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AR100-S, AR110-S, AR120-S, AR150-S, AR160-S, AR200-S, AR1200-S, AR2200-S, and AR3200-S V200R009 CLI-based Configuration Guide - Ethernet Switching

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Basic Principles of Transparent Bridging

Basic Principles of Transparent Bridging

Forwarding Entry Learning

Transparent bridging uses a forwarding table to forward packets. A network bridge's forwarding table records the mapping between the MAC address and the packet's outbound interface. If an Ethernet frame arrives, the network bridge takes the following actions to forward it:
  • Obtain the source MAC address of the valid Ethernet frame.
  • Add the mapping relationship between the source MAC address and the interface to the forwarding table to generate a forwarding entry.

As shown in Figure 15-1, PC1, PC2, PC3, and PC4 are located on two LANs. PC1 connects to bridge port Port1 and PC2 connects to bridge port Port2. When PC1 sends an Ethernet frame to PC2, both Port1 and PC2 receive the frame.

Figure 15-1  PC1 transmitting information to PC2 on LAN1

After Port2 receives the frame, the network bridge learns that PC1 connects to Port1 because the frame is received from Port1. Then the mapping between the MAC address of PC1 and Port1 is added to the network bridge table, as shown in Figure 15-2.

Figure 15-2  Network bridge learning that PC1 connects to Port1

When PC2 responds to the frame from PC1, the network bridge also detects the frame from PC2 and learns that PC2 connects to Port2 because the frame is received from Port2. The mapping between the MAC address of PC2 and Port2 is added to the network bridge table, as shown in Figure 15-3.

Figure 15-3  Network bridge learning that PC2 connects to Port2

The network bridge learns the mappings between all MAC addresses and bridge interfaces, as shown in Figure 15-4.

Figure 15-4  Last network bridge address table

If a MAC address establishes a mapping relationship with more than one interface, the more recent mapping relationship overrides the earlier one. This ensures each MAC address is related with only one outbound interface.

The transparent bridge can perform dynamic MAC address learning. Learned MAC address entries are deleted when their aging time expires.

Packet Processing

The transparent bridge processes received data frames in either of the following modes:

  • Unicast frame

    If the received data frame's destination MAC address can be found in the forwarding table, and the inbound and outbound interfaces of the frame are different, the outbound interface forwards the data frame.

  • Broadcast

    If the received data frame's destination MAC address is a unicast MAC address and cannot be found in the forwarding table, or the destination MAC address of the data frame is a multicast or broadcast MAC address, the data frame is forwarded to all interfaces in the corresponding bridge group on the same VLAN, except the frame's inbound interface.

NOTE:
When packets enter the network bridge, the following BPDUs will be discarded:
  • If the network bridge interface is configured with selective QinQ, the packets with the destination MAC address of 0180-C200-0002 will be discarded.
  • If the network bridge interface is not configured with selective QinQ, the packets with the destination MAC addresses of 0180-C200-000x and 0180-C200-002x will be discarded. x represents 1-bit hexadecimal integer.
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Updated: 2019-05-17

Document ID: EDOC1000174108

Views: 55289

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