Understanding Selecting Network Protocols

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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 needed—a 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.

IS-IS Routing

The steps for IS-IS routing are as follows:

1.  Each End System (ES or host) belongs within a particular logical area. An end station discovers the nearest Intermediate System (IS or router) by listening for IS hello packets.
2.  When an End System wants to send a packet to another End System, it sends the packet to one of the Intermediate Systems on its directly attached network.
3.  The Intermediate System (router) looks up the destination address and forwards the packet along the best route.
4.  If the destination end station is on the same subnetwork, the local Intermediate System will know this from listening to the End Station hellos and will forward the packet appropriately.
5.  The IS might also provide a redirect (RD) message back to the source to tell it that a more direct route is available.
6.  If the destination address is an ES on another subnetwork in the same area, the IS will know the correct route and will forward the packet appropriately.
7.  If the destination address is an ES in another area, the Level 1 IS sends the packet to the nearest Level 2 IS.
8.  Forwarding through Level 2 ISs continues until the packet reaches a Level 2 IS in the destination area.
9.  Within the destination area, ISs forward the packet along the best path until the destination ES is reached.

IS-IS Timers

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:

1.  Each IS generates an update with IP addresses specifying the ESs and ISs to which it is connected, as well as the associated metrics.
2.  This update is then sent to all neighboring ISs, which forward (flood) it to their neighbors, and so on.
3.  Using these updates, each IS can build a complete topology of the network. Each update has a sequence number that is used to distinguish old updates from new ones.
4.  When the network topology changes, new updates are sent as the process begins again.

IS-IS Metrics

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:

  Delay. The delay cost metric reflects the amount of delay on the link.
  Expense. The expense cost metric reflects the communications cost associated with using the link.
  Error. The error cost metric reflects the error rate of the link.

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 hello packets (ISHs)
  Link-state packets (LSPs)
  Sequence numbers packets (SNPs)

IS-IS Logical Packet Format

Each of the three IS-IS packet types has a complex format with the following three different logical parts:

  Common header. An 8-byte fixed header shared by all three-packet types.
  Fixed length packet specific header. Packet-type-specific portion with a fixed format.
  Variable length packet specific header. Also packet-type-specific, but of variable length.

Figure 3-3 shows the logical format of IS-IS packets:


Figure 3-3  IS-IS common header fields.


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OSPF Network Design Solutions
OSPF Network Design Solutions
ISBN: 1578700469
EAN: 2147483647
Year: 1998
Pages: 200
Authors: Tom Thomas

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