A provider edge (PE) device can learn an IP prefix from the following sources:
A customer edge (CE) device by static configuration
A Border Gateway Protocol (BGP) session with the CE device
A Routing Information Protocol (RIP) exchange with the CE device
The IP prefix is a member of the IPv4 address family. After the PE device learns the IP prefix, the PE converts it into a
VPN-IPv4 prefix by combining it with an 8-byte route distinguisher (RD). The generated prefix is a member of the VPN-IPv4
address family. It uniquely identifies the customer address, even if the customer site is using globally nonunique (unregistered
private) IP addresses. The route distinguisher used to generate the VPN-IPv4 prefix is specified by a configuration command
associated with the virtual routing and forwarding (VRF) instance on the PE device.
BGP distributes reachability information for VPN-IPv4 prefixes for each VPN. BGP communication occurs at two levels:
Within an IP domains, known as an autonomous system (interior BGP [IBGP])
Between autonomous systems (external BGP [EBGP])
PE-PE or PE-RR (route reflector) sessions are IBGP sessions, and PE-CE sessions are EBGP sessions. In an Enhanced Interior
Gateway Routing Protocol (EIGRP) PE-CE environment, when an EIGRP internal route is redistributed into BGP by one PE, and
then back into EIGRP by another PE, the originating router ID for the route is set to the router ID of the second PE, replacing
the original internal router ID.
BGP propagates reachability information for VPN-IPv4 prefixes among PE devices by means of the BGP multiprotocol extensions
(refer to RFC 2283,
Multiprotocol Extensions for BGP-4), which define support for address families other than IPv4. Using the extensions ensures that the routes for a given VPN
are learned only by other members of that VPN, enabling members of the VPN to communicate with each other.