You can configure a PPPOE client on either a physical interface or a subinterface,
but not both at the same time. Only one PPPoE subinterface is supported on a
physical interface. Before you begin configuring a PPPoE client, ask your ISP what
VLAN tag to use for your connection. You will enter that tag when you configure the
subinterface number and Tag. The following task assumes you have already configured
a Layer 3 Ethernet interface on the firewall with a security zone.