3
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.
Multiprotocol BGP (MP-BGP) can solve this problem by advertising VPN-IPv4 addresses (also
called VPNv4 addresses).
As shown in
, 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 2 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.
MCE working mechanism
As shown in
, the MCE exchanges private routes with VPN sites and PE 1, and adds the
private routes to the routing tables of corresponding VPN instances.
Type
2 bytes
4 bytes
IPv4 address prefix
6 bytes
Route Distinguisher (8 bytes)
Assigned number subfield
Administrator subfield
