Route filtering is particularly important for the Border Gateway Protocol on the global Internet, where it is used for a variety of reasons.
When a site is multihomed, announcing non-local routes to a neighbour different from the one it was learned from amounts to advertising the willingness to serve for transit, which is undesirable unless suitable agreements are in place.
In some cases, routers have insufficient amounts of main memory to hold the full global BGP table.
A simple work-around is to perform input filtering, thus limiting the local route database to a subset of the global table.
However, this practice is not recommended, as it can cause suboptimal routing[2] or even communication failures with small networks[citation needed], and frustrate the traffic-engineering efforts of one's peers.