Layer 3 Switching, Routing, and IP Services
Layer 3 switches route between VLANs using SVIs (VLAN interfaces) or routed ports. Know the packet path: frame arrives, L2 lookup to SVI MAC, L3 lookup in FIB, rewrite, forward. Routing protocols (OSPF, BGP) and static routes populate the RIB/FIB. IP services include DHCP relay (ip helper-address), ARP, proxy ARP, and first-hop redundancy (VRRP/HSRP).
Traps: (1) ip helper-address must be on the client-facing SVI to relay DHCP broadcasts. (2) VRRP/HSRP virtual MAC and priority/preemption behavior-higher priority wins, preempt must be enabled to reclaim. (3) Asymmetric routing breaks stateful firewalls. (4) ARP entries age; proxy ARP can mask misconfiguration. (5) SVI stays down unless at least one access port is up in that VLAN.
Rule of thumb: place the default gateway as a VRRP/HSRP virtual IP; enable preemption and track uplinks so failover is automatic.
Worked example: Clients in VLAN 10 need DHCP from a server in VLAN 20. Configure ip helper-address <server> on the VLAN 10 SVI. Without it, DHCP DISCOVER broadcasts stay local and clients get APIPA addresses. Verify the relay counter increments.