In this Community Roundtable episode, returning guests Russ White and Nick Russo continue our three part deep dive into the Border Gateway Protocol, or BGP, with a look at the mechanisms within the protocol to perform traffic engineering. Show Notes Influence Ingress Classic bestpath options to influence ingress AS-path prepend outbound to influence inbound traffic Why AS Path prepend doesn’t always work In many areas, ISPs are in a full or almost full mesh and connected to common backbones making AS Path prepend largely irrelevant Providers normally use their own local preference for outbound traffic back to a customer MED MED is a hint, it’s often stripped or ignored MED only works if the AS Path is the same on all routes MED is non-transitive and doesn’t mean anything beyond the next hop Longest Match Be careful about this, as it pollutes the DFZ DFZ = default free zone A router belongs to the DFZ if it doesn’t need a 0.0.0.0 route to reach everything on the internet Tragedy of the commons here An enterprise can force inbound traffic to be load-balanced better but it pushes the processing of that traffic engineering onto the internet This is the “big hammer” Using RFC 1998 communities for influence ingress traffic This is a way to signal your provider to take some sort of BGP action You need to find the specific communities used by each provider Make certain the provider accepts communities on their eBGP edge Influence egress Local Pref Overrides pretty much everything other than weight Used to implement hot/cold potato routing hot potato routing is when a provider chooses to get the traffic out of its network as quickly as possible at the closest egress point cold potato routing is when a provider chooses to control some traffic as long as possible for some reason  Weight Local to a device Other handy stuff: Cost community: IGP and pre-bestpath POI Accumulated IGP (AIGP) iBGP tie breakers Using RFC 1998 communities for influence ingress traffic This is a way to signal your provider to take some sort of BGP action You need to find the specific communities used by each provider Make certain the provider accepts communities on their eBGP edge BGP deterministic MED   Russ White Guest

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