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US7070447-001, Rev 01
7-
1
Chapter 7
Bridging Concepts
Basic Bridging Concepts
A bridge operates at the Data Link Layer of the OSI model by forwarding or
flooding packets based on the destination MAC addresses. A bridge normally
operates in promiscuous mode and learns all packets on the network. The bridge
then builds an internal Forwarding table based on the source MAC addresses.
When a bridge receives a packet, it compares the destination address with
addresses in the Forwarding Table with one of the following results occurring:
❒ If the destination address is on the same LAN as its source address, the bridge
will discard that packet.
❒ If the destination address is in the Bridge Forwarding Table, the packet will
get forwarded onto the necessary port.
❒ If the destination address in NOT in the Bridge Forwarding Table, the packet
will get flooded out all ports except the port on which the packet came in on.
This also applies to broadcast, multicast and unknown unicast frames.
How Spanning Tree Works
Bridging loops occur when there are redundant bridge links in the network. To
correct for this, the Spanning Tree algorithm is used to detect and prevent loops in
the network. The version of Spanning Tree supported on the HiSpeed switches is
IEEE 802.1D.
Spanning Tree is a configuration algorithm and protocol that ensures that no data
loops exist within a single broadcast domain. Figure 7-1 shows bridges 1 and 2 in
a loop; in this configuration without Spanning Tree, the network is unusable