet header. IS-IS uses these mappings to compute routes through the internetwork.
IS-IS Packet Formats
IS-IS uses three basic packet formats:
IS-IS Logical Packet Format
Each of the three IS-IS packet types has a complex format with the following three different logical parts:
Figure 3-3 shows the logical format of IS-IS packets: