Logical Switching and Routing
Segments and routers
Logical switching uses segments. An overlay segment is backed by GENEVE and is independent of VLANs; a VLAN-backed segment maps to a VLAN and is often used for physical connectivity or migration. Transport zones determine which transport nodes can participate. Routing is distributed: a Tier-1 gateway has a distributed router on hosts, so east-west inter-subnet traffic is routed locally at the source host when possible. Tier-0 connects to the physical network through Edge nodes and provides north-south routing, NAT, and centralized services. Tier-1 typically connects to Tier-0; route advertisement and static or dynamic routing control reachability. A Tier-1 can be linked to Tier-0 with a transit segment; check link status.
Traps: assuming all routing is centralized at Edge; forgetting that a Tier-1 needs an Edge cluster only for centralized services; confusing segment VLAN with transport VLAN; expecting a Tier-1 to provide north-south without Tier-0.
Rule: same segment means switching; different segments mean routing; leaving the data center means Tier-0 and Edge. Worked example: VM A and VM B are on different overlay segments on the same host. Their default gateway is a Tier-1 distributed router. The first packet is routed by the host distributed router and forwarded directly to VM B vNIC; it does not go to the Edge.