MPLS L3VPN concepts
Site
A site has the following features:
A site is a group of IP systems with IP connectivity that does not rely on any service provider networks.
The classification of a site depends on the topology relationship of the devices, rather than the geographical positions. However, the devices at a site are, in most cases, adjacent to each other geographically.
The devices at a site can belong to multiple VPNs, which means that a site can belong to multiple VPNs.
A site is connected to a provider network through one or more CEs. A site can contain multiple CEs, but a CE can belong to only one site.
Sites connected to the same provider network can be classified into different sets by policies. Only the sites in the same set can access each other through the provider network. Such a set is called a VPN.
VPN instance
VPN instances implement route isolation, data independence, and data security for VPNs.
A VPN instance has the following components:
A separate Label Forwarding Information Base (LFIB).
An IP routing table.
Interfaces bound to the VPN instance.
VPN instance administration information, including route distinguishers (RDs), route targets (RTs), and route filtering policies.
To associate a site with a VPN instance, bind the VPN instance to the PE's interface connected to the site. A site can be associated with only one VPN instance, and different sites can be associated with the same VPN instance. A VPN instance contains the VPN membership and routing rules of associated sites.
VPN-IPv4 address
Each VPN independently manages its address space. The address spaces of VPNs might overlap. For example, if both VPN 1 and VPN 2 use the addresses on subnet 10.110.10.0/24, address space overlapping occurs.
BGP cannot process overlapping VPN address spaces. For example, if both VPN 1 and VPN 2 use the subnet 10.110.10.0/24 and each advertise a route destined for the subnet, BGP selects only one of them. This results in the loss of the other route.
Multiprotocol BGP (MP-BGP) can solve this problem by advertising VPN-IPv4 addresses (also called VPNv4 addresses).
As shown in Figure 77, a VPN-IPv4 address consists of 12 bytes. The first eight bytes represent the RD, followed by a four-byte IPv4 prefix. The RD and the IPv4 prefix form a unique VPN-IPv4 prefix.
Figure 77: VPN-IPv4 address structure
An RD can be in one of the following formats:
When the Type field is 0, the Administrator subfield occupies two bytes, the Assigned number subfield occupies four bytes, and the RD format is 16-bit AS number:32-bit user-defined number. For example, 100:1.
When the Type field is 1, the Administrator subfield occupies four bytes, the Assigned number subfield occupies two bytes, and the RD format is 32-bit IPv4 address:16-bit user-defined number. For example, 172.1.1.1:1.
When the Type field is 2, the Administrator subfield occupies four bytes, the Assigned number subfield occupies two bytes, and the RD format is 32-bit AS number:16-bit user-defined number, where the minimum value of the AS number is 65536. For example, 65536:1.
To guarantee global uniqueness for a VPN-IPv4 address, do not set the Administrator subfield to any private AS number or private IP address.
Route target attribute
MPLS L3VPN uses route target community attributes to control the advertisement of VPN routing information. A VPN instance on a PE supports the following types of route target attributes:
Export target attribute—A PE sets the export target attribute for VPN-IPv4 routes learned from directly connected sites before advertising them to other PEs.
Import target attribute—A PE checks the export target attribute of VPN-IPv4 routes received from other PEs. If the export target attribute matches the import target attribute of a VPN instance, the PE adds the routes to the routing table of the VPN instance.
Route target attributes define which sites can receive VPN-IPv4 routes, and from which sites a PE can receive routes.
Like RDs, route target attributes can be one of the following formats:
16-bit AS number:32-bit user-defined number. For example, 100:1.
32-bit IPv4 address:16-bit user-defined number. For example, 172.1.1.1:1.
32-bit AS number:16-bit user-defined number, where the minimum value of the AS number is 65536. For example, 65536:1.