Various methods enable you to control the routing information sent between routers. These methods include the following:
• Passive interfaces—An interface that does not participate in the routing process. In RIP and IGRP, the process listens but will not send updates. In OSPF and EIGRP, the process neither listens nor sends updates because no neighbor relationship can form.
The interfaces that participate in the interior routing process are controlled by the interface configuration. During configuration, the routing process is instructed via the network command on which interfaces to use. Because most protocols express the networks at the major boundary, interfaces that have no reason to send this protocol's updates propagate the data across the network. This is not only a waste of bandwidth, but in many cases, it also can lead to confusion.
• Default routes—A route used if there is no entry in the routing table for the destination network. If the lookup finds no entry for the desired network and no default network is configured, the packet is dropped.
If the routing process is denied the right to send updates, the downstream routers will have a limited understanding of the network. To resolve this, use default routes. Default routes reduce overhead, add simplicity, and can remove loops.
• Static routes—A route that is manually configured. It takes precedence over routes learned via a routing process.
If no routing process is configured, static routes may be configured to populate the routing table. This is not practical in a large network because the table cannot learn of changes in the network topology dynamically. In small environments or for stub networks, however, this is an excellent solution.
• The null interface—An imaginary interface that is defined as the next logical hop in a static route. All traffic destined for the remote network is carefully routed into a black hole.
This can be used in a similar way as the passive interface, but it allows for greater granularity in the denied routes.
It is also used to feed routes into another routing protocol. Its allows another mask to be set and, therefore, is useful when redistribution occurs between a routing protocol that uses VLSM and one that does not. In this way, it aggregates routes as shown in the previous chapter.
• Distribution lists—Access lists applied to the routing process, determining which networks will be accepted into the routing table or sent in updates.
When communicating to another routing process, it is important to control the information sent into the other process. This control is for security, overhead, and management reasons. Access lists afford the greatest control for determining the traffic flow in the network.
• Route maps—Complex access lists permitting conditional programming. If a packet or route matches the criteria defined in a match statement, then changes defined in the set command are performed on the packet or route in question.
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