Chapter 3: Bridging Configuration Guide
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DIGITAL GIGAswitch/Router User Reference Manual
Bridging Modes (Flow-Based and Address-Based)
The GSR provides the following types of wire-speed bridging:
Address-based bridging
- The GSR performs this type of bridging by looking up the
destination address in an L2 lookup table on the line card that receives the bridge packet
from the network. The L2 lookup table indicates the exit port(s) for the bridged packet. If
the packet is addressed to the GSR's own MAC address, the packet is routed rather than
bridged.
Flow-based bridging
- The GSR performs this type of bridging by looking up an entry in
the L2 lookup table containing both the source and destination addresses of the received
packet in order to determine how the packet is to be handled.
The GSR ports perform address-based bridging by default but can be configured to
perform flow-based bridging instead, on a per-port basis. A port cannot be configured to
perform both types of bridging at the same time.
The GSR performance is equivalent when performing flow-based bridging or address-
based bridging. However, address-based bridging is more efficient because it requires
fewer table entries while flow-based bridging provides tighter management and control
over bridged traffic.
VLAN Overview
Virtual LANs (VLANs) are a means of dividing a physical network into several logical
(virtual) LANs. The division can be done on the basis of various criteria, giving rise to
different types of VLANs. For example, the simplest type of VLAN is the port-based
VLAN. Port-based VLANs divide a network into a number of VLANs by assigning a
VLAN to each port of a switching device. Then, any traffic received on a given port of a
switch
belongs
to the VLAN associated with that port.
VLANs are primarily used for broadcast containment. A layer-2 (L2) broadcast frame is
normally transmitted all over a bridged network. By dividing the network into VLANs,
the
range
of a broadcast is limited, i.e., the broadcast frame is transmitted only to the
VLAN to which it belongs. This reduces the broadcast traffic on a network by an
appreciable factor.
