Principles of the Layer 2 forwarding policy feature

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Access devices, as Layer 2 network devices, support transparent transmission or forwarding of packets at Layer 2. In traditional Layer 2 forwarding, packets are forwarded based on virtual local area network (VLAN) information and MAC addresses, that is, VLAN+MAC address forwarding. If the destination MAC address of a packet is invalidated due to dynamic MAC address aging, VLAN+MAC address searching will fail. The packet becomes an unknown unicast packet and is broadcast within the VLAN, which poses a security threat. In addition, VLAN+MAC address forwarding is subject to MAC address spoofing and attacks, which lead to security problems.
To address the preceding problems, you can use S-VLAN+C-VLAN (S+C) forwarding instead. In S-VLAN+C-VLAN forwarding, 2 VLAN IDs form a Layer 2 forwarding mapping relationship. Packets are forwarded based on VLANs rather than learned MAC addresses.
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Other related questions:
Are packets in a bridge group forwarded at Layer 2 or Layer 3
Packets in a bridge group are forwarded at Layer 2. Only interfaces supporting Layer 3 functions can be added to a bridge. Data in a bridge, however, is forwarded at Layer 2.

Whether the firewall supports Layer 2 or Layer 3 forwarding
The USG2000&5000&6000 support implementing the Layer 2 forwarding function in transparent mode. When the firewall implements route-based forwarding, the Layer 3 forwarding function is used.

Default Layer 2 multicast forwarding mode on an S series switch
For S series switches excluding the S1700, after Layer 2 multicast is enabled, the S1720, S2710SI, S2700SI, S2700EI, S2720EI, S2750EI, S5700LI, and S5700S-LI (S5700S-28X-LI-AC and S5700S-52X-LI-AC) forward multicast data based on MAC addresses, and other models forward multicast data based on IP addresses by default.

Does a bridge group on an AR router forward packets in Layer 2 or Layer 3

A bridge group on an AR router forwards packets in Layer 2.
Only interfaces with Layer-3 functions can be added to a bridge. However, a bridge forwards data in Layer 2.

How to check whether forwarding is looped on a Layer 2 network
The following events indicate that forwarding is looped on a Layer 2 network: Massive congestion and full-bandwidth storm. Run the display interface ethernet brief | include up command to check whether the incoming and outgoing traffic of a port reaches the maximum. Massive MAC address flapping is found during MAC flapping detection. Run the display trap logbuffer command to check whether MAC address flapping alarms are generated.

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