Seamless MPLS Fundamentals

Usage Scenario

Seamless MPLS networking solutions are as follows:

• Intra-AS seamless MPLS: The access, aggregation, and core layers are within a single AS. Intra-AS seamless MPLS applies to mobile bearer networks.

• Inter-AS seamless MPLS: The access and aggregation layers are within a single AS, whereas the core layer in another AS. Inter-AS seamless MPLS is mainly used to transmit enterprise services.

• Inter-AS seamless MPLS+HVPN: A cell site gateway (CSG) and an aggregation (AGG) node establish an HVPN connection, and the AGG and a mobile aggregate service gateway (MASG) establish a seamless MPLS LSP. The AGG provides hierarchical L3VPN access services and routing management services. Seamless MPLS+HVPN combines the advantages of both MPLS and HVPN. Seamless MPLS allows any two nodes on an inter-AS LSP to transmit services at the access, aggregation, and core layers, providing high service scalability. HVPN enables carriers to reduce network deployment costs by deploying devices with layer-specific capacities to meet service requirements.

Intra-AS Seamless MPLS

Table 1 Intra-AS seamless MPLS networking

Network Deployment		Description
Control plane	Deploy routing protocols.	Figure 1 Deploying routing protocols for the intra-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 -> core ABR1 -> MASG1 is used in the following example. An IBGP peer relationship is established between each of the following pairs of devices: CSG and AGG AGG and core ABR Core ABR and MASG The AGG and core ABR are configured as route reflectors (RRs) so that the CSG and MASG can obtain routes destined for each other's loopback addresses. Because IBGP peer relationships are established between devices, you need to set BGP next hops on AGGs and core ABRs to themselves.
	Deploy tunnels.	Figure 2 Deploying tunnels for the intra-AS seamless MPLS networking As shown in Figure 2, tunnels are deployed as follows: A public network tunnel is established using LDP, LDP over TE, or TE in each IGP area. The path CSG1 -> AGG1 -> core ABR1 -> MASG1 is used in the following example. An IBGP peer relationship is established between each of the following pairs of devices: CSG and AGG AGG and core ABR Core ABR and MASG These devices 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, an E2E BGP LSP is established between the CSG and MASG.
Forwarding plane		Figure 3 Forwarding plane for the intra-AS seamless MPLS networking Figure 3 illustrates the forwarding plane of the intra-AS seamless MPLS 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 CSG1 -> AGG1 -> core ABR1 -> MASG1. 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. The AGG removes the access-layer MPLS tunnel labels from the packets and swaps the existing BGP LSP labels for new labels. The AGG then pushes an aggregation-layer MPLS tunnel label into each packet. The AGG proceeds to forward the packets to the core ABR. If the penultimate hop popping (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. The core ABR removes aggregation-layer MPLS tunnel labels from the VPN packets and swaps the existing BGP LSP labels for new labels. The core ABR pushes a core-layer MPLS tunnel label to each packet and forwards the packets to the MASG. The MASG removes MPLS tunnel labels and BGP LSP labels from the VPN packets. If the PHP function is enabled on the MASG, the core ABR has removed the core-layer MPLS tunnel labels from the packets, and therefore, the MASG receives packets without MPLS tunnel labels. The VPN packet transmission along the intra-AS seamless MPLS tunnel is complete.

Inter-AS Seamless MPLS

Table 2 Inter-AS seamless MPLS networking

Network Deployment		Description
Control plane	Deploy routing protocols.	Figure 4 Deploying routing protocols for the inter-AS seamless MPLS networking As shown in Figure 4, 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 5 Deploying tunnels for the inter-AS seamless MPLS networking As shown in Figure 5, tunnels are deployed as follows: A public network tunnel is established using LDP, LDP over TE, 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 route-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 6 Forwarding plane for the inter-AS seamless MPLS networking with a BGP LSP established in the core area Figure 6 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. 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. 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. 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. 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. 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 7 Forwarding plane for the inter-AS seamless MPLS networking without a BGP LSP established in the core area Figure 7 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.

Inter-AS Seamless MPLS+HVPN

Table 3 Inter-AS seamless MPLS+HVPN networking

Network Deployment		Description
Control plane	Deploy routing protocols.	Figure 8 Deploying routing protocols for the inter-AS seamless MPLS+HVPN networking As shown in Figure 8, 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 9 Deploying tunnels for the inter-AS seamless MPLS+HVPN networking As shown in Figure 9, 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 route-policy. After they exchange BGP routes, a BGP LSP can be established between each pair of an AGG and MASG.
Forwarding plane		Figure 10 Forwarding plane of the inter-AS seamless MPLS+HVPN networking Figure 10 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. The CSG pushes an MPLS tunnel label into each VPN packet and forwards the packets to the AGG. 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. 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. 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. 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.

Reliability

Seamless MPLS network reliability can be improved using a variety of functions. If a network fault occurs, devices with reliability functions enabled immediately detect the fault and switch traffic from active links to standby links.

The following examples demonstrate the reliability functions used on an inter-AS seamless MPLS network.

• A fault occurs on a link between a CSG and an AGG.

  As shown in Figure 11, the active link along the primary path between CSG1 and AGG1 fails. After BFD for LDP or BFD for CR-LSP detects the fault, the BFD module uses LDP FRR, TE Hot-standby or BGP FRR to switch traffic from the primary path to the backup path.

  Figure 11 Traffic protection triggered by a fault in the link between the CSG and AGG on the inter-AS seamless MPLS network
  

• A fault occurs on an AGG.

  As shown in Figure 12, BGP Auto FRR is configured on CSGs and AGG ASBRs to protect traffic on the BGP LSP between CSG1 and MASG1. If BFD for LDP or BFD for TE detects AGG1 faults, the BFD module switches traffic from the primary path to the backup path.

  Figure 12 Traffic protection triggered by a fault in an AGG on the inter-AS seamless MPLS network
  

• A fault occurs on the link between an AGG and an AGG ASBR.

  As shown in Figure 13, a fault occurs on the link along the primary path between AGG1 and AGG ASBR1. After BFD for LDP or BFD for CR-LSP detects the fault, the BFD module uses LDP FRR, TE hot-standby or BGP FRR to switch traffic from the primary path to the backup path.

  Figure 13 Traffic protection triggered by a fault in the link between an AGG and an AGG ASBR on the inter-AS seamless MPLS network
  

• A fault occurs on an AGG ASBR.

  As shown in Figure 14, BFD for LDP or BFD for TE is configured on AGG1, and BFD for interface is configured on core ASBR1. If AGG ASBR1 fails, the BFD modules on AGG1 and core ASBR1 detect the fault and trigger the BGP Auto FRR function. BGP Auto FRR switches both upstream and downstream traffic from the primary path to backup paths.

  Figure 14 Traffic protection triggered by a fault in an AGG ASBR on the inter-AS seamless MPLS network
  

• A fault occurs on the link between an AGG ASBR and a core ASBR.

  As shown in Figure 15, BFD for interface is configured on AGG ASBR1 and core ASBR1. If the BFD module detects a fault in the link between AGG ASBR1 and core ASBR1, the BFD module triggers the BGP Auto FRR function. BGP Auto FRR switches both upstream and downstream traffic from the primary path to backup paths.

  Figure 15 Traffic protection triggered by a fault in the link between an AGG ASBR and a core ASBR on the inter-AS seamless MPLS network
  

• A fault occurs on a core ASBR.

  As shown in Figure 16, BFD for interface and BGP Auto FRR are configured on AGG ASBR1. BGP Auto FRR and BFD for LDP (or BFD for TE) are configured on MASGs to protect traffic on the BGP LSP between CSG1 and MASG1. If the BFD module detects a fault in core ASBR1, it switches both upstream and downstream traffic from the primary path to backup paths.

  Figure 16 Traffic protection triggered by a fault in a core ASBR on the inter-AS seamless MPLS network
  

• A link fault occurs in the core area.

  As shown in Figure 17, BFD for LDP or BFD for CR-LSP is configured on core ASBR1. If the BFD module detects a fault in the link between core ASBR1 and MASG1, it triggers the LDP FRR, TE Hot-standby or BGP FRR function. LDP FRR, TE FRR, or BGP FRR switches both upstream and downstream traffic from the primary path to the backup path.

  Figure 17 Traffic protection triggered by a link fault in a core area on the inter-AS seamless MPLS network
  

• A fault occurs on an MASG.

  As shown in Figure 18, BFD for BGP tunnel is configured on CSG1. BFD for BGP tunnel is implemented in compliance with relevant standards "Bidirectional Forwarding Detection (BFD) for MPLS Label Switched Paths (LSPs)." BFD for BGP tunnel monitors E2E BGP LSPs, including a BGP LSP connected to an LDP LSP. When MASG1 that functions as a provider edge (PE) device fails, BFD for BGP tunnel can rapidly detect the fault and trigger VPN FRR switching. The BFD module then switches both upstream and downstream traffic from the primary path to the backup path.

  Figure 18 Traffic protection triggered by a fault in an MASG on the inter-AS seamless MPLS network