Routing design
There is no change in the way the SD-WAN nodes establish BGP sessions between them. The difference is only in the content of the BGP advertisements: we use a different BGP address family (VPNv4), and every advertised BGP route now includes the following important elements:
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The prefixes are prepended with a route distinguisher (RD), to make them look unique.
Remember that IP overlap is permitted between different segments (VRFs). On the diagram in the Segmentation over single overlay topic, we see how the two VRFs both contain the same LAN prefix 10.0.2.0/24. By prepending two unique RDs, "65000:11" and "65000:12" respectively, we make these prefixes look unique: "65000:11:10.0.2.0/24" and "65000:12:10.0.2.0/24". Even though both BGP routes are originated by the same Spoke (and hence having the same BGP NH), these are nevertheless two different BGP routes now.
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An extended community called route target (RT) is attached to each route, and it signals from which VRF this route is exported (advertised) and into which VRF the receiving node must import it.
The combination of RD and RT allows us to advertise Customer LAN prefixes, while preserving their VRF information across the entire SD-WAN overlay network.
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The exact values of RT/RD don't matter, as long as they are configured consistently on all the SD-WAN nodes. These values will never be readvertised to any peers outside of the SD-WAN overlay network. Therefore, they will not conflict with any existing MP-BGP deployments (such as existing BGP/MPLS L3VPNs in Customer network). |