Routing and Switching
Core concepts
Aruba's switching exam expects fluency in Layer 2 and Layer 3 forwarding: VLANs and 802.1Q tagging, trunk vs. access ports, spanning tree (MSTP is the default and the exam favorite), link aggregation (LACP), and the Aruba VSX/MC-LAG redundancy model on AOS-CX. On the routing side, know static routes, OSPF (areas, cost, passive interfaces, DR/BDR election), and the distinction between routed ports and SVIs.
Traps to watch
- MSTP vs. RSTP vs. PVST: Aruba defaults to MSTP; questions often imply rapid convergence without stating it. Confirm which mode is active before reasoning about timers.
- VSX split-brain: If the ISL fails but keepalive succeeds, VSX stays up but traffic may black-hole; if both fail, both peers go primary. Know which link does what.
- OSPF cost math: Cost = reference bandwidth / interface bandwidth. Default reference is 100 Mbps, so anything 100 Mbps or faster ties at cost 1 unless you change it.
- Native VLAN mismatch on a trunk silently merges broadcast domains - a classic troubleshooting distractor.
Worked example
Two AOS-CX switches form a VSX pair with a 40 Gbps ISL. A question asks what happens when the ISL fails but the keepalive link is healthy. Answer: the secondary peer shuts down its VSX-enabled links (split-brain prevention), so downstream dual-homed hosts lose one path but do not loop. If the keepalive also fails, both peers assume primary - the dangerous case the exam tests.
Rule of thumb
For any redundancy question, draw the failure domain first: which link failed, which peer is primary, and which ports get disabled.