Episode 19 – BGP: Traffic Engineering

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