|
|
This section introduces some of the factors that you should take into account when designing the layout and connectivity of your organization's networks. It is by no means an exhaustive discussion of this topic, nor does it cover the features or configuration of the router hardware solutions that are available from third-party vendors.
Before starting to configure routers for your network, you need to decide on the number of routers that you will need and to choose the type of routing (static or dynamic) that they will use.
The number of routers that you need to configure depends on the number of networks you want to interconnect, on how you want to connect the networks, and on the level of activity on the connected networks.
For example, consider the networks shown in ``Multiple networks connected by multiple routers''.
Multiple networks connected by multiple routers
Suppose that systems on the rivers and cities networks need to communicate with each other and with the places network. To connect the rivers network directly to the places network, you would use a single router (nile). To connect the cities network directly to the places network, you could configure another router (cairo). Assuming the proper routes are defined, the systems on all three networks can communicate. However, if the places network is very busy, communication between the rivers and cities networks may suffer unacceptable delays.
A more cost-effective solution would be to use a single router that is connected directly to all three networks as shown in ``Multiple networks connected by a single router''.
Multiple networks connected by a single router
In general, the number of networks you connect through a single router is limited by the number of network adapter cards that the routing system can support.
Several problems potentially arise with using a single router between multiple networks:
An important consideration when planning your organization's network layout is how many separate internal networks are needed. If there are sufficient network hosts (say, more than 30), it is common practice to assign separate networks to individual departments. This concentrates traffic on the networks where it is relevant, and prevents it loading other networks. If you group server machines on the same network as the host machines that will access them, you can avoid creating unnecessary traffic on other networks and reduce load on the intermediate routers. These departmental servers can provide access to networked filesystems, user mailboxes, departmental web pages, and directory services.
If a server machine must be accessed by hosts in two or more networks, consider making this machine directly accessible to these networks by configuring it with separate interfaces to each one. A centralized server can provide access to the same facilities as a departmental server. Additionally, it is suitable for hosting an organization's web server and other intranet services.
Note that you do not have to configure the server as a router to make it accessible from the locally attached networks, even if it does have more than one network interface. In fact, it may be inadvisable to make a server also function as a router as some portion of its processing power must inevitably be expended routing traffic between the attached networks. If the server also runs a routing daemon, configure this to run in quiet mode if the server will not act as a router. If possible, use a non-server machine as a router between the networks.
Non-routing server connected to multiple networks
``Non-routing server connected to multiple networks'' shows a server named archie connected directly via its three network interfaces to the three networks introduced in the previous examples. Forwarding of IP datagrams has been disabled on archie, so that only cairo, and nile will act as routers between the networks. Hosts on each of the three networks have direct access to archie without going through an intermediate router.
Once you have chosen a network topology and its connectivity, you must decide on the type of routing to configure: static or dynamic.
Use static routing if your networks are small-scale, simply connected (no more than one router per network), or their configuration rarely changes.
Use dynamic routing if your networks are large-scale, multiply connected, or their configuration may change frequently.
You can also use a combination of static and dynamic routing. For example, you might configure a static definition on a router that leads to an otherwise unconnected network, while allowing routes to networks with complex topologies to be updated by routing daemons. Any static routes that you create are not advertised to other routers, and they are not updated by the routing daemons.
If you configure dynamic routing, choose the routing daemon according to the type of router (interior or exterior) and the protocols that it must support:
``Examples of router configuration'' shows examples of how routers might be configured in three organizations:
Examples of router configuration