3Com 8807 Configuration Manual page 533

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The routes for other VPNs will not appear in the VPN's routing table by using VPN
Target attribute to filter routing information received at PE router, so the
CE-transmitted data will only be forwarded within the VPN.
BGP/MPLS VPN BGP/MPLS VPN works on this principle: It uses BGP to propagate VPN private
Implementation routing information on carrier backbone network, and uses MPLS to forward VPN
service traffic.
The following are introductions to BGP/MPLS implementation from two aspects:
advertising VPN routing information and forwarding VPN packets.
Advertising VPN routing information
Routing information exchange has the following four types:
1 Between CE and PE
A PE router can learn routing information about the CE connected to it through
static route, RIP (supporting multi-instance), OSPF (supporting multi-instance) or
EBGP, and imports it in a vpn-instance.
2 Between ingress PE and egress PE
The ingress PE router uses MP-BGP to send information across public network: It
advertises routing information learned from CE to the egress PE router (with MPLS
label) and learns the CE routing information learned at the egress PE router.
The internal connectivity among the VPN internal nodes is ensured through
enabling IGP (for example, RIP and OSPF) or configuring static routes on the PEs.
3 LSP setup between PEs
LSPs must be set up between PEs for VPN data traffic forwarding with MPLS LSP.
The PE router which receives packets from CE and create label protocol stack is
called Ingress LSR, while the BGP next hop (Egress PE router) is Egress LSR. Using
LDP to create fully connected LSPs among PEs.
4 Between PE and CE
A CE can learn remote VPN routes from the PE connected through static routes,
RIP, OSPF or EBGP.
With above-mentioned steps, reachable routes can be established between CEs,
for transmission of VPN private routing information over public network.
Forwarding VPN packets
On the ingress PE, two-layer label stack is formed for each VPN packet:
Interior-layer label, also called MPLS label, is at the bottom of the label stack and
distributed by M-BGP when the egress PE advertises routing information (in VPN
forwarding table) to ingress GE. When VPN packets from public network reach the
CE, they can be forwarded from the designated interface to the designated CE or
site by searching for the target MPLS forwarding table according to the labels
contained.
BGP/MPLS VPN Overview 533