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How Does Bridge Mode Work?

Bridges operate at layer 2 of the OSI model, therefore adding a bridge to an existing network is completely transparent and does not require any changes to the network's structure.

Each bridge maintains a forwarding table, which consists of <MAC Address, Port> associations. When a packet is received on one of the bridge ports, the forwarding table is automatically updated to map the source MAC address to the network port from which the packet originated, and the gateway processes the received packet according to the packet's type.

See Also

Overview

Multiple Bridges and Spanning Tree Protocol

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When a bridge receives an IP packet, the gateway processes the packet as follows:

  1. The destination MAC address is looked up in the bridge's forwarding table.
  2. If the destination MAC address is found in the forwarding table, the packet is forwarded to the corresponding port.
  3. If the destination MAC address is not found in the forwarding table, the destination IP address is searched for in all the defined bridge IP address ranges.
  4. If the destination IP address is found in the bridge IP address range of exactly one port, the IP address is transmitted to that port.
  5. If the IP address is found in the bridge IP address range of more than one port, the packet is dropped. The gateway then sends an ARP query to each of the relevant ports.
  6. If a host responds to the ARP request packet with an ARP reply, the forwarding table is updated with the correct <MAC Address, Port> association. Subsequent packets will be forwarded using the forwarding table.

If a bridge receives a non-IP packet, and the bridge is configured to forward non-IP protocol Layer-2 traffic, the gateway processes the packet as follows:

  1. The destination MAC address is looked up in the bridge's forwarding table.
  2. If the destination MAC address is found in the forwarding table, the packet is forwarded to the corresponding port.
  3. If the destination MAC address is not found in the forwarding table, the packet is flooded to all the ports on the bridge.