3Com S7906E Configuration Manual page 1522

Figure 1-5 Network diagram for hub and spoke networking scheme
In Figure 1-5, the spoke sites communicate with each other through the hub site. The arrows in the
figure indicate the advertising path of routes from Site 2 to Site 1:
The hub PE can receive all the VPN-IPv4 routes advertised by spoke PEs.
All spoke PEs can receive the VPN-IPv4 routes advertised by the hub PE.
The hub PE advertises the routes learnt from a spoke PE to the other spoke PEs. Thus, the spoke
sites can communicate with each other through the hub site.
The import target attribute of any spoke PE is distinct from the export VPN targets of the other
spoke PEs. Therefore, any two spoke PEs can neither directly advertise VPN-IPv4 routes to each
other nor directly access each other.
Extranet networking scheme
The extranet networking scheme can be used when some resources in a VPN are to be accessed by
users that are not in the VPN.
In this kind of networking scheme, if a VPN needs to access a shared site, the export target attribute
and the import target attribute of the VPN must be contained respectively in the import target attribute
and the export target attribute of the VPN instance of the shared site.
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