| Revision as of 04:31, 16 December 2008 editJec (talk | contribs)Extended confirmed users1,130 edits →Technical reasons← Previous edit | Revision as of 07:52, 23 December 2008 edit undoRobert Brockway (talk | contribs)Extended confirmed users, Pending changes reviewers, Rollbackers4,749 editsm →Reasons to filter: Fix typoNext edit → | ||
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| 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. | 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 |
===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. | 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. | ||
Revision as of 07:52, 23 December 2008
In the context of network routing, 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 BGP on the global Internet, where it is used for a variety of reasons.
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, and 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.
This practice is not recommended, as it can cause suboptimal routing or even communication failures with small networks, and frustrate the traffic-engineering efforts of one's peers.
See also
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