Page 70 - DCAP406_DCAP_207_Computer Networks

Unit 5: Networking Devices

Notes
Figure 5.5: Bridged LAN Network
Bridge

From architecture point of view bridges are protocol independent devices and are very simple.
They do not perform complex processes on the data packets traveling through them such as the
evaluation of the network as a whole in order to make end-to-end routing decisions. They
simply read the destination address of the incoming data packet and forward it along its way to
the next link. Therefore, bridges are inexpensive and fast. There are bridges called cascading
bridges are used to support multiple LANs connected by multiple media.
Dissimilar LANs such as Ethernet-to-token ring can also be connected with the help of bridge
known as encapsulating bridge. The function of encapsulating bridge is also very simple. It
encapsulates the originating LAN data along with control information of the end user LAN.
Bridges with routing function between LANs are also available.

Figure 5.6: Filtering and Forwarding in Bridged Network
1 2 A B

Bridge

3 C
Computer 1 in the Figure 5.6 wishes to talk to Computer 3 on the same network.
The packet sent by Computer 1 will contain the physical address of Computer 3 that will also
received by the bridge device connecting the two LAN segments. The bridge will read the
physical address contained in the packet and observe that this address belongs to the computer
on the same LAN segment. Hence bridge will filter this packet and will not allow it to be
transmitted on other side of the network. In case Computer 1 wishes to talk with Computer C on
other segment, the bridge will know from its table of addresses that this address belongs to the
computer attached to other segment of the network. In this case, this will be forwarded to the
other segment of the LAN. The bridge learns location of computers attached to the network by
watching frames. This will be explained later on in the subsequent discussion. Note that in case
of broadcast and multicast packets, bridge forwards these packets to all computers attached to
the segment on both sides.

Media Access Control (MAC) Bridge

This is used to connect dissimilar LANs such as Ethernet-to-token ring using encapsulation or
translation. This bridge translates the original packet format from the requesting LAN segment
by encapsulating or enveloping with control data specific to the protocol of the destination LAN
segment.

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