Basic Principles of Seamless MPLS - Part 3

Inter-AS Seamless MPLS+HVPN

Control plane

   Deploy routing protocols : 

Figure 1, Deploying routing protocols for the inter-AS seamless MPLS+HVPN networking

As shown in Figure 1, routing protocols are deployed on devices as follows:

• An IGP (IS-IS or OSPF) is enabled on devices at each of the access, aggregation, and core layers to implement intra-AS connectivity.
• An IBGP peer relationship is established between each of the following pairs of devices:

  • AGG and an AGG ASBR
  • Core ASBR and MASG
• An EBGP peer relationship is established between the AGG ASBR and core ASBR.
• An MP-IBGP peer relationship is established between the CSG and AGG, and a multi-hop MP-EBGP peer relationship is established between the AGG and MASG.

    Deploy tunnels : 

Figure 2, Deploying tunnels for the inter-AS seamless MPLS+HVPN networking

As shown in Figure 2, tunnels are deployed as follows:

• A public network tunnel is established using LDP or TE in each IGP area.
• The AGGs, AGG ASBRs, core ASBRs, and MASGs are enabled to advertise labeled routes. They assign labels to BGP routes that match a specified routing policy. After they exchange BGP routes, a BGP LSP can be established between each pair of an AGG and MASG.

Forwarding plane

Figure 3, Forwarding plane of the inter-AS seamless MPLS+HVPN networking

Figure 3 illustrates the forwarding plane of the inter-AS seamless MPLS+HVPN networking. Seamless MPLS is mainly used to transmit VPN packets. The following example demonstrates how VPN packets, including labels and data, are transmitted from a CSG to an MASG along the path CSG2 -> AGG1 -> AGG ASBR1 -> core ASBR1-> MASG1.

1. The CSG pushes an MPLS tunnel label into each VPN packet and forwards the packets to the AGG.
2. The AGG removes the access-layer MPLS tunnel labels from the packets and pushes a BGP LSP label. It then adds aggregation-layer MPLS tunnel labels to the packets and then proceeds to forward them to the AGG ABR. If the PHP function is enabled on the AGG, the CSG has removed the MPLS tunnel labels from the packets, and therefore, the AGG receives packets without MPLS tunnel labels.
3. The AGG ASBR then removes the MPLS tunnel labels from packets and swaps the existing BGP LSP label for a new label in each packet. It then forwards the packets to the core ASBR. If the PHP function is enabled on the AGG ASBR, the AGG has removed the MPLS tunnel labels from the packets, and therefore, the AGG ASBR receives packets without MPLS tunnel labels.
4. After the core ASBR receives the packets, it swaps a BGP LSP label for a new label and adds a core-layer MPLS tunnel label to each packet. It then forwards the packets to the MASG.
5. The MASG removes MPLS tunnel labels, BGP LSP labels, and VPN labels from the packets. If the PHP function is enabled on the MASG, the core ASBR has removed the MPLS tunnel labels from the packets, and therefore, the MASG receives packets without MPLS tunnel labels.
   The VPN packet transmission along the seamless MPLS tunnel is complete.