Campus and Branch LAN Design
This domain covers Juniper EX and QFX LAN design for campuses and branches: hierarchy, VLANs, spanning tree, MC-LAG, PoE, and access security.
Core rules
- Use a hierarchical access-aggregation-core model; collapse core and aggregation in small branches.
- Keep Layer 2 domains small; use RSTP or MSTP with root guard, BPDU guard, and loop guard.
- Use LAG or MC-LAG for resilient uplinks; avoid single-homed access switches.
- Design VLANs by function and policy, not by physical location.
- Budget PoE and uplink oversubscription; enable 802.1X and DHCP snooping at the edge.
Worked example
A three-floor office has 300 users, 120 IP phones, and 40 access points. Design: EX access switches per floor, dual 10G uplinks with MC-LAG to a collapsed QFX core, VLANs for data, voice, guest, and AP management, 802.1X with voice VLAN fallback, and PoE budget at 80% of capacity. Reason: MC-LAG gives active-active resilience without STP blocking; a separate voice VLAN enables QoS; PoE headroom covers AP and phone growth.
Traps
- Flat VLANs and large broadcast domains.
- STP without edge protections.
- Ignoring PoE budget and uplink oversubscription.
- Using MC-LAG without consistent configuration on peers.