239
Figure 66 VPN-IPv4 address structure
Upon receiving an IPv4 route from a CE, a PE changes the route to a VPN route by adding an RD
and then advertises the VPN route to the peer PE. The RD ensures the uniqueness of the VPN route.
Each service provider can independently assign unique RDs. A PE can advertise routes for VPNs
even if the VPNs are from different service providers and are using the same IPv4 address space.
Configure a distinct RD for each VPN instance on a PE, so that routes to the same CE use the same
RD. The VPN-IPv4 address with an RD of 0 equals a globally unique IPv4 address.
By prefixing a distinct RD to a specific IPv4 address prefix, you get a globally unique VPN IPv4
address prefix.
An RD can be related to an autonomous system (AS) number, in which case it is the combination of
the AS number and a discretionary number. Or it can be related to an IP address, in which case it is
the combination of the IP address and a discretionary number.
An RD can be in one of the following formats distinguished by the Type field:
•
When the value of 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 value of 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 value of 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 an RD, do not set the Administrator subfield to any private AS
number or private IP address.
BGP extended community attributes
MPLS L3VPN uses the these extended community attributes: route target attributes and the Site of
Origin (SoO) attribute.
MPLS L3VPN uses route target 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
advertised from other PEs. If the export target attribute matches the import target attribute of the
VPN instance, the PE adds the routes to the VPN routing table.
In other words, route target attributes define which sites can receive VPN-IPv4 routes, and from
which sites that a PE can receive routes.
Like RDs, route target attributes can be 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.
Type
2 bytes
4 bytes
IPv4 address prefix
6 bytes
Route Distinguisher (8 bytes)
Assigned number subfield
Administrator subfield