PLANET XGS3-24042 - Manual (Page 679)

81-33
If a VPN user wants to provide some site resource of this VPN to outside users, the Extranet Networking
resolution can solve the problem.
In this networking if a VPN needs to access the sharing site, its Export Target should be included in the Import
Target of the sharing site VPN instances, and its Import Target should be included in the Export Target of the
sharing site VPN instances.
In the above figure, site3 of VPN1 can be accessed by VPN1 and VPN2:
 PE3 can receive the VPN-IPv4 routes advertised by PE1 and PE2
 PE1 and PE2 can receive the VPN-IPv4 routes advertised by PE3
 Based on the above two conditions, site1 and site3 of VPN1 can intercommunicate, so do the site2
of VPN2 and site3 of VPN1.
PE3 won’t advertise VPN-IPv4 routes from PE1 to PE2, or advertise the VPN-IPv4 route from PE2 to PE1 (the
routes learnt from an IGBP neighbor won’t be sent to other IBGP neighbors), so site1 of VPN1 and site2 of
VPN2 can’t intercommunicate.
81.1.5 BGP/MPLS VPN Route Advertisement
In basic BGP/MPLS VPN networks, VPN route advertisement concerns CE and PE, since P routers only
maintains routes of the backbone network, and doesn’t need any VPN route information. PE routers only
maintain the VPN route information directly connected to it, not all VPN routes. SO the BGP/MPLS VPN
network is easy to extend.
The VPN route advertisement process includes three parts to create a reachable route from the local CE to
the remote CE, enabling the advertisement of VPN private network route information in the backbone network:
from local CE to ingress PE, from the ingress PE to the egress PE, from egress PE to the remote CE.
The followings are introduction to the three parts:
The route information switch from the local CE to the ingress PE
CE will send the local VPN route to the PE directly connected to it after establishing an adjacency to the latter.
CE can use static routes, RIP, OSPF, IS-IS or EBGP to send routes to PE, all in the form of standard IPv4
routes.
The route information switch from the ingress PE to the egress PE
PE will add RD and VPN target attributes to the VPN routes it learns from CE, then store these VPN-IPv4
routes into the VPN instances created for CE.
The ingress PE will advertise the VPN-IPv4 routes to the egress PE via MP-BGP. The egress PE will
determine whether to add this route into the route table of VPN instance according to the routes’ Export Target
attribute and the import Target of the VPN instances it maintains.
Different PE ensure the intercommunication between them via IGP.
The route information switch between the egress PE to the remote CE
Like the route information switch from the local CE to the ingress PE, there are many available methods for
the remote CE to learn VPN routes the egress PE, including static route, RIP, OSPF, IS-IS and EBGP.
81.1.6 Multi-AS VPN Introduction