3Com 8800 Configuration Manual page 489

3Com Switch 8800 Configuration Guide
Set up IBGP adjacency between PE1 and PE2 or PE1 and PE3, but not between
PE2 and PE3, that is, VPN routing information cannot be exchanged between PE2
and PE3.
Create two VPN-instances on PE1, import VPN routes of VPN-target 100:11 and
100:12, set VPN-target for VPN routes advertised as 100:2.
Create a VPN-instance on PE2, import VPN routes of VPN-target 100:2, set
VPN-target for VPN routes advertised as 100:11.
Create a VPN-instance on PE3, import VPN routes of VPN-target 100:2, set
VPN-target for VPN routes advertised as 100:12.
Then PE2 and PE3 can only learn their neighbor's routes through PE1.
Note:
In this case the configuration is focused on four points:
Route advertisement can be controlled by VPN-target settings on different PEs.
Routing loop is permitted only once, so that PE can receive route update messages
with AS number included from CE.
In Hub&Spoke networking, vpn-target of VPN-instance (VPN-instance3) which is
used to release route on the PE1 cannot be the same with any vpn-target of
VPN-instance (VPN-instance2) which is used to import route on PE1.
In Hub&Spoke networking, route-distinguisher rd2 (100:3) of VPN-instance which is
used to release route on the PE1 cannot be the same with the route-distinguisher
rd1 (100:1) or rd4 (100:4) of corresponding VPN-instances on each PE2 and PE3;
rd 1 and rd4 can be the same or not.
Chapter 37 BGP/MPLS VPN Configuration
37-45