Complex case: Multi-homed customer

• If the customer is multi-homed, then they may also use addresses from other ISPs

  • e.g. Satellite downlink from ISP A, uplink to ISP B

• ISPs can still block the forged packets

  • Need to have a list of valid addresses

• Use generic filtering features, such as cisco access lists

  • Not just one trivial command, but still worth doing