Why cannot a switch forward incoming traffic from an upstream device that performs an active/standby switchover

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When an upstream device performs an active/standby switchover, its IP address remains unchanged, but its MAC address changes. The downstream switch cannot detect the MAC address change in traffic, and cannot update ARP entries. Incoming traffic, therefore, cannot be forwarded. The aging time of ARP entries on the switch is 20 minutes. After 20 minutes, the switch learns ARP entries of the upstream device again, and the traffic can be forwarded. To shorten the service interruption period, set a shorter aging time for ARP entries. In V100R006 and later versions, run the mac-address update arp command to enable the switch to update ARP entries when the MAC addresses of upstream devices change, which shortens traffic interruption period (within several seconds).

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