Route Targets - Avaya 8800 Planning And Engineering, Network Design

Figure 119: VPN-IPv4 Address
The RD is configured on each and every VRF created on the PE nodes and must be configured
such that no other VRF on any other PE in the backbone has the same value. RDs are encoded as
part of the Network Layer Reachability Information (NLRI) in the BGP Update messages.
Please note that the RD is simply a number that you configure. It provides a means to identify a PE
node which may contain one or more VRFs. It does not identify the origin of the route nor does it
specify the set of VPNs or VRFs to which the routes are distributed. Its sole purpose is to provide a
mechanism to support distinct routes to a common IPv4 address prefix. By allowing different RDs to
support the same IPv4 addresses, overlapping addresses are supported.

Route targets

When an VPN-IPv4 route advertised from a PE router is learned by a given PE router, it is
associated with one or more Route Target (RT) attributes. The RT, which is configured on the PE
router as either import, export, or both, is the glue which determines whether a customer VPN-IPv4
route being advertised by one PE router can be accepted by another remote PE router resulting in
the formation of a logical IP VPN end to end. These routes are accepted by a remote PE providing
the remote PE has a matching import RT configured on one of its VRFs.
A Route Target attribute can be thought of as identifying a set of sites, though it would be more
precise to think of it as identifying a set of VRFs. Each VRF instance is associated with one or more
Route Target (RT) attributes. Associating a particular Route Target attribute with a route allows that
route to be placed in the VRFs that are used for routing traffic among the sites in that VPN. Note
that a route can only have one RD, but it can have multiple Route Targets. RTs also enhance the
PE scaling capability since a given PE node only accepts VPN-IPv4 routes for which it has local
VRFs belonging to that IP VPN; any other VPN-IPv4 routes are not accepted.
Each VPN-IPv4 route contains a route target extended community that is advertised or exported by
the PE router export policy. Any PE router in the network configured with a matching route target in
its import policy imports the route for that particular VRF.
June 2016 Planning and Engineering — Network Design
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