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Routing TCP/IP, Volume I (CCIE Professional Development) by Jeff Doyle

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Filter Placement Considerations

For the best performance, you must consider not only the efficient design of the access list itself but also the placement of the filter on the router and in the internetwork.

As a rule of thumb, security filters usually are incoming filters. Filtering unwanted or untrusted packets before they reach the routing process, prevents spoofing attacks—wherein a packet fools the routing process into thinking it has come from somewhere it hasn't. Traffic filters, on the other hand, usually are outgoing filters. This approach makes sense when you consider that the point of a traffic filter is to prevent unnecessary packets from occupying a particular data link.

Aside from these two rules of thumb, another factor to consider ...

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