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Proxy ARP & Gratuitous ARP: Advanced ARP Patterns

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Advanced ARP: proxy, gratuitous, and cache behavior

Basic ARP (address resolution for local network hosts) is just the beginning. Proxy ARP lets routers answer ARP requests on behalf of remote hosts, often masking misconfigured subnet masks. Gratuitous ARP broadcasts IP-to-MAC bindings proactively — used in VRRP failover and VM migrations.

Understanding these behaviors explains why VRRP failover is near-instant, how MITM attacks exploit ARP, and why large Layer 2 domains are problematic.

Packets in motion — click to inspect:

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