Switching, Routing, and Overlay Technologies
Domain 2 covers the mechanics underneath a modern data center fabric: VLANs and trunking, link aggregation, Layer 2 edge features, Layer 3 underlay routing, and VXLAN-EVPN overlays. Questions test whether you know where a frame is encapsulated, which table answers a lookup, and what breaks when MTU or ARP handling is wrong.
Underlay and overlay
- The underlay is a routed Layer 3 fabric, typically eBGP, carrying loopback-to-loopback reachability between VTEPs.
- VXLAN encapsulates the original frame in UDP (destination port 4789) with a 24-bit VNI, supporting about 16 million segments.
- EVPN is the control plane: type 2 carries MAC/IP, type 3 carries the inclusive multicast (IMET) route for BUM traffic, and type 5 carries IP prefixes for symmetric IRB.
- Anycast gateway lets every leaf own the same distributed gateway IP and MAC, so a host can move racks without changing its default gateway.
Layer 2 edge
- MLAG, vPC, and VLT pair two switches so a dual-homed host sees one logical LAG. The peer link carries control and orphan traffic, not normal data.
- ARP suppression and ND suppression reduce BUM flooding by answering from the EVPN table.
Worked example
Tenant MTU is 9000. VXLAN overhead is 50 bytes: outer Ethernet 14, outer IP 20, UDP 8, VXLAN 8. The underlay must therefore carry at least 9050 bytes, so set the underlay to 9100 or 9216. If the underlay stays at 9000, the largest tenant frame that survives is 8950 bytes. Symptom: small pings pass, large transfers or iSCSI stall, and PMTUD black holes appear when ICMP is filtered.
Traps that cost marks
- Calling an EVPN type 5 route a MAC route; type 5 is an IP prefix route.
- Forgetting the 50-byte encapsulation overhead when sizing underlay MTU.
- Assuming the MLAG peer link is a normal forwarding path for dual-homed traffic.
- Believing anycast gateway requires unique MAC addresses per leaf.
- Designing BUM handling without a type 3 route or multicast underlay.