Table of Contents
Advertisement
QTECH
3.1
Introduction to Bridging
A bridge is a store-and-forward device that connects and transfers traffic between local area network (LAN)
segments at the data-link layer. In some small-sized networks, especially those with dispersed distribution of users,
the use of bridges can reduce the network maintenance costs, without requiring the end users to perform special
configurations on the devices.
In applications, there are four major kinds of bridging technologies : transparent bridging, source-route
bridging (SRB), translational bridging, and source-route translational bridging (SR/TLB).
Transparent bridging is used to bridge LAN segments of the same physical media type, primarily in Ethernet
environments. Typically, a transparent bridging device keeps a bridge table, which contains mappings between
destination MAC addresses and outbound interfaces.
Presently the devices support the following transparent bridging features :
·
Bridging over Ethernet
·
Bridging over point-to-point (PPP) and high-level data link control (HDLC) links
·
Bridging over X.25 links
·
Bridging over frame relay (FR) links
·
Inter-VLAN transparent bridging
·
Routing and bridging are simultaneously supported
3.2
Major Functionalities of Bridges
3.2.1
A bridge relies on its bridge table to forward data. A bridge table consists two parts : MAC address list and
interface list. Once connected to a physical LAN segment, a bridge listens to all Ethernet frames on the segments.
When it receives an Ethernet frame, it extracts the source MAC address of the frame and creates a mapping entry
between this MAC address and the interface on which the Ethernet frame was received.
As shown in I. Figure 1, Hosts A, B, C and D are attached to two LAN segments, of which LAN segment 1
is attached to bridge interface 1 while LAN segment 2 is connected with bridge interface 2. When Host A sends an
Ethernet frame to Host B, both bridge interface 1 and Host B receive this frame.
MAC address table management
Chapter 3
Maintaining the bridge table
Software Configuration Manual
3-41
Advertisement
Table of Contents
