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LAN Segmentation

As the files transiting the network become larger and larger, network response time grows longer. One way to alleviate this problem is to segment the LAN into two or more pieces. By splitting the users in half, there will be less contention for the Ethernet, therefore; response time should improve. Once the Ethernet is segmented, we open a can of worms on how to connect these segments. We call this can of worms bridging and routing.

Bridging

A bridge may connect two or more LANs and performs minor routing and filtering functions. The bridge receives data packets, scans the network address, and passes the packet to the appropriate network. A self-learning bridge can filter information by monitoring the traffic on the networks connected to it and learning the addresses that are associated with each network. By doing this, the bridge isolates the traffic destined to remain on the local segment of the network and broadcasts the rest to the other networks. Bridges are ideally suited for interconnecting similar networks in which protocol conversion is not required, security concerns are minimal, and where filtering is required.

The filtering capabilities enable the network manager to restrict the types of packets that go over the bridge, thereby alleviating traffic bottlenecks and limiting access to certain types of network resources. Exhibit 3-5-4 shows two bridging configurations for Ethernet LANs.


Exhibit 3-5-4.  Bridging for Ethernet Lans

In Exhibit 3-5-4, should workstation A need to communicate with the server, the traffic would stay local on the Ethernet segment. If workstation F wanted to access the server, the bridge would forward that traffic to the appropriate Ethernet segment. On the other hand, if F wanted to communicate with H, the traffic would not be forwarded to the other segment.

Routing

A router is similar to a bridge, but it offers a more sophisticated network management and functionality. It involves two basic activities: path determination and transport. Routing algorithms keep track of information like the desirability of a path, status of links, and next hop for a given destination. Once the router knows the best path for a given packet and finds that that path is available, the packet will be transported either to its destination or to the next hop on the way to its destination. Routing occurs at the Network Layer (Layer 3), thereby allowing devices connected to different types of LANs to communicate. Bridging occurs at the Data Link Layer (Layer 2).

Protocols like TCP/IP or IPX are commonly routed because they conform to the specifications of the Network Layer. These protocols have routing protocols built in. Some protocols like LAT must be bridged because they do not conform to Network Layer specifications. If a device at one IP address wants to communicate with a server at a different IP address, a router will look at the destination address, look up which port the address belongs to, and route the data packets to the appropriate port. Exhibit 3-5-5 shows some typical router configurations.


Exhibit 3-5-5.  Routing Configurations

Both bridging and routing allow for extension of the LANs across the wide area. Routing not only allows for different protocols to traverse the same network, but also allows for devices on different LAN media (i.e., Ethernet and Token Ring) to communicate. Multiprotocol routers are the primary backbone data networking devices today.

LAN Switching

As router networks grow, performance issues begin to pop up. Protocols have hop count limitations, which may be exceeded in the case of a failure in the network. As the load on the network increases, it becomes impossible to predict delay end-to-end through multiple hops. Also, if there are many protocols being routed across the network, routing table updates can cause huge overhead (especially upon a failure and/or recovery) which can have serious effects on performance. New ways to internetwork are emerging that offload most of the local router traffic and let the routers route traffic across the WAN only. This is called LAN switching. Exhibit 3-5-6 shows shared LANs feeding into a LAN switch.


Exhibit 3-5-6.  Switched Ethernet

LAN switching handles most of the traffic within a building or campus. It allows traffic from each segment to be “switched” at Layer 2 (MAC address) to any other segment (provided the media is the same). Ethernet switching allows for each device to be on its own 10M-bps segment. Switches are connected together through an Ethernet port or through a high-speed uplink, like 155M-bps ATM or 100M-bps Ethernet. Exhibit 3-5-7 depicts this backbone.


Exhibit 3-5-7.  Switched Ethernet with ATM Backbone

A routing table is built within the switch to map a MAC address with a physical port. The switch looks at the destination address within a frame, then “switches” the frame to the appropriate port. These ports may have shared or dedicated segments (i.e., each port may have only 1 MAC address, or several). There are many Ethernet segment switches on the market today from all of the major vendors. Some of these switches have “auto-learning” features that require minimal configuration. The switch learns which devices are attached upon start-up of the device and stores these addresses in its table. The switch may also group devices together in a virtual LAN (VLAN) based on which devices need to communicate with each other.

Should a device move from one port to another, there is no configuration of the switch. The switch will automatically update its table and maintain the same VLAN independent of the physical segment where the device resides. VLAN technology simplifies moves, adds, and changes for facilities people as well as network managers. Be careful, there are many different VLAN implementations today that are not compatible. Check each product thoroughly for the features and functions provided.


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