When BGP is used as the provider edge (PE) or customer edge (CE) routing protocol, the peering sessions are configured as
an external peering between the VPN provider autonomous system (AS) and the customer network autonomous system. The L3VPN
iBGP PE-CE feature enables the PE and CE devices to exchange Border Gateway Protocol (BGP) routing information by peering
as internal Border Gateway Protocol (iBGP) instead of the widely used external BGP peering between the PE and the CE. This
mechanism applies at each PE device where a VRF-based CE is configured as iBGP. This eliminates the need for service providers
(SPs) to configure autonomous system override for the CE. With this feature enabled, there is no need to configure the virtual
private network (VPN) sites using different autonomous systems.
The introduction of the neighbor internal-vpn-client command enables PE devices to make an entire VPN cloud act like an internal VPN client to the CE devices. These CE devices
are connected internally to the VPN cloud through the iBGP PE-CE connection inside the VRF. After this connection is established,
the PE device encapsulates the CE-learned path into an attribute called ATTR_SET and carries it in the iBGP-sourced path
throughout the VPN core to the remote PE device. At the remote PE device, this attribute is assigned with individual attributes
and the source CE path is extracted and sent to the remote CE devices. ATTR_SET is an optional transitive attribute that
carries a set of BGP path attributes. It can include any BGP attribute that can occur in a BGP update message as received
from the source CE device.