Users Guide

BGP 1353
If two VRFs use the same IPv4 address prefix, the router translates these into
unique VPN-IPv4 address prefixes by prepending the RD (configured per
VRF) to the address. The purpose of the RD is to allow the router to install
unique routes with an identical IPv4 address prefix. The structuring of the
RD provides no semantics. When BGP compares two such addresses, it
ignores the RD structure completely and compares it as a 12-byte entity. It is
recommended that each VPN within a site utilize a unique RD.
A router may be configured to associate routes that belong to a particular
VRF with a particular RD. When BGP redistributes these routes, BGP
prepends the configured RD value to the route and re-distributes them as
VPNv4 routes. The router that receives these VPNv4 routes installs them into
the global BGP table along with the RD. If two routes have the same address
prefix but different RD values, only the last route is installed to the RTO
table of the router that imports the route and the rest are overwritten.
Dell EMC Networking BGP doesn't advertise routes in the traditional IPv4
NLRI format when a neighbor is activated in VPNv4 address family mode.
Controlling Route Distribution
This section describes the methods by which VPNv4 route redistribution may
be controlled.
The Route Target Attribute (RT)
A Route Target attribute identifies a set of VRFs belonging to a VPN. Every
VRF is associated with one or more "Route Target" attributes that define the
VPNs to which it belongs. Route targets are advertised using the VPNV4
address family.
When a VPNv4 route is advertised by a router, the "Route Target" attributes
are carried in the BGP advertisement as attributes of the route.
An MP-BGP router that receives a VPNV4 route compares it with the Import
Route Target attributes configured for one or multiple VRFs and depending
on the match installs the route into the matching VRF table. When a BGP
router advertises a route to a BGP neighbor, it attaches one or more Export
Route Target attributes to the route (as configured for that VRF).
The Export Route Target attributes and the Import Route Target attributes
are distinct sets and may or may not be the same in a VRF.
A BGP route can only have one RD but can have multiple Route Targets.