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VPN-IPv4 Addresses
Because each VPN has its own private address space, the same IP address might be
used in several VPNs. To provide for more than one route to a given IPv4 address
(each route unique to a single VPN), BGP/MPLS VPNs use route distinguishers (RDs)
followed by an IPv4 address to create unique VPN-IPv4 addresses. A route can have
only one RD.
The RD contains no routing information; it simply enables you to create unique
VPN-IPv4 address prefixes. You can specify the RD in either of the following ways:
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An autonomous system (AS) number followed by a 32-bit assigned number. If
the AS number is from the public address space, it must have been assigned to
the service provider by the Internet Assigned Numbers Authority (IANA). The
service provider can choose the assigned number. We recommend you do not
use numbers from the private AS number space.
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An IP address followed by a 16-bit assigned number. If the IP address is from
the public IP address space, it must have been assigned to the service provider
by IANA. The assigned number may be chosen by the service provider. Use of
numbers from the private IP address space is strongly discouraged.
You can create unique VPN-IPv4 addresses by assigning a unique RD to each VRF
in your network. However, the optimal strategy depends on the configuration of your
network. For example, if each VRF always belongs to only one VPN, you might use
a single RD for all VRFs that belong to a particular VPN.
Route Targets
A route-target extended community, or route target, is a type of BGP extended
community that you use to define VPN membership. The route target appears in a
field in the update messages associated with VPN-IPv4.
You create route-target import lists and route-target export lists for each VRF. The
route targets that you place in a route target export list are attached to every route
advertised to other PE routers. When a PE router receives a route from another PE
router, it compares the route targets attached to each route against the route-target
import list defined for each of its VRFs. If any route target attached to a route matches
the import list for a VRF, then the route is imported to that VRF. If no route target
matches the import list, then the route is rejected for that VRF.
Depending on your network configuration, the import and export lists may be
identical. Typically, you do the following:
■
Allocate one route-target extended-community value per VPN.
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Configure the import list and the export list to include the same information: the
set of VPNs comprising the sites associated with the VRF.
For more complicated scenarios—for example, hub-and-spoke VPNs—the route-target
import list and the route-target export list might not be identical.
Overview
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Chapter 5: Configuring BGP-MPLS Applications