3Com Switch 8800 Configuration Guide Chapter 37 BGP/MPLS VPN Configuration
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3) Each P router on LSP forwards MPLS packets using exterior-layer label to the
penultimate-hop router, namely the P router before PE2. The penultimate-hop
router extracts the exterior-layer and sends MPLS packet to PE2.
4) PE2 looks up in the MPLS forwarding table according to the interior-layer label and
destination address to determine the egress interface for labeling operation and
the packet. It then extracts the interior-layer label and forwards through the egress
interface the IPv4 packet to CE2.
5) CE2 looks up in the routing table and sends the packet in normal IPv4 packet
forwarding mode to the site2.
37.1.3 Nested BGP/MPLS VPN Implementation
When implementing a nested BGP/MPLS VPN, pay attention to the following items:
z No address overlap is allowed between user's internal sub-VPNs.
z To ensure the VPN routing information is correctly advertised over the backbone
network, the VPN-Targets of the user VPN and the internal sub-VPNs cannot be
overlapped and must be specified by the service provider.
z The provider PE and the customer PE must be directly connected and cannot
exchange VPNv4 route in Multihop-EBGP mode.
Before configuring a nested BGP/MPLS VPN, you must complete the following tasks:
z Configuring IGP on the MPLS backbone network (including provider PE and P
routers) to implement the IP connectivity on the backbone network.
z Configuring basic MPLS capability on the MPLS backbone network.
z Configuring MPLS LDP and setting up LDP LSP on the MPLS backbone network.
z Configuring BGP on the MPLS backbone network (create EBGP peers between
provider PEs).
z Configuring basic MPLS capability on user-end network (including customer PEs).
37.1.4 Hierarchical BGP/MPLS VPN Implementation
As PE is required to aggregate multiple VPN routes on a BGP/MPLS VPN, it is prone to
forming a bottleneck in a large-scale deployment or in the case that PE capacity is
small.
Hierarchical BGP/MPLS VPN divides an MPLS VPN into several MPLS VPNs in a
hierarchical network structure. Each VPN takes on a role depending on its level. There
are high performance requirements in routing and forwarding on the PEs at the higher
level of MPLS VPN, because they are primarily used for connecting the backbone
networks and providing access service for huge VPN clients. However, such
requirements are relatively low for PEs at the lower level of the network as they
primarily function to access the VPN clients at the edges. Congruous with the IP
network model, HoVPN model improves the scalability of BGP/MPLS VPN, and hence
allows lower-layer MPLS VPNs comprising low-end equipment to provide MPLS VPN
accessing and interconnect through the high-end MPLS VPN backbone.