Brocade Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide (Supporting R05.6.00) - Manual (Page 643)

Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide 619
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BGP or MPLS VPN operation
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The PE device is connected to the MPLS domain through one or more interfaces. The PE must
advertise the routes that it has available in it’s VRF tables across the MPLS domain to its PE peers.
Available routes in the VRF are prepended with a Route Distinguisher (RD) and advertised across
the MPLS domain using IBGP. The PEs can either be configured for IBGP as either full mesh or with
a route reflector to allow greater scalability. Routes that are advertised from other PEs in the VPN
are received at the PE and collected in the VRF table. This procedure establishes which other PEs
are in the VPN and what networks are available through them.
OSPF is used a the Interior Gateway Protocol (IGP) within the service provider’s MPLS domain to
provide connectivity. OSPF also populates the traffic engineering (TE) used by RSVP-TE.
Labeled Switch Paths (LSPs) are then created using Label Distribution Protocol (LDP), Resource
Reservation Protocol (RSVP) configurations in the MPLS domain. Using this protocol, the PE obtains
an LSP required to switch traffic to the other PEs. The network is now populated with all of the
routes required to forward packets between the customer’s networks.
FIGURE 71 BGP or MPLS VPN route discovery
Routing a packet through a BGP or MPLS VPN
This section and the diagram in Figure 72 describe how a packet is forwarded through a BGP or
MPLS VPN.
When a packet is forwarded from a CE to a PE, a bottom label is attached to the packet by the PE
that is associated with the final destination. This label is obtained from the egress PE as part of the
route discovery conducted by IBGP. Then, the top label which is obtained by the LSP connecting to
the egress PE is added to the packet. The packet is then forwarded through the MPLS domain and
is switched using the top label. At the penultimate device in the LSP, the top label is removed and
the packet is forwarded to the egress PE. The egress PE uses the inner label to identify the CE to
which the packet must be forwarded. The egress PE removes the inner label and forwards the
packet to the correct CE.
FIGURE 72 Routing a packet through a BGP or MPLS VPN