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Cisco IP Solution Center, 3.0: MPLS VPN Management User Guide, 3.0
OL-4344-01
Chapter1 About Cisco IP Solution Center
About MPLS VPNs
•The MPLS VPN backbone relies on the appropriate Interior Gateway Protocol (IGP) that is
configured for MPLS, for example, EIGRP, or OSPF. When you issue a show ip route command on
a PE, you see the IGP-derived routes connecting the PEs together. Contrast that with the show ip
route vrf VRF_name command, which displays routes c onnecting customer sites in a particular
VPN.
Creating a VRF Instance
The configuration commands to create a VRF instance are as follows:
Route Distinguishers and Route Targets
MPLS-based VPNs employ BGP to communicate between PEs to facilitate customer routes. This is
made possible through extensions to BGP that carry addresses other than IPv4 addresses. A notable
extension is called the route distinguisher (RD).
The purpose of the route distinguisher (RD) is to make the prefix value unique across the backbone.
Prefixes should use the same RD if they are associated with the same set of route targets (RTs) and
anything else that is used to select routing policy. The community of interest association is based on the
route target (RT) extended community attributes distributed with the Network Layer Reachability
Information (NLRI).The RD value must be a globally unique value to avoid conflict with other prefixes.
The MPLS label is part of a BGP routing update. The routing update also carries the addressing and
reachability information. When the RD is unique across the MPLS VPN network, proper connectivity is
established even if different customers use non-unique IP addresses.
For the RD, every CE that has the same overall role should use a VRF with the same name, same RD,
and same RT values. The RDs and RTs are only for route exchange between the PEs running BGP. That
is, for the PEs to do MPLS VPN work, they have to exchange routing information with more fields than
usual for IPv4 routes; that extra information includes (but is not limited to) the RDs and RTs.
The route distinguisher values are chosen by the ISC software.
•CEs with hub connectivity use bgp_AS:value.
•CEs with spoke connectivity use bgp_AS:value + 1
Each spoke uses its own RD value for proper hub and spoke connectivity between CEs; therefore, the
ISC software implements a new RD for each spoke that is provisioned.
Command Description
Step1 Router# configure terminal
Router(config)#
Enter global configuration mode.
Step2 Router(config)# ip vrf vrf_name For example, ip vrf CustomerA initiates a VPN routing table
and an associated CEF table named CustomerA. The command
enters VRF configuration submode to configure the variables
associated with the VRF.
Step3 Router(config-vrf)# rd RD_value Enter the eight-byte route descriptor (RD) or IP address. The PE
prepends the RD to the IPv4 routes prior to redistributing the
route into the MPLS VPN backbone.
Step4 Router(config-vrf)# route-target import |
export | both community
Enter the route-target information for the VRF.