What will be Covered:

• Bridges

• Segmentation

• Microsegmentation.

• Benefits of LAN switching.

• Symmetric and asymmetric switching.

• Cut-through, fast-forward and store-and-forward switching.

• Spanning-Tree Protocol.

Switch Configuration

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.

Bridging

A network bridge connects multiple network segments at the data link layer (Layer 2) of the OSI model. In Ethernet networks, the term bridge formally means a device that behaves according to the IEEE 802.1D standard. A bridge and switch are very much alike; a switch being a bridge with numerous ports. Switch or Layer 2 switch is often used interchangeably with bridge.

Bridges are similar to repeaters or network hubs, devices that connect network segments at the physical layer (Layer 1) of the OSI model; however, with bridging, traffic from one network is managed rather than simply rebroadcast to adjacent network segments. Bridges are more complex than hubs or repeaters. Bridges can analyze incoming data packets to determine if the bridge is able to send the given packet to another segment of the network.

Bridges

Wireless BridgingWireless Bridging is quite commonly used in scenarios where a physical cable is not able to be used.

Most home style routers come with the ability to act like a bridge in either wireless or wired mode.

We’ll cover this scenario further in a practical.

Pros -

• Unlike hubs and repeaters, switches allow multiple data streams to pass simultaneously.

• LAN switches are used to interconnect multiple LAN segments. LAN switching provides dedicated, collision-free communication between network devices, with support for multiple simultaneous conversations.

• Collisions: Switches reduce collisions on network segments because they provide dedicated bandwidth to each network segment and each connected segment is in a separate collision domain.

• Bandwidth: LAN switches provide excellent performance for individual users by allocating dedicated bandwidth to each switch port (for example, each network segment). This technique is known as microsegmenting. An Ethernet LAN switch improves bandwidth by separating collision domains and selectively forwarding traffic to the appropriate segments.

• Dedicated Bandwidth: Switches deliver dedicated bandwidth to users through high-density group switched and switched 10BaseT or 100BaseT Ethernet.

• VLANs: LAN switches can group individual ports into logical switched workgroups called VLANs, thereby restricting the broadcast domain to designated VLAN member ports. VLANs are also known as switched domains and autonomous switching domains. Communication between VLANs requires a router.

Pros and Cons of LAN Segmentation with Switches

Cons –

• Segmenting a network can impact the performance of the network. An example would be having a server that has the backup service running on one segment and then the actual data on another segment. When the backup runs you’ll end up slowing down the whole network due to the enormous amount of traffic going through the network device that is performing the segmentation.

• Too many segments makes a network too complex to manage and causes confusion. Always segment smartly – example, segment via departments like Sales, Marketing, Production etc.

• Network segmentation is costly to have. Several extra pieces of network hardware like switches, hubs, routers, firewalls, repeaters and bridges may be needed.

Pros and Cons of LAN Segmentation with Switches

Microsegmentation is network segmenation of a collision domain into as many segments as there are circuits minus one. So the number of segments equals the number of circuits minus one. The microsegmentation in this scenario is performed by the switch; the collision domain is cut down so that only two nodes coexist within each collision domain. Therefore you cut down on collisions due to only having two NICs which are directly connected.

Microsegmentation

Layer 2 switching is highly efficient because there is no modification to the data packet, only to the frame encapsulation of the packet, and only when the data packet is passing through dissimilar media (such as from Ethernet to FDDI). Layer 2 switching is used for workgroup connectivity and network segmentation (breaking up collision domains). This allows a flatter network design with more network segments than traditional 10BaseT shared networks. Layer 2 switching has helped develop new components in the network infrastructure

Server farms — Servers are no longer distributed to physical locations because virtual LANs can be created to create broadcast domains in a switched internetwork. This means that all servers can be placed in a central location, yet a certain server can still be part of a workgroup in a remote branch, for example.

Intranets — Allows organization-wide client/server communications based on a Web technology.

These new technologies allow more data to flow off from local subnets and onto a routed network, where a router's performance can become the bottleneck.

Benefits of LAN Switching

LAN switches also can be characterized according to the proportion of bandwidth allocated to each port. Symmetric switching provides evenly distributed bandwidth to each port, while asymmetric switching provides unlike, or unequal, bandwidth between some ports.

An asymmetric LAN switch provides switched connections between ports of unlike bandwidths, such as a combination of 10BaseT and 100BaseT. This type of switching is also called 10/100 switching. Asymmetric switching is optimized for client/server traffic flows in which multiple clients simultaneously communicate with a server, requiring more bandwidth dedicated to the server port to prevent a bottleneck at that port.

A symmetric switch provides switched connections between ports with the same bandwidth, such as all 10BaseT or all 100BaseT. Symmetric switching is optimized for a reasonably distributed traffic load, such as in a peer-to-peer desktop environment.

Symmetric and Asymmetric Switching

The three different types of switching involve various different means of error checking, speed and redundancy:

• Cut-through switching - With the cut-through switching method, the LAN switch copies only the destination address (the first 6 bytes following the preamble) into its on-board buffers. It then looks up the destination address in its switching table, determines the outgoing interface, and forwards the frame toward its destination. A cut-through switch provides reduced latency because it begins to forward the frame as soon as it reads the destination address and determines the outgoing interface.

• Fast-forward switching – This offers the lowest level of latency by immediately forwarding a packet after receiving the destination address. Because fast-forward switching does not check for errors, there may be times when frames are relayed with errors. Although this occurs infrequently and the destination network adapter discards the fault frame upon receipt. In networks with high collision rates, this can negatively affect available bandwidth.

Cut-through, fast-forward and store-and-forwarding switching

• Store-and-forwarding switching - With the store-and-forward switching method, the LAN switch copies the entire frame into its onboard buffers and computes the cyclic redundancy check (CRC). The frame is discarded if it contains a CRC error or if it is a runt(less than 64 bytes, including the CRC) or a giant (more than 1518 bytes, including the CRC). If the frame does not contain any errors, the LAN switch looks up the destination address in its forwarding, or switching, table and determines the outgoing interface. It then forwards the frame toward its destination.

Cut-through, fast-forward and store-and-forwarding switching

Spanning-Tree Protocol is a link management protocol that provides path redundancy while preventing undesirable loops in the network. For an Ethernet network to function properly, only one active path can exist between two stations.

Multiple active paths between stations cause loops in the network. If a loop exists in the network topology, the potential exists for duplication of messages. When loops occur, some switches see stations appear on both sides of the switch. This condition confuses the forwarding algorithm and allows duplicate frames to be forwarded.

To provide path redundancy, Spanning-Tree Protocol defines a tree that spans all switches in an extended network. Spanning-Tree Protocol forces certain redundant data paths into a standby (blocked) state. If one network segment in the Spanning-Tree Protocol becomes unreachable, or if Spanning-Tree Protocol costs change, the spanning-tree algorithm reconfigures the spanning-tree topology and reestablishes the link by activating the standby path.

Spanning-Tree Protocol operation is transparent to end stations, which are unaware whether they are connected to a single LAN segment or a switched LAN of multiple segments.

Introduction to Spanning Tree Protocol

We will be setting up a basic network using multiple computers and a switch.

• Everyone needs to setup Windows XP on their computer and change the computer’s name to Internetworking with a number at the end, eg Internetworking1, Internetworking2.

• Then you will need to all join the same Workgroup, we will call our Workgroup: TAFE.

• Assign a Static IP to your computer. We will use the 192.168.0.x range.

• Setup a File Share and attempt to access it from another computer.

• Setup a Printer on one computer and share it to all the other computers.

Practical