Campus and Data Center Networking Fundamentals
The fundamentals domain checks that you can explain why a design works, not just name products. Questions cluster around loop prevention, redundancy models, oversubscription and where Layer 2 ends.
Layer 2 and loop prevention
- RSTP/MSTP blocks redundant links: three switches in a triangle leave one link idle.
- MLAG, and VOSS vIST/SMLT, make two chassis appear as one logical node, so both uplinks forward and no port is blocked.
- Switch Engine stacking builds one logical switch from up to eight members, numbered by slot, with ports written slot:port such as 1:1.
- Fabric Connect replaces STP entirely: IS-IS computes shortest paths and every fabric link forwards, giving equal-cost multipathing.
Layer 3 and first-hop redundancy
- VRRP provides a shared virtual gateway; VSPs use SMLT/RSMLT for sub-second failover.
- Underlays are routed with OSPF or IS-IS; EVPN fabrics peer BGP between spines and leaves.
- An anycast gateway places the same SVI IP and MAC on every leaf, so hosts never leave the rack to reach their gateway.
Data center design math
- Spine-leaf: every leaf uplinks to every spine, so host-to-host paths are one leaf-spine-leaf hop and ECMP spreads flows.
- Oversubscription = total downlink bandwidth divided by total uplink bandwidth. A leaf with 48 x 25G downlinks (1200G) and 6 x 100G uplinks (600G) is 2:1.
- VXLAN adds a 50-byte outer header; size the underlay MTU at 1600 or jumbo (9000) or you will see large-packet drops and mysterious application hangs.
Worked example
Two spines and four leaves for 96 servers at 25G. Each leaf gives 48 x 25G to servers (1200G down) and 2 x 100G per spine (400G up), a 3:1 ratio. That is acceptable for general virtualization but thin for storage traffic; the reasoning is to match the ratio to the workload, not to apply a fixed rule.
Traps
- Assuming VSPs stack; they pair with vIST instead.
- Forgetting MTU on EVPN/VXLAN underlays.
- Designing PoE without checking the budget: 802.3bt ports can draw 90W, so a 48-port closet may need load balancing or a larger power supply.