Case Study 3: Implementing Layer 2 VPNs over Inter-AS Topologies Using Layer 2 VPN Pseudo-Wire Switching

Case Study 3 Implementing Layer 2 VPNs over Inter AS Topologies Using Layer 2 VPN Pseudo Wire Switching

Service providers SP1 and SP2 are required by Customer A to provide Layer 2 VPN connectivity between multiple sites belonging to Customer A, as illustrated in Figure 14-22.

Figure 14-22. Case Study 3 Topology and Base Configurations

To enable the implementation of Layer 2 VPN (AToM) over an Inter-AS topology, as shown in Figure 14-22, the ASBR Routers PE2-AS1-ASBR1 and PE1-AS2-ASBR1 are configured for layer2vpn pseudo-wire switching.

Figure 14-22 also outlines the basic configurations for the devices in the SP domain, namely, PE1-AS1, PE2-AS1-ASBR1, P1-AS1, PE2-AS2, PE1-AS2-ASBR1, P1-AS2, and CE routers CE1-A and CE2-A. It is assumed that MPLS forwarding has been enabled on the appropriate interfaces, and IP addressing performed as illustrated in Figure 14-22. The PE routers are configured for OSPF as the IGP. The ASBR routers are configured for Inter-AS implementation between one another. CE routers CE1-A and CE2-A are configured with OSPF process of 10 with all networks in area 0. Only the configurations on the routers to implement Inter-AS have been depicted. Note that the Inter-AS implementation uses IPv4 BGP for label distribution between AS boundaries. Configurations pertaining to the implementation of Layer 2 VPN pseudo-wire switching are shown later in this section.

Layer 2 VPN Pseudo-Wire Switching Theory and Configuration

Layer 2 VPN pseudo-wire switching enables extension of Layer 2 VPN pseudo-wires across an Inter-AS boundary or across two separate MPLS networks. Layer 2 VPN pseudo-wire switching connects two or more contiguous pseudo-wire segments to form an end-to-end multi-hop pseudo-wire. This end-to-end pseudo-wire functions as a single point-to-point pseudo-wire. Layer 2 VPN pseudo-wire switching enables the SP to keep the IP addresses of the edge PE routers private across Inter-AS boundaries, using the IP addresses of the ASBRs, the ASBRs join the pseudo wires of the two domains.

AToM packets forwarded between two pseudo wires are treated the same as any other MPLS packet excluding the following exceptions:

• The outgoing virtual circuit (VC) label replaces the incoming VC label in the packet. New Internal Gateway Protocol (IGP) labels and Layer 2 encapsulation are added.
• The incoming VC label Time To Live (TTL) field is decremented by one and copied to the outgoing VC label TTL field.
• The incoming VC label EXP value is copied to the outgoing VC label EXP field.
• The outgoing VC label "Bottom of Stack" S bit in the outgoing VC label is set to 1.
• AToM control word processing is not performed at the Layer 2 VPN pseudo-wire switching aggregation point or ASBR. Sequence numbers are not validated.

The configurations required on the PE routers in either domain, as well as the ASBRs implementing Inter-AS, are shown in the configuration flowchart in Figure 14-23.

Figure 14-23. Configuration Flowchart for PE and ASBR Routers to Implement Layer 2 VPN Pseudo-Wire Switching

Following the configuration flowchart shown in Figure 14-23, the additional configurations for the PE and PE-ASBR devices in Figure 14-22 are as illustrated in Figure 14-24.

Figure 14-24. Configuration in Case Study 3 for PE and ASBR Routers to Implement Layer 2 VPN Pseudo-Wire Switching

 

Verifications for Case Study 3

Figure 14-25 outlines the verification involved in the implementation of L2 VPN pseudo-wire switching on the PE, PE-ASBR, and CE routers in Case Study 3.

Figure 14-25. Case Study 3: Layer 2 VPN Pseudo-Wire Switching Verification

 

Final Configurations for Case Study 3

The final configurations for the devices in Case Study 3 are shown in Figure 14-26.

Figure 14-26. Case Study 3: Final Configurations