Transit Traffic
It is helpful to distinguish between two kinds of traffic, as seen from a given AS. Local traffic is traffic that either originates or terminates at that AS; this is traffic that “belongs” to that AS. At leaf sites (that is, sites that connect only to their ISP and not to other sites), all traffic is local. The other kind of traffic is transit traffic; the AS is forwarding it along on behalf of some nonlocal party. For ISPs, most traffic is transit traffic. A large almost-leaf site might also carry a small amount of transit traffic for one particular related (but autonomous!) organization. The decision as to whether to carry transit traffic is a classic example of an administrative choice, implemented by BGP’s support for routing policies. Most real-world BGP configuration issues relate to the carriage (or non-carriage) of transit traffic.
BGP Filtering and Routing Policies
Asstatedabove,oneofthegoalsofBGPistosupportroutingpolicies;thatis,routingbasedonmanagerial or administrative concernsin addition to technical ones. A BGP speaker may be aware of multiple routes to a destination. To choose the one route that we will use, it may combine a mixture of optimization rules and policy rules. Some examples of policy rules might be: • do not use AS13 as we have an adversarial relationship with them • do not allow transit traffic BGPimplementspolicythroughfilteringrules–thatis,rulesthatallowrejectionofcertainroutes–atthree different stages: 1. Importfilteringis applied to the lists of routes a BGP speaker receives from its neighbors. 2. Best-pathselection is then applied as that BGP speaker chooses which of the routes accepted by the first step it will actually use. 3. Export filtering is done to decide what routes from the previous step a BGP speaker will actually advertise. A BGP speaker can only advertise paths it uses, but does not have to advertise every such path. While there are standard default rules for all these (accept everything imported, use simple tie-breakers, exporteverything),asitewillusuallyimplementatleastsomepolicyrulesthroughthisfilteringprocess(eg “prefer routes through the ISP we have a contract with”). As an example of import filtering, a site might elect to ignore all routes from a particular neighbor, or to ignoreallrouteswhoseAS-pathcontainsaparticularAS,ortoignoretemporarilyallroutesfromaneighbor that has demonstrated too much recent “route instability” (that is, rapidly changing routes). Import filtering can also be done in the best-path-selection stage, by having the best-path-selection process ignore routes from selected neighbors.
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