3Com MSR 50 Series Configuration Manual page 1280

1280 C 75: M
HAPTER ULTICAST
VPN C
ONFIGURATION
Figure 368 Typical application of MPLS L3VPNs
VPN B
Site 6
CE 6
PE 3
VPN A
Site 5
As shown in Figure 368, VPN A comprises Site 1, Site 2 and Site 3, while VPN B
comprises Site 4, Site 5 and Site 6. A VPN involves the following three types of
devices:
Provider (P) device: device in the core of the provider backbone network. A P
■
device does not directly interface with CE devices, but it implements MPLS
forwarding.
Provider edge (PE) device: edge device in the provider backbone network.
■
Directly interfacing with one or more CE devices, a PE device processes VPN
routing as a main MPLS L3VPN implementer.
Customer edge (CE) device: edge device on a customer network. A CE device
■
can be a router, a switch, or a host, that implements route distribution on the
customer network.
In an MPLS L3VPN environment, between any two sites that belong to the same
VPN, packets are transmitted labeled across the public network. The PE device at
the entrance to the provider backbone attaches two labels to the packets, one
inner label and the other outer label:
Outer label: the label used for switching within the backbone, representing a
■
tunnel from the local PE to the peer PE. With this label, a packet can arrive to
the peer PE along a label switched path (LSP).
Inner label: The inner label represents a tunnel between two CE devices
■
interconnected over the backbone network. It identifies the site to which the
packet belongs. The PE forwards the packet to the target CE based on the
inner label.
VPN A
Site 1
PE 1
CE 1
P 1
Core
layer
P 3
CE 5
VPN B
Site 2
CE 2
CPE
layer
Edge
layer
P 2
CE 3
PE 2
CE 4
VPN B
Site 4
VPN A
Site 3