Basic Principles of Seamless MPLS - Part 2

Inter-AS Seamless MPLS

Control plane 

  Deploy routing protocols:

Figure 1 Deploying routing protocols for the inter-AS seamless MPLS 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.
• The path CSG1 -> AGG1 -> AGG ASBR1 -> core ASBR1 -> MASG1 is used in the following example. A BGP peer relationship is established between each of the following pairs of devices:

  • CSG and AGG
  • AGG and AGG ASBR
  • AGG ASBR and core ASBR
  • Core ASBR and MASG
  An EBGP peer relationship is established between the AGG ASBR and core ASBR, and IBGP peer relationships are established between other devices.
• The AGG is configured as an RR so that IBGP peers can exchange BGP routes, and the CSG and MASG can obtain BGP routes destined for each other's loopback addresses.
• If the AGG ASBR and core ASBR are connected indirectly, an IGP neighbor relationship between them must be established to implement inter-area connectivity.

Deploy tunnels:

Figure 2 Deploying tunnels for the inter-AS seamless MPLS 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. An LDP LSP or a TE LSP is established if more than one hop exists between the AGG ASBR and core ASBR.
• The CSG, AGG, AGG ASBR, and core ASBR are enabled to advertise labeled routes and assign labels to BGP routes that match a specified routing policy. After the devices exchange labeled BGP routes, a BGP LSP is established between the CSG and core ASBR.
• Either of the following tunnel deployment methods can be used in the core area:

  • A BGP LSP between the core ASBR and MASG and combined with the BGP LSP between the CSG and core ASBR to form an E2E BGP LSP. The route to the MASG's loopback address is installed into the BGP routing table and advertised to the core ASBR using the IBGP peer relationship. The core ASBR assigns a label to the route and advertises the labeled route to the AGG ASBR.
  • No BGP LSP is established between the core ASBR and MASG. The core ASBR runs an IGP to learn the route destined for the MASG's loopback address and installs the route to the routing table. The core ASBR assigns a BGP label to the route and associates the route with an intra-AS tunnel. The BGP LSP between the CSG and core ASBR and the MPLS tunnel in the core area are combined into an E2E tunnel.

  
  

Forwarding plane

Figure 3 Forwarding plane for the inter-AS seamless MPLS networking with a BGP LSP established in the core area

Figure 3 illustrates the forwarding plane of the inter-AS seamless MPLS networking with a core-layer BGP LSP established. 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 CSG1 -> AGG1 -> AGG ASBR1 -> core ASBR1 -> MASG1.

1. The CSG pushes a BGP LSP label and an MPLS tunnel label in sequence 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 swaps the existing BGP LSP labels for new labels. The AGG pushes an aggregation-layer MPLS tunnel label into each packet and then proceeds to forward the packets to the AGG ASBR. 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 inter-AS seamless MPLS tunnel is complete.

Figure 4 Forwarding plane for the inter-AS seamless MPLS networking without a BGP LSP established in the core area 

Figure 4 illustrates the forwarding plane for the inter-AS seamless MPLS networking without a BGP LSP established in the core area. The process of transmitting packets on this network is similar to that on a network with a BGP LSP established. The difference is that without a BGP LSP in the core area, the core ASBR removes BGP labels from packets and add MPLS tunnel labels to these packets.