Keep in mind that the objective of any network design is the delicate balance of meeting business and application requirements while minimizing the cost to meet the objective.
The phenomenal growth of internetworks has predicated the move from bridges to routers and now switches. There are four basic devices used in building an internetwork. Understanding the functions of each is important in determining the network design. These four devices are: Hubs, bridges, routers and switches.
- Network Devices and Capabilities
Hubs are often called concentrators and made possible centralized LAN topologies. All the LAN devices are connected to the hub. The hub essentially regenerates the signal received form one port to another acting as a repeater. These devices operate at the physical layer (Layer 1) of the OSI Reference Model.
Bridges connect autonomous LAN segments together as a single network and operate at the data link layer (Layer 2) of the OSI Reference Model. These devices use the Media Access Control (MAC) address of the end station for making a decision forwarding the packet. Bridges are protocol independent.
Routers performing a routing function operate at the network layer (Layer 3) of the OSI Reference Model. These devices connect different networks and separate broadcast domains. Routers are protocol dependent.
Switches were first advanced multiport bridges with the ability to separate collision domains. Layer 2 switches enhancing performance and functionality through virtual LANs have replaced hubs. The second incarnation of switches, enable them to perform Layer 3 routing decisions thereby performing the function of a router.
Bridging for this discussion is concerned with transparent bridging. This is opposed to Source-Route Bridging (SRB) which is closer to routing than bridging. Bridging occurs at the MAC sublayer of IEEE 802.3/802.5 standard applied to the data link layer of the OSI Reference Model. Routing takes place at the Network layer of the OSI Reference Model. Bridging views the network as a single logical network with one hop to reach the destination. Routing enables multiple hops to and between multiple networks. This leads to four distinct differences between the routing and bridging:
- Bridging and Routing
Data-link packet header does not contain the same information fields as network layer packets.
Bridges do not use handshaking protocols to establish connections. Network layer devices utilize handshaking protocols.
Bridges do not reorder packets from the same source while network layer protocols expect reordering due to fragmentation.
Bridges use MAC addresses for end node identification. Network layer devices such as routers, use a network layer address associated with the wire connecting to which the device is attached.
While there are these differences between bridging and routing there are times where bridging may be required or preferred over routing and vice-a-versa.
Advantageous of bridging over routing:
Transparent bridges are self-learning therefore require minimal, if any, configuration. Routing requires definitions for each interface for the assignment of a network address. These network addresses must be unique with in the network.
Bridging has less overhead for handling packets than does routing.
Bridging is protocol independent while routing is protocol dependent.
Bridging will forward all LAN protocols. Routing only uses network layer information and therefore can only route packets.
In contrast routing has the following advantageous over bridging:
Routing allows the best path to be chosen between source and destination. Bridging is limited to a specific path.
Routing is a result of keeping updated complete network topology information in routing tables on every routing node. Bridging maintains a table of devices found off its interfaces. This causes bridges to learn the network slower than routing thereby enabling routing to provide a higher level of service.
Routing uses network layer addressing which enables a routing device to group the addresses into areas or domains creating a hierarchical address structure. This leads to an unlimited amount of supported end nodes. Bridging devices maintain data link layer MAC addresses, therefore they can not be grouped, and hence results in a limited number of supported end nodes.
Routing devices will block broadcast storms from being propagated to all interfaces. Bridging spans the physical LAN segment to multiple segments and therefore forward a broadcast to all attached LAN segments.
Routing devices will fragment large packets to the smallest packet size for the selected route and then reassemble the packet to the original size for delivery to the end device. Bridges drop packets that are too large to send on the LAN segment without notification to the sending device.