Steering Flows to a Direct Next-Hop

Figure 1 illustrates the traffic forwarding from a Virtual Network Function (VNF1) residing in a host in a Data Centre (DC1) to outside of the customer network via the remote peering Point Of Presence (POP1).

The traffic is forwarded over a segment routing capable backbone. DC1 and POP1 do not support segment routing and MPLS while the DC Edge routers do not support segment routing. Hence, MPLS packets of VNF1 flows are tunneled over a UDP/IP or GRE/IP tunnel and a static label route is configured on POP Edge3/4 to steer the decapsulated packets to the needed external BGP peer.

Figure 1. Traffic Steering to a Direct Next-hop using a Static Label Route

The intent is to override the BGP routing table at the peering routers (POP Edge3 and Edge4) and force packets of a flow originated in VNF1 to exit the network using a primary external BGP peer Peer1 and a backup external BGP peer Peer2, if Peer1 is down. This application is also referred to as Egress Peer Engineering (EPE).

The following are the data path manipulations of a packet across this network:

  1. DC Edge1/2 receives a MPLS-over-UDP (or a MPLS-over-GRE) encapsulated packet from the host in the DC with the outer IP destination address set to the remote POP Edge3/4 routers in peering POP1 (anycast address). The host also pushes the static label 20001 for the remote external BGP Peer1 it wants to send to.

  2. This label 20001 is configured on POP Edge3/4 using the MPLS forwarding policy feature with primary next-hop of Peer1 and backup next-hop of Peer2.

  3. There is EBGP peering between DC Edge1/2 and LER1/2, and between POP Edge3/4 and LER3/4, and between POP Edge3/4 and Peer1/2.

  4. LER1/LER2 pushes the anycast SID of remote LER3/4 as part of the BGP route resolution to a SR-ISIS tunnel or SR-TE policy.

  5. LER3/4 removes the anycast SID and forwards the GRE packet to POP Edge3/4.

  6. POP Edge3/4 removes UDP/IP (or GRE/IP) header and swaps the static label 20001 to implicit null and forwards to Peer1 (primary next-hop) or to Peer2 (backup next-hop).

The following CLI commands configure the static label route to achieve this use case. It creates a label-binding policy with a single NHG containing a primary and backup direct next-hops and is applied to peering routers POP Edge3/4. 

    —  
    — config>router>mpls-labels
        — reserved-label-block static-label-route-lbl-block
            — start-label 20000 end-label 25000
    —  
    — config>router>mpls
        — forwarding-policies
            — forwarding-policy static-label-route-direct
                — binding-label 20001
                — revert-timer 10
                — next-hop-group 1 resolution-type direct
                    — primary-next-hop 
                        — next-hop fd84:a32e:1761:1888::1
                    — backup-next-hop
                        — next-hop fd22:9501:806c:2387::2
Parent topic: Configuring Static Label Routes using MPLS Forwarding Policy