Transparent Bridging

Transparent bridges are devices which connects more than one network segments with other bridges to make all routing decisions.Transparent bridges are sometimes called learning bridges. When they are turned on and receive data packets from a network segment they:
- Learn the relation between MAC address and segment/port, and
- Forward the packet to all other segments/ports.

There are two types of Transparent Bridge Modes:
- Store-and-Forward: Stores the entire frame and verifies the CRC before forwarding the frame. If a CRC error is detected, the frame is discarded.
- Cut-Through: Forwards the frame just after it reads the destination MAC address without performing a CRC check.

Transparent bridges are so named because their presence and operation are transparent to network hosts. When transparent bridges are powered on, they learn the workstation locations by analyzing the source address of incoming frames from all attached networks.

How Transparent Bridging Operates ?
The bridge uses its table as the basis for traffic forwarding. When a frame is received on one of the bridge’s interfaces, the bridge looks up the frame’s destination address in 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 indicated port. If no association is found, the frame is flooded to all ports except the inbound port. Broadcasts and multi casts also are flooded in this way.
Transparent bridges successfully isolate intra-segment traffic, thereby reducing the traffic seen on each individual segment. This is called filtering and occurs when the source and destination MAC addresses reside on the same bridge interface.

Benefits of Transparent Bridging :
Transparent bridging can deliver many benefits including more efficient usage of finite network resources, improved responsiveness, reduced wait delays, fewer network errors resulting from collisions, plug ‘n’ play installation capabilities, reduced administrative managed requirements and overheads, superior network bandwidth allocation, reduced instances of the deleterious effects of broadcast storms, loop and redundancy issues.

Network Bridging

Bridging is a forwarding technique used in packet-switched computer networks. Unlike routing, bridging makes no assumptions about where in a network a particular address is located. Instead, it depends on flooding and examination of source addresses in received packet headers to locate unknown devices. Once a device has been located, its location is recorded in a table where the MAC address is stored so as to preclude the need for further broadcasting. The utility of bridging is limited by its dependence on flooding, and is thus only used in local area networks.
A network bridge is a device which connects two parts of a network together at the data link layer (layer 2 of the OSI model).

Situations Where Bridging Is Appropriate :
- Connecting Networks.
- Filtering/Traffic Shaping Firewall.
- Network Tap.
- Layer 2 VPN : Two Ethernet networks can be joined across an IP link by bridging the networks to an EtherIP tunnel or a tap(4) based solution such as OpenVPN.
- Layer 2 Redundancy : A network can be connected together with multiple links and use the Spanning Tree Protocol to block redundant paths.

Advantages :
- Self-configuring.
- Isolate collision domain.
- Reduce the size of collision domain by micro segmentation in non-switched networks.
- Transparent to protocols above the MAC layer.
- LANs interconnected are separate, and physical constraints such as number of stations, repeaters and segment length don't apply.
- Helps minimize bandwidth usage.

Disadvantages :
- It does not limit the scope of broadcasts.
- It does not scale to extremely large networks.
- Buffering and processing introduces delays.
- Bridges are more expensive than repeaters or hubs.