In the context of

network routing Routing is the process of selecting a path for traffic in a network or between or across multiple networks. Broadly, routing is performed in many types of networks, including circuit-switched networks, such as the public switched telephone netw ...

, route filtering is the process by which certain routes are not considered for inclusion in the local route database, or not advertised to one's neighbours. Route filtering is particularly important for the

Border Gateway Protocol Border Gateway Protocol (BGP) is a standardized exterior gateway protocol designed to exchange routing and reachability information among autonomous systems (AS) on the Internet. BGP is classified as a path-vector routing protocol, and it make ...

on the global

Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, pub ...

, where it is used for a variety of reasons. One way of doing route filtering with external-resources in practice is using

Routing Policy Specification Language The Routing Policy Specification Language (RPSL) is a language commonly used by Internet Service Providers to describe their routing policies. The routing policies are stored at various whois databases including RIPE, RADB and APNIC. ISPs (using a ...

in combination with

Internet Routing Registry An Internet Routing Registry (IRR) is a database of Internet route objects for determining, and sharing route and related information used for configuring routers, with a view to avoiding problematic issues between Internet service providers. The ...

databases.

Types of filtering

There are two times when a filter can be naturally applied: when learning routes from a neighbour, and when announcing routes to a neighbour.

Input filtering

In input filtering, a filter is applied to routes as they are learned from a neighbour. A route that has been filtered out is discarded straight away, and hence not considered for inclusion into the local routing database.

Output filtering

In output filtering, a filter is applied to routes before they are announced to a neighbour. A route that has been filtered out is never learned by a neighbour, and hence not considered for inclusion in the remote route database.

Reasons to filter

Economic 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. Applying output filtering on these routes avoids this issue.

Security reasons

An ISP will typically perform input filtering on routes learned from a customer to restrict them to the addresses actually assigned to that customer. Doing so makes address hijacking more difficult. Similarly, an ISP will perform input filtering on routes learned from other ISPs to protect its customers from address hijacking.

Technical reasons

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. This can be done by filtering on prefix length (eliminating all routes for prefixes longer than a given value), on AS count, or on some combination of the two; security is the most important point for this. However, this practice is not recommended, as it can cause suboptimal routing or even , and frustrate the traffic-engineering efforts of one's peers.

References

Routing {{Telecom-stub