Page 69 - DCAP207_NETWORKS_DCAP406_COMPUTER_NETWORKS

Computer Networks/Networks

Notes 5.1.2 Router Protocols

Router protocols consists of both bridging and routing protocols as listed below:
1. Inter-router Protocols: These are router-to-router protocols that can operate over
dissimilar networks. This protocol routes information and stores data packets during
periods of idleness.
2. Serial Line Protocols: This protocol is widely used over serial or dialup links connecting
unlike routers. Examples include HDLC, SLIP (Serial Line Interface Protocol), and PPP
(Point-to-Point Protocol).

3. Protocol Stack Routing and Bridging Protocols: This advises the router as to which
packets should be routed and which should be bridged.

5.2 Bridges

Like repeaters, bridges are used to connect similar LANs together, for example, Ethernet-to-
Ethernet and operate at the bottom two layers of the OSI model i.e. physical layer and data link
layer as shown in Figure 5.4. As it operates on second layer of the OSI model therefore it relays
only necessary data to other signals. MAC addresses (physical addresses) are used to determine
whether data is necessary to transmit to other LAN segments or not. It passes information from
one LAN segment to another based on the destination address of the packet. In other words,
when a bridge receives data through one of its ports, it checks the data for a MAC address. If this
address matches that of the node connected to other port, the bridge sends this data through this
port. This action is called forwarding. If the address does not match with any node connected to
other port, the bridge discards it. This action is called filtering. This is shown in Figure 5.5.
Unlike repeaters, bridges have buffers to store and forward packets in the event that the destination
link is congested with traffic.

Figure 5.4: Bridge and OSI Reference Model Correspondence

Application Layer Application Layer
Presentation Layer Presentation Layer
Session Layer Session Layer
Transport Layer Transport Layer
Network Layer Bridge Network Layer
Data Link Layer Data Link Layer
Physical Layer Physical Layer

The main advantage of bridge over repeater is that it has filtering action. If any noise on
Ethernet occurs because of collision or disturbance in electrical signal, the bridge will consider
it as an incorrectly formed frame and will not forward to the segment connected to other port of
the bridge.

Notes Note that bridge can relay broadcast packets and packets with unknown destination.

As you know that the maximum four repeaters can be used to connect multiple Ethernet segments.
However, if a bridge is provided between repeaters, this limit of four is increased. The maximum
number of bridges is not specifically limited.

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