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DES-7200 Configuration Guide Chapter 2 Configuring BGP IP VPN
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The VPN Routing and Forwarding table (VRF) is used to address the conflicts of local routes.
Each connection between a PE and CE is associated with a VRF. One PE can have several
VRFs to exchange route information with CEs. You can consider a VRF as a virtual router. Each
virtual router should be connected to a CE to receive route information from the CE or notify the
CE of the VPN route information. The VRF addresses the conflicts of local routes due to the
adoption of the same address space by different VPNs. One VRF includes the following:
1) An independent routing table
2) A group of interfaces that belong to the VRF
3) A group of routing protocols that are used in the VRF
The VRF has two important attributes: Route Distinguisher (RD) and Route-Target (RT)
attributes.
RD
The RD is introduced to address the conflict of routes during the transmission.
You can consider the RD as a distinguisher. If different VPNs use the same network address and
advertise their route information on the backbone network through BGP, the BGP module
chooses and advertises only the best route from the overlapped addresses. As a result, some
VPNs cannot obtain their route information. If the RD values are added to the overlapped
addresses, the BGP module identifies the same network addresses based on the different RDs
carried in the VPN information. In this manner, each VPN can obtain its own route information.
The RD only serves as a distinguisher to identify the same network addresses. If address
overlapping does not exist for different VPNs, you can configure no RD values.
Generally speaking, one VPN is specified with a unique RD value. In this manner, different VPNs
have different RDs, facilitating the transmission of routing information on the backbone network.
The RD value is generally defined as xx: xx, such as RD 1: 100, among which 1 stands for the
AS number of the backbone network and 100 is a number specified by the user. One VPN route
can carry only one RD value.
The RD consists of three fields: type, administrator, and assigned number. Based on the value of
the type field, the encoding formats are classified into the following three types: