The loopback interface is not strictly bound to any of the physical interfaces and can load-balance between them based on the ECMP hash. In standard mode, traffic coming from the underlay can reach the loopback IP address via each of the physical links. Figure 2. Connecting to the MPLS cloud via two physical interfaces This scenario often happens when the service provider has two CE routers onside, and we want to have extra resiliency in the underlay by connecting each vEdge to both CEs. Figure 2 shows a typical use-case where vEdge-4 has two interfaces connected to the same MPLS cloud - ge0/6 with IP address 10.10.2.1/30 and ge0/7 with addresses 10.10.2.5/30. We use the standard mode in scenarios where a vEdge router has multiple physical interfaces to the same WAN provider. The following figure visualizes this use case: Figure 1. Loopback TLOC over two physical interfacesĭepending on the specific scenario, there are two modes for a loopback TLOC - standard mode and bind mode. The loopback interface serves as a tunnel endpoint and is marked with a single TLOC color. Using a loopback interface as a local TLOC is a technique that allows a vEdge router to have multiple physical interfaces attached to the same WAN provider and utilize ECMP across them. That is why the Cisco SD-WAN solution allows us to configure a loopback interface as a local TLOC instead of a physical interface. However, this can overcomplicate the overlay topology with restricted colors and tunnel groups. Therefore, in scenarios where a vEdge has multiple interfaces connected to the same WAN provider, we must use different colors on each interface.
We have seen earlier in this chapter that we cannot assign a particular TLOC color to more than one interface per vEdge because the color uniquely identifies a single WAN link.
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