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At the most basic level, there are only three BGP policies: pushing traffic through a specific exit point; pulling traffic through a specific entry point; preventing a remote AS (more than one AS hop away) from transiting your AS to reach a specific destination. In this series I’m going to discuss different reasons for these kinds of policies, and different ways to implement them in interdomain BGP.

In the following network—

There are many reasons an operator might want to select which neighboring AS through which to send traffic towards a given reachable destination (for instance, 100::/64). Each of these examples assumes the AS in question has learned multiple paths towards 100::/64, one from each peer, and must choose one of the two available paths to forward along.

Examining this from AS65006’s Perspective …

Assuming AS65006 is an edge operator (commonly called enterprise, but generally just originating and terminating traffic, and never transiting traffic), there are several reasons the operator may prefer one exit point (through an upstream provider), including:

• An automated system may determine AS65004 has some sort of brownout; in this case, the operator at 65006 has configured the system to prefer the exit through AS65005
• The traffic destined to 100::/64 may require a class of service (such as video transport) AS65004 cannot support (for instance, because the link between AS65006 and 65005 has low bandwidth, high delay, or high jitter)

The most common way this kind of policy would be implemented is by setting the BGP LOCAL_PREFERENCE (called preference throughout the rest of this document) on routes learned from AS65005 higher than the preference on routes learned from AS65004.

Another common case is AS65006 would prefer to send traffic to AS65005 only when the destination is in an AS directly connected to AS65005 itself, while sending all other traffic through AS65004. This is common when a one provider has good local and poor global coverage, while the other provider has good global but poor local coverage.

For instance, if AS65006 is in a somewhat isolated part of the world, such as some parts of the South Pacific or Central America, there may be a local provider, such as AS65004, that has solid connectivity to most of the other edge operators in the local geographic region but charges a high cost for transiting to the rest of the global Internet. A second provider, such as AS65005, charges less to reach destinations beyond the local geographic region but is relatively expensive to use when sending traffic to other edge operators within the local region.

Preference, by itself, would be difficult to use in this case, because the operator would need to distinguish between geographically local and geographically distant routes. To implement this kind of policy, the operator would accept partial routes from the geographically local provider (AS65004 in this case) and set a high preference on these routes. Partial routes are typically those the local provider learns only from other directly connected autonomous systems, and hence would only include operators in the local geographic region. The operator would then accept full routes, or the entire Internet global routing table, from the second provider (AS65005 in this case) and set a lower preference.

An alternative way to implement geographic preference is using communities. Many transit providers mark individual reachable destinations with information about where the route originated. NTT, for instance, describes their geographic marking here. An operator can create filters using regular expressions to change the preference of a route based on its geographic origin.

This is not a common way to solve the problem because the filtering rules involved can become complex—but it might be deployed if local providers do not offer partial routes for some reason.

Another alternate to implement geographic preference is to use a regular expression filter to set the preference for each reachable destination based on the length of the AS Path. Theoretically, routes originating within the local region should have an AS Path of one or two hops, while those originating outside a region should have longer AS Paths.
This generally does not work for two reasons. First, the average length of an AS Path (after prepending is factored out) is about 4 hops in the entire global Internet—and it is easy to reach four hops even within a local region in some situations. Second, many operators prepend the AS Path to manage inbound entry point preference; these prepended hops must be factored out to use this method.

Next week I’ll continue this series on BGP interdomain policies… feel free to leave a comment if you think I’ve explained something incorrectly, etc.