Fundamentals of EVPN Seamless MPLS

Background

The popularity of EVPN MPLS networks poses increasing requirements for the service scalability of the network architecture. The different metro networks of a service provider or the collaborative backbone networks of different service providers often span multiple ASs. In this case, EVPN seamless MPLS can be used to establish an inter-AS E2E BGP LSP to carry EVPN services.

Implementation

On a seamless MPLS network, the EVPN services to be transmitted need to be encapsulated using signaling only at service access points. In addition, if a network-side fault triggers EVPN service convergence, the same transport layer convergence technology is used to converge the services, without the service layer being aware of the fault.

Application Scenarios

EVPN seamless MPLS supports the following networking solutions:

• EVPN intra-AS seamless MPLS: The access, aggregation, and core layers are deployed within a single AS. This solution mainly applies to mobile transport networks.
• EVPN inter-AS seamless MPLS: The access and aggregation layers are deployed in a single AS, whereas the core layer is deployed in a different AS. This solution mainly applies to enterprise networks.

EVPN Intra-AS Seamless MPLS

Table 1 EVPN intra-AS seamless MPLS networking

Network Deployment		Description
Control plane	Deploying routing protocols	In Figure 1, routing protocol deployment on devices is 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 network connectivity. 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 AGGs and core ABRs are configured as RRs to reflect routes to CSGs and MASGs, respectively, so that the CSGs and MASGs can obtain the route to each other's loopback address. The AGGs and core ABRs set the next-hop IP addresses in BGP routes to their own addresses to prevent the public routes of other IGP areas from being advertised. Figure 1 Deploying routing protocols for the EVPN intra-AS seamless MPLS networking
	BGP EVPN peer relationship establishment and route advertisement	As shown in Figure 1, AGGs and core ABRs function as RRs. BGP EVPN peer relationships need to be established between CSGs and AGGs, between MASGs and core ABRs, and between AGGs and core ABRs. Then, EVPN MAC/IP routes (Type 2) and IP prefix routes (Type 5) need to be transmitted between the peers to transmit MAC and IP routing information.
	Deploying tunnels	On the network shown in Figure 2, tunnels are deployed as follows: A public network tunnel is established using LDP, TE, or LDP over TE in each IGP area. 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 route-policy. After the devices exchange labeled BGP routes, an E2E BGP LSP is established between each pair of a CSG and MASG. Figure 2 Deploying tunnels for the EVPN intra-AS seamless MPLS networking
Forwarding plane		Figure 3 illustrates the forwarding plane of EVPN intra-AS seamless MPLS networking. Seamless MPLS is mainly used to transmit EVPN packets. The following example demonstrates how EVPN packets, including labels and packet content, 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 EVPN packet and forwards the packets to the AGG. Upon receipt, 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 and 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. Upon receipt, the core ABR removes aggregation-layer MPLS tunnel labels from the EVPN 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 EVPN 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 EVPN packet transmission along the intra-AS seamless MPLS LSP is complete. Figure 3 Forwarding plane for the EVPN intra-AS seamless MPLS networking

EVPN Inter-AS Seamless MPLS

Table 2 EVPN inter-AS seamless MPLS networking

Network Deployment		Description
Control plane	Deploying routing protocols	In Figure 4, routing protocol deployment on devices is as follows: An IGP (IS-IS or OSPF) is enabled on devices at each of access, aggregation, and core layers to implement intra-AS network connectivity. 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 the other pairs of devices. The AGGs are configured as RRs to reflect routes so that IBGP peers can exchange BGP routes, and the CSGs and MASGs 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. Figure 4 Deploying routing protocols for the EVPN inter-AS seamless MPLS networking
	BGP EVPN peer relationship establishment and route advertisement	On the network shown in Figure 4, BGP EVPN peer relationships need to be established between the following pairs of devices: CSG and AGG AGG and AGG ASBR AGG ASBR and core ASBR Core ASBR and MASG The peers exchange MAC/IP routes (Type 2) and IP prefix routes (Type 5) to advertise MAC and IP routing information.
	Deploying tunnels	On the network shown in Figure 5, tunnels are deployed as follows: A public network tunnel is established using LDP, TE, or LDP over TE in each IGP area. An LDP LSP or a TE LSP must be established if more than one hop exists between each pair of an AGG ASBR and core ASBR. The CSGs, AGGs, AGG ASBRs, and core ASBRs 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 each pair of a CSG and core ASBR. Either of the following tunnel deployment methods can be used in the core area: A BGP LSP between a core ASBR and MASG is 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 MPLS tunnel. The BGP LSP between the CSG and core ASBR and the MPLS tunnel in the core area are combined into an E2E tunnel. Figure 5 Deploying tunnels for the EVPN inter-AS seamless MPLS networking
Forwarding plane		Figure 6 illustrates the forwarding plane of the EVPN inter-AS seamless MPLS networking with a core-layer BGP LSP established. EVPN seamless MPLS is mainly used to transmit EVPN packets. The following example demonstrates how EVPN packets, including VPN labels and packet 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 EVPN packet and forwards the packets to the AGG. Upon receipt, 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 and 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. Upon receipt, the AGG ASBR removes the MPLS tunnel labels from the EVPN 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. Upon receipt, the core ASBR swaps a BGP LSP label for a new label and pushes a core-layer MPLS tunnel label into each packet. It then forwards the packets to the MASG. Upon receipt, 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 EVPN packet transmission along the inter-AS seamless MPLS LSP is complete. Figure 6 Forwarding plane for the EVPN inter-AS seamless MPLS networking with a core-layer BGP LSP established Figure 7 illustrates the forwarding plane for the EVPN inter-AS seamless MPLS networking without a BGP LSP established in the core area. The process of transmitting EVPN packets on this network is similar to that on a network with a BGP LSP established in the core area. The difference is that without a BGP LSP in the core area, the core ASBR removes (rather than swaps) BGP labels from packets and pushes MPLS tunnel labels into these packets. Figure 7 Forwarding plane for the EVPN inter-AS seamless MPLS networking without a BGP LSP established in the core area

Reliability

EVPN 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 the active link to the standby link.

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

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

  On the EVPN inter-AS seamless MPLS network shown in Figure 8, the active link along the primary path between CSG1 and AGG1 fails. After BFD for LDP LSP 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 8 Traffic protection triggered by a fault of the CSG-AGG link on the EVPN inter-AS seamless MPLS network
  

• A fault occurs on an AGG.

  On the EVPN inter-AS seamless MPLS network shown in Figure 9, 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 an AGG1 fault, the BFD module switches traffic from the primary path to the backup path.

  Figure 9 Traffic protection triggered by an AGG fault on the EVPN inter-AS seamless MPLS network
  

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

  On the EVPN inter-AS seamless MPLS network shown in Figure 10, a fault occurs on the link between AGG1 and AGG ASBR1. After BFD for LDP LSP or BFD for CR-LSP detects the fault, the BFD module instructs LDP FRR, TE hot standby, or BGP FRR to switch traffic from the primary path to the backup path.

  Figure 10 Traffic protection triggered by a fault of 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 11, 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 11 Traffic protection triggered by a fault of an AGG ASBR on the EVPN inter-AS seamless MPLS network
  

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

  As shown in Figure 12, BFD for interface is configured on AGG ASBR1 and core ASBR1. If the BFD module detects a fault of 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 12 Traffic protection triggered by a fault of the link between an AGG ASBR and a core ASBR on the EVPN inter-AS seamless MPLS network
  

• A fault occurs on a core ASBR.

  On the EVPN inter-AS seamless MPLS network shown in Figure 13, BFD for interface and BGP Auto FRR are configured on AGG ASBR1. BGP Auto FRR and BFD for LDP (or for TE) are configured on MASGs to protect traffic on the BGP LSP between CSG1 and MASG1. If the BFD module detects a fault on core ASBR1, it switches both upstream and downstream traffic from the primary path to backup paths.

  Figure 13 Traffic protection triggered by a fault of a core ASBR on the EVPN inter-AS seamless MPLS network
  

• A link fault occurs in the core area.

  On the EVPN inter-AS seamless MPLS network shown in Figure 14, BFD for LDP or BFD for TE is configured on core ASBR1. If the BFD module detects a fault on the link between core ASBR1 and MASG1, it instructs the LDP FRR, TE hot standby, or BGP FRR function to switch both upstream and downstream traffic from the primary path to the backup paths.

  Figure 14 Traffic protection from a link fault in the core area on the EVPN inter-AS seamless MPLS network
  

• A fault occurs on an MASG.

  As shown in Figure 15, BFD for BGP tunnel is configured on CSG1. BFD for BGP tunnel is implemented in compliance with a standard titled "Bidirectional Forwarding Detection (BFD) for MPLS Label Switched Paths (LSPs)." BFD for BGP tunnel monitors E2E BGP LSPs, including a BGP LSP stitched with an LDP LSP. If MASG1 that functions as a remote PE fails, BFD for BGP LSP 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 15 Traffic protection triggered by a fault of an MASG on the EVPN inter-AS seamless MPLS network
  

• A fault occurs on an access-side link.

  On the inter-AS seamless MPLS network shown in Figure 16, if an E-Trunk in single-active mode detects a link failure, the E-Trunk switches traffic from the primary path to the backup path and PE2's interface connected to CE1 is unblocked. Then upstream traffic on CE1 is switched to PE2. For BUM traffic on the network side, PE1 sends a per-ES A-D route withdraw message to PE2, and PE2 is elected as the DF to forward BUM traffic. After receiving the MAC route advertised by PE2, PE3 switches unicast traffic to PE2.

  If an E-Trunk in active-active mode detects a link failure, PE1 sends a per-ES A-D route withdraw message to PE3, and PE3 switches unicast traffic to PE2.

  Figure 16 Traffic protection triggered by an access-side link fault on the EVPN inter-AS seamless MPLS network
  

• A PE on the access side fails.

  If PE1 fails, the original EVPN detection mechanism is triggered, which is similar to that triggered when an access-side link fails. Other PEs switch traffic after detecting PE1's down state rather than receiving a route withdraw request.