BROCADE IP PRIMER

MPLS Layer 3 VPNsTo validate the configuration for VPLS, enter the following:BR-Switch# show mpls VPLS detailThis command should provide complete details on the VPLS Peers and theircondition (up, down, etc.).MPLS Layer 3 VPNsLet’s wrap up MPLS with Layer 3 VPNs. With Layer 3 VPNs, the service providerparticipates in the customer's Layer 3 routing. The customer's CE router ateach of their sites speaks a routing protocol, such as BGP or OSPF, to the provider'sPE router, and the IP prefixes advertised at each customer site arecarried across the provider’s MPLS network.In the MPLS VPN backbone, the label stack is used to forward VPN packets. Asthe packet leaves the Customer Edge router, it is forwarded to the ProviderEdge Router (LER). The LER pushes two tags onto the packet. The inner labelused for the VPN, informs the Peer LER to send the packet to an outgoing interfaceor a Virtual Routing and Forwarding Table (VRF). The outer label is theLDP label that guarantees that the packet will reach the peer LER. If the secondlabel points to a VRF, the LER will perform a label lookup to find the targetVRF and then perform an IP Lookup within the VRF. The packet is then routedto the customer edge router. Note that penultimate hop popping can be performedin frame based MPLS. Penultimate hop popping removes the outerlabel at the last LSR prior to the LER. This allows the LER to perform a fasterand simpler lookup.As stated above, the LER provides the inner and outer label, but for the innerlabel to provide details of the VPN, it must know every VPN attached to thepeer LER. This is accomplished by the peer LER providing the inner label to theLER that contains a summarized VPN route table to every attached customeredge router. The mechanism used for the peer LER to provide this inner labelto LER is Multi Protocol - Border Gateway Protocol (MP-BGP). Every MP-BGPupdate carries a label assigned by the peer LER together with the 96-bitVPNv4 prefix. MPLS VPN packet forwarding requires that the LER specified asthe BGP next hop in the incoming BGP update is the same LER as the one thatassigned the second label to the label stack.Brocade IP Primer 267