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ZyWALL 5 User’s Guide
378 Chapter 24 Maintenance
The bridge gradually builds a host MAC-address-to-port mapping table such as in the
following example, during the learning process.
For example, if a bridge receives a frame via port 1 from host A (MAC address
00a0c5123478), the bridge associates host A with port 1. When the bridge receives another
frame on one of its ports with destination address 00a0c5123478, it forwards the frame
directly through port 1 after checking the internal table.
The bridge takes one of these actions after it checks the destination address of an incoming
frame with its internal table:
• If the table contains an association between the destination address and any of the bridge's
ports aside from the one on which the frame was received, the frame is forwarded out the
associated port.
• If no association is found, the frame is flooded to all ports except the inbound port.
Broadcasts and multicasts also are flooded in this way.
• If the associated port is the same as the incoming port, then the frame is dropped
(filtered).
24.7 Transparent Firewalls
A transparent firewall (also known as a transparent, in-line, shadow, stealth or bridging
firewall) has the following advantages over “router firewalls”:
1
The use of a bridging firewall reduces configuration and deployment time because no
networking configuration changes to your existing network (hosts, neighboring routers
and the firewall itself) are needed. Just put it in-line with the network it is protecting. As
it only moves frames between ports (after inspecting them), it is completely transparent.
2
Performance is improved as there's less processing overhead.
3 As a transparent bridge does not modify the frames it forwards, it is effectively “stealth”
as it is invisible to attackers.
Table 139 MAC-address-to-port Mapping Table
HOST MAC ADDRESS PORT
00a0c5123456 3
00a0c5123478 (host A) 1
00a0c512349a 3
00a0c51234bc 2
00a0c51234de 4