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When choosing an underlay for an EVPN/VXAN network, the prevailing wisdom is that BGP is the best choice for the underlay routing protocol. And overall, I think that’s true. But OSPF can make a compelling underlay too, as it has a few logistical advantages over BGP in certain cases.
When building out EVPN/VXLAN networks, I like to break the build into four components. They are layers that are built one-by-one on top of each other.
- Topology (typically leaf/spine)
- Underlay (provides IP connecitivity for loopbacks)
- Overlay (exchanges EVPN routes)
- EVPN services (these are the Layer 2 and Layer 3 networks internal hosts and external networks connect to)
This article is exclusively about the underlay portion. It’s a very simple routed network that has one job, and job only:
Provide routes to enable IP connectivity from any loopbacks on a device to any loopback on any other device.
That’s it.
In normal operation the routing table will be incredibly static. The only time the routing table would change is when a switch is added or removed, or a link goes down, or a switch is upgraded, etc. In regular operation it won’t change.
The underlay is important, but the underlay isn’t Continue reading