3Com 8807 Configuration Manual page 532

532 C 49: BGP/MPLS VPN C
HAPTER ONFIGURATION
VPN is just a private network, so it can use the same IP address to indicate different
sites. But the IP address is supposed as unique when MP-BGP advertises CE routes
between PE routers, so routing errors may occur for the different meaning in two
systems. The solution is to switch IPv4 addresses to VPN-IPv4 address to generate
globally unique addresses before advertising them, so PE routers is required to
support MP-BGP.
A VPN-IPv4 address consists of 12 bytes, and the first eight bytes represent the RD
(Route Distinguisher), which are followed by a 4-byte IPv4 address. The service
providers can distribute RD independently. However, their special AS (Autonomous
System) number must be taken as a part of the RD. After being processed in this
way, even if the 4-byte IPv4 address contained in VPN-IPv4 address has been
overlapped, the VPN-IPv4 address can still maintain globally unique. RD is only
used within the carrier network to differentiate routes. When the RD is 0, a
VPN-IPv4 address is just a IPv4 address in general sense.
The route received by PE from CE is the IPv4 route that needs to be redistributed
into VPN-instance routing table, and in this case a RD needs to be added. It is
recommended that the same RD be configured for all routes from the same user
site.
VPN Target attribute
VPN Target attribute is one of the MBGP extension community attributes and is
used to limit VPN routing information advertisement. It identifies the set of sites
that can use some route, namely by which Sites this route can be received, and the
PE router can receive the route transmitted by which Sites. The PE routers
connected with the site specified in VPN Target can all receive the routes with this
attribute.
For PE routers, there are two sets of VPN Target attributes: one of them, referred
to as Export Targets, is added to the route received from a direct-connect site in
advertising local routes to remote PE routers. And the other one, known as Import
Targets, is used to decide which routes can be imported into the routing table of
this site in receiving routes from remote PE routers.
When matching the VPN Target attribute carried by the route to filter the routing
information received by the PE router, if the export VPN target set of the received
route contains identical items with the import VPN target set of the local end, the
route is imported into the VPN routing table and then advertised to the connected
CE . Otherwise, the route will be rejected.
Figure 129 Route filtering through matching VPN Target attribute
ERT: Export Route Targets
RD IPv4 address ... ERT1 ERT2 ... ERTn
MPLS VPN Route
Import Route Targets:
( IRT1, IRT2, ... ,IRTm )