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Routing & Switching · Intermediate

Unknown Unicast Flooding

Forwarding a frame out all ports in its VLAN (except the source) when the destination MAC is not in the MAC table.

Definition

Flooding ensures delivery until the destination replies and is learned. Excessive flooding occurs with asymmetric routing where MAC entries age out before ARP entries. Storm control can limit it, and EVPN can suppress it.

Example

A switch floods a frame to 00aa.bb11.2233 because the MAC aged out.

Exam tip

Mismatched ARP (4 hours) and MAC (5 minutes) timers cause unicast flooding in asymmetric designs.

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