Fragmentation
Tuesday 06 December 2011 at 1:10 pm.
To minimize collisions network adminstrators segment the network into various pieces which only affect collisions within each segment, not the entire network.
With layer-2 switching devices, switches and bridges, layer-2 devices, can be used to split a big network into two separate collision domains. A broadcast domain works at layer 3 of the OSI model with routers. A packet sent to a network's broadcast address will resonate throughout a broadcast domain.
ARP and RARP ARP (Address Resolution Protocol) turns an IP (network) address into a hardware address (MAC address). It finds the hardware address of a computer with the given IP address.
RARP (Reverse Address Resolution Protocol) does the opposite, i.e. turn a MAC address into an IP address. Remember that. ARP works by flooding the network with a broadcast saying "What is the MAC address for this IP address?" The entire network hears the question, but only the computer with that IP address is supposed to respond.
WAN technologies - PPP, frame relay, and ISDN.
PPP (Point-to-Point Protocol) is the protocol you use to connect to your ISP over a phone line, consisting of two protocols: LCP (Link Control Protocol), and NCP (Network Control Protocol). LCP is responsible for establishing and configuring the point-to-point connection. NCP is really just an abstraction of several protocols to let PPP work with things like IP.
ISDN (Integrated Services Digital Network) was the primary form of direct digital telphony interface, before DSL, T1 and cable offered far greater bandwidths.
Frame relay uses FECN (Forward-Explicit Congestion Notification) and BECN (Backward-Explicit Congestion Notification) to let other systems on the network know of congestion. FECN is used to let systems ahead of you know that the network is congested and to expect slower transmissions from you; BECN is used to advise systems transmitting to you that the network is congested. BECN is a request for the sending system to slow down (similar to a "source quench" message). Frame relay also uses something called DE (Discard Eligibility) to help control congestion. DE actually uses one bit of each frame on the network to indicate whether DE is on or off; If DE is turned on for a frame, that frame is considered disposable. When a switch on a frame relay network gets congested, frames with the DE bit turned on will be thrown away to conserve bandwidth.
Non-OSI network models - The 7-layer model is the industry standard for abstraction of a network component's function, however, there are two other standards involved in Cisco certification, the Cisco hierarchical model and The DoD model.
Cisco's networking model has 3 layers: Core, Distribution, and Access. The Core layer is the very core or backbone of the network. The Core layer's routers must be fast and have high-bandwidth connections. The Access layer is the point where people actually use the network; The workstations that users use are in the Access layer, as well as local workgroup's switches or hubs. The Distribution layer is a concept midway between the Core and the Access layers. Cisco explicitly defnes this model to stress that breaking up collision domains should ONLY be done at the Access layer. Cisco gets very excited about this topic.
The U.S. Department of Defense (DoD), the folks who originally created TCP, also created a four-layer network model which basically combines several of the OSI model layers into single layers: The bottom layer of the DoD model is the Network Access layer. It coextends with the Physical and Data-Link layers of the OSI model. The second layer of the DoD model is the Internet layer. It corresponds to the Network layer of the OSI model. The third layer of the DoD model is the Host-to-Host layer. It corresponds to the Transport layer of the OSI model. The top layer of the DoD model is the Process/Application layer. It coextends with the Session, Presentation, and Application layers of the OSI model.
Virtual LANs (VLANs) A virtual LAN (VLAN) is one of those concepts which is not usually very well explained, but is actually pretty simple once you can grasp the concepts behind it. To make an analogy, if you're familiar with PC support, you're probably aware of hard disk partitioning and how it works; You can have one single physical hard disk drive in a computer, but it can be divided into separate partitions, each of which is treated as a separate "logical" or "virtual" disk drive. Even though they all exist on the same physical device, they are treated as separate entities. A VLAN works much the same way.
It is network segmentation achieved through a single switch. You configure the switch so that some computers are connected to each other, and others are not. The result is that each connected community of computers becomes a virtual LAN, seeming to exist separately, even though they all really run through the same physical switch. VLANs create a small problem by separating computers that sometimes need to talk to each other. Virtual LANs are mainly organizational in their intent; They do not actually achieve anything useful other than making things a little more nicely logical and organized on the network. Creating a VLAN is easy; Just use the vlan command from global configuration mode. The syntax is as follows: vlan # name [name] ...Where # is the number of the VLAN, and [name] is the actual name you want to give it. For example, to create VLAN number 4 and name it "Accounting" to mark it as belonging to the accounting department, you'd type: vlan 4 name Accounting Cisco networking equipment uses VTP (Virtual Trunking Protocol) to keep on top of what VLANs exist. VTP is like RIP for VLANs; It propagates VLAN information throughout the entire network, so you don't have to. VTP has three modes of operation: Server, Client, and Transparent.
