Switches and VLANs

VXLAN

Also called virtual extensible LAN, VNI, VXLAN network identifier, VTEP, tunnel endpoint, UDP 4789, flood and learn, head-end replication, EVPN, BGP EVPN

Ethernet frames carried inside UDP across a routed network, so one segment can span racks or sites.

A VTEP (VXLAN tunnel endpoint, usually the leaf switch) wraps each frame in an outer IP and UDP header (port 4789) and an 8-byte VXLAN header holding a 24-bit VNI, the segment's number: about 16 million segments, where VLAN IDs stop at 4094. The routers in between see only the outer addresses. Broadcasts and unknown destinations are copied to every VTEP of the VNI (head-end replication, or a multicast group), and VTEPs learn which MAC is behind which VTEP from the frames they receive (flood and learn). BGP EVPN is the control plane that replaces flooding: VTEPs announce their MACs and IPs in BGP. The encapsulation adds 50 bytes, so the underlay needs an MTU 50 bytes larger than the tenants', or tenants get 1450.

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