How does STP process MAC and ARP entries after the network topology changes

If the network topology changes, the Spanning Tree Protocol (STP) clears media access control (MAC) addresses, and ages Address Resolution Protocol (ARP) entries by default.

In case of a topology change, the STP will delete the MAC entries and adopt the aging mechanism for the ARP entries by default.

principals of ARP entry update upon MAC address change on S series switches (except S1700): Every device on a network has an IP address, which is used to communicate with other devices. On an Ethernet, hosts, switches, or routers send and receive Ethernet data frames based on MAC addresses. ARP provides mappings between IP addresses and MAC addresses. When devices on different network segments communicate, ARP entries must be used to map IP addresses to correct MAC addresses and outbound interfaces. If you change the location of a host to connect the host to another interface of a switch, the host's MAC address will be learned on this interface and the outbound interface corresponding to the MAC address will change. However, the outbound interface in the ARP entry will be updated only after the aging time expires. Before the aging time expires, the switch will use the incorrect ARP entry for communication. After the mac-address update arp command is configured in the system view, the outbound interface in an ARP entry can be updated based on the outbound interface in an MAC address entry. Precautions: 1. This command is valid only for dynamic ARP entries instead of static ARP entries. 2. The mac-address update arp command will not take effect after the arp anti-attack entry-check enable command is executed to configure fixed ARP. 3. After ARP entry update upon MAC address change is enabled, an ARP entry is updated only when the outbound interface in the corresponding MAC address entry changes. 4. Configuring ARP entry update upon MAC address change will cause the gratuitous ARP packet discarding function to become ineffective. 5. S series switches running versions earlier than V100R006C00 do not support ARP entry update upon MAC address change.

When the STP topology changes, an S series switch (except the S1700) processes ARP and MAC address entries as follows: 1. By default, all ports enter the Discarding state after STP is enabled. When the STP topology changes, dynamic ARP entries and MAC addresses on all ports are deleted. 2. When a port enters the Discarding state during normal running of the system, dynamic ARP and MAC address entries on the port are deleted. In addition, the device sends a TC packet to the upstream device, requesting it to delete the dynamic ARP and MAC address entries. When the root node receives the TC packet, the topology of the entire network converges.

If MFF gateway detection is enabled on a switch, the switch detects gateway MAC address changes and updates the gateway MAC address entries. In this way, user services are not interrupted. However, if MFF gateway detection is disabled, the switch does not update the gateway MAC address entries and user traffic may be interrupted. To detect gateway MAC address changes, enable MFF gateway detection on the switch.