Zte ZXR10 GER User Manual page 313

VPN forwarding table contains a label corresponding to the VPN-
VPN
IP address. Label is used to send the data to the corresponding
forwarding
table destination. Since the label is used instead of the IP address, a
user can maintain its dedicated address structure, without the
need of data transfer by means of Network Address Translation
(NAT). According to the data ingress, the corresponding router
will select a special VPN forwarding table that only contains a
valid destination address in VPN.
CE advertises routing information on the user's network to the
PE by means of static route, default route or routing protocols
RIP, OSPF, IS-IS and BGP.
Multi-Protocol Meanwhile extended multi-protocol BGP is used between PEs to
BGP transfer VPN-IP information and the corresponding label (VPN
label, called internal layer label hereinafter). Traditional IGP is
used between PE and P to learn the routing information from
each other, and the LDP is used for the binding of routing
information and label (a label on the backbone network, called
external layer label hereinafter).
PE In this case, basic network topology and routing information of
CE, PE and P routers have already been formed. A PE router has
the routing information of the backbone network and the routing
information of each VPN.
CE When a CE user on a VPN enters the network, the system can
identify to which VPN the CE belongs on the interface between
the CE and the PE, and will further read the next-hop address
information in the routing table of the VPN. In addition,
forwarded packets will be marked with a VPN label (internal
layer label).
External Layer In this case, the next-hop address obtained is the address of a
Label PE that is the peer of this PE. To reach the destination PE,
routing information of backbone network should be read from
source PE to obtain the address of the next P router, and
meanwhile, forwarded user's packets will be tagged with a
backbone network label (external layer label).
In the backbone network, all P routers after the source PE read
the external layer label to determine the next hop. Therefore,
only simple label switching is performed on the backbone
network.
Destination When a packet reaches the last P router before arriving at the
destination PE, the external layer label will be cancelled. After
the packet reaches the destination PE, the PE will read the
internal layer label, find the next-hop CE in the corresponding
VRF, send the packet to the related interface and further
transfer the data to the CE network of the VPN.
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Chapter 20 MPLS VPN Configuration
291