Campus Access Switching and Routing Configuration
Expect configuration-level questions on VLANs, trunking, link aggregation, spanning tree, and Layer 3 routing. On Aruba switches, a trunk carries multiple tagged VLANs; an access port carries one untagged VLAN. Native/untagged VLAN mismatches are a classic failure: both ends must agree on which VLAN is untagged, and allowed-VLAN lists must include every VLAN you expect to pass.
For Layer 2 resilience, know RSTP/MSTP roles and that MSTP maps VLANs to instances for load sharing. For multichassis designs, aggregation (LAG) provides active-active forwarding without blocked ports. For Layer 3, distinguish static routes (administrative distance default, simple) from OSPF (areas, cost based on bandwidth, neighbor adjacency requirements: matching area, hello/dead timers, MTU, authentication). Remember that OSPF cost is derived from interface bandwidth, so a faster link is preferred unless cost is manually set.
Traps: assuming a routed port and an SVI behave identically for all features; forgetting that an SVI stays down until a member port in that VLAN is up; misreading show output where a LAG is up but a member is down; and confusing inter-VLAN routing on a switch with routing that must hairpin through a firewall. Also watch DHCP relay: the relay (helper) address must point at the DHCP server, and the server needs a scope for that subnet.
Worked example: two switches, VLANs 10 and 20, need load sharing and no blocked links. Configure an aggregation group with both uplinks, allow both VLANs on the LAG, and use MSTP (or the multichassis feature) so both links forward. Verify with the LAG and spanning-tree show commands that both members are forwarding and the root bridge is where you intend.