Routing Procotols - Distance-Vector Versus Link-State:
- Distance-vector routing gets all topological data fram the routing table informtion of its neighbors. Link- State routing obtains a complete view of the network topology by accumulating information from LSAs (link-state advertisements) from both neighboring and distant routers.
- Distance-vector routing determines the best path by adding to the metric value it receives as tables move from router to router. For link-state routing, each router works separately to calculate its own shortest path to destinations.
- With most distance-vector routing protocols, updates for topology changes come in periodic table updates. These tables pass from router to router, often resulting in slower convergence. With link-state routing protocols, updates usually are triggered by topology changes. Relatively small LSAs are passed to all other routers usually result in faster time to converge on any network topology change.
Segment - Protocol Data Unit: | Top
Be careful with this term. In context - a single transport-layer unit of information.
Shielding: | Top
In cable that employs shielding, a metal braid or foil surrounds each wire pair or group of wire pairs. This shielding acts as a barrier to any interfering signals. However, as with increasing the size of conductors, using braid or foil covering increases the diameter of the cable and the cost as well. Therefore, cancellation is more commonly used technique to protect the wire from undesirable interference.
Simplex Transmission: | Top
The capability for tranmission in only one direction between a sending and a receiving station. Broadcast television is an example of simplex technology.
STP - Spanning-Tree Protocol: | Top
The main function of Spanning-Tree Protocol is to allow duplicate switched/bridged paths without incurring the latency effects of loops in the network.
Bridges and switches make their forwarding decisions for unicast frames based on the destination MAC address in the frame. If the MAC address is unknown, the device floods the frame out all ports in an attempt to reach the desired destination. It also does this for all broadcast frames.
The Spanning-Tree Algorithm, implemented by the Spanning-Tree Protocol, prevents loops by calculating a stable spanning-tree network topology. When creating fault-tolerant networks, a loop-free path must exisit between all Ethernet nodes in the network. The Spanning-Tree Algorithm is used to calculate a loop-free path. Spanning-tree frames, called "bridge protocol data units" (BPDUs), are sent and received by all switches in the netowrk at regular intervals and are used to determine the spanning-tree topology.
A switch uses Spanning-Tree Protocol on all Ethernet and Fast Ethernet based VLANs. Spanning-Tree Protocol detects and breaks loops by placing some connections in standby mode, which are activated in the event of an active connection failure.
The Spanning-Tree Protocol states are:
- Blocking - No frames forwarded, BPDUs heard.
- Listening - No frames forwarded, listening for frames.
- Learning - No frames forwarded, learning addresses.
- Forwarding - Frames forwarded, learning addresses.
- Disabled - No frames forwarded, no BPDUs heard.
Spike: | Top
A spike is an impulse that produces a voltage overload on the power line. Generally speaking, spikes last between .5 and 100 microseconds. In simple terms, when a spike occurs, it means that the power line has momentarily been struck with a powerful hit of at least 240 V on a power supply rated for 120 V (100 percent increase).
SPX - Sequenced Packet Exchange (Novell): | Top
A reliable, connection-orientated protocol that supplements the datagram service provided by network layer protocols.
Star Topology: | Top
A LAN topology in which endpoints on a network are connected to a common central switch by point-to-point links. A ring topology that is organized as a star implements a unidirectional closed-loop star, instead of point-to-point links.
Static Route: | Top
A route that is explicitly configured and entered into the routing table by default. Static routes take precedence over routes chosen by dynamic routing protocols. Key concept - use less overhead compared with Dynamic Routes.
Surge: | Top
A surge is a voltage increase above 110 percent of the normal voltage carried by a power line. Typically, such incidents last only a few seconds; however, this type of power disruption is responsible for nearly all hardware damage that computer users experience. This is because most computer power supplies that run 120 V are not built to handle more than 132 V for any length of time. Hubs are particularly vulnerable to electrical surges because of the sensitive love-voltage data lines.
Stub Network | Top
A network that has only a single connection to a router.
Switch: | Top
A network device that filters, forwards and floods frames based on the destination address of each frame. The switch operates at the data-link layer of the OSI model.
Switching Modes: | Top
Switching is the process of taking an incoming frame from one interface and delivering it out through another interface.
Two switching modes can be used to forward a frame through a switch:
- Store-and-forward - The entire frame is received before any forwarding takes place. The destination and/or the source addresses are read and filters are applied before the frame is forwarded. Latency occurs while the frame is being received; the latency is greater with larger frames because the entire frame takes longer to read. Error detection is high because of the time available to the switch to check for errors while waiting for the entire frame to be received.
- Cut-through - The switch reads the destination address before receiving the entire frame. The frame is then forwarded before the entire frame arrives. This mode decreases the latency of the transmission and has poor error detection. Fast-forward and fragment-free are two forms of cut-through switching:
- Fast-forward switching 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, the destination network adapter discards the faulty frame upon receipt. In networks with high collision rates, this can negatively affect available bandwidth. Use the fragment-free option to reduce the number of collision frames forwarded with errors. In fast-forward mode, latency is measured from the first bit received to the first bit transmitted, or first in, first out (FIFO).
- Fragment-free switching filters out collision fragments, which are the majority of packet errors, before forwarding begins. In a properly functioning network, collision fragments must be smaller than 64 bytes. Anything greater than 64 bytes is a valid packet and is usually received without error. Fragement-free switching waits until the recived packet has been determined not to be a collision fragment before forwarding the packet. In fragment-free mode, latency is measured as FIFO.
The latency of each switching mode depends on how the switch forwards the frames. The faster the switching mode, the smaller the latency in the switch. To accomplish faster frame forwarding, the switch takes less time to check for errors. The tradeoff is less error checking, which can lead to a higher number of retranmissions.
Synchronous Transmission: | Top
Digital signals that are transmitted with precise clocking. Such signals have the same frequency, with individual characters encapsulated in control bits (called start bits and stop bits) that designated the beginning and end of each character.
Subnet Mask: | Top
A 32-bit address mask used in IP to indicate the bits of an IP address that are being used for the subnet address.
TCP - Transmission Control Protocol: | Top
A connection-orientated transport-layer protocol that provides reliable full-duplex data transmission.
Telnet: | Top
Is a standard terminal emulation protocol. It is used for remote connections, enabling users to log in to remote systems to use resources as if they were connected to a local system.
Throughput: | Top
The rate information arriving at, and possibly passing through, a particular point in a network system.
Timing Jitter: | Top
All digital systems are clocked, meaning clock pulses cause everything to happen. Clock pulses cause a CPU to calculate, data to store in memory, and the NIC to send bits. If the clock on the source host is not synchronized with the destination, which is quite likely, timing jitter is the result.
Token: | Top
A frame that contains control information. Possession of the token allows a network device to transmit data onto a network.
Token Ring: | Top
A "token" passing LAN developed and supported by IBM. Token Ring runs at 4 or 16 Mbps over a ring topology. Similar to 802.5. (Deterministic - taking turns)
Trace: | Top
Uses TTL values to generate messages from each router used along the path. This command is very powerful in its capability to locate failures in the path from the source to the destination. You must understand trace command responses:
- !H - The probe was received by the router but not forwarded, which is usually due to an access list issue.
- P - The protocol was unreachable.
- N - The network was unreachable.
- U - The port was unreachable.
- * - Timed out.
Trailer | Top
Control information appended to data when encapsulating the data for network transmission.
Tree Topology: | Top
A LAN topology similar to a bus topology, except that tree networks can contain branches with multiple nodes. Transmissions from a station propagate the length of the medium and are received by all other stations.
Time-To-Live: | Top
A field in an IP header that indicates how long a packet is considered valid.
Twisted-Pair Cable: | Top
Is a 4 or 8-conductor (pins) copper transmission medium:
STP - Shield Twisted-Pair:- A two-pair wiring medium used in a variety of network implementations. STP has a layer of shielded insulation to reduce EMI.
UTP - Unshielded Twisted-Pair:- A four-pair wiring medium used in a variety of network implementations. It falls into five grades:
- Category 1 - good for telephone communications.
- Category 2 - good for transmitting data up to 4 Mbps.
- Category 3 - good for transmitting data up to 10 Mbps.
- Category 4 - good for transmitting data up to 16 Mbps. (Token Ring)
- Category 5 - good for transmitting data up to 100 Mbps. (Most commonly used today)
Note: There are other grades of UTP but they are not referred to within the material.
UDP - User Datagram Protocol: | Top
A connectionless transport-layer protocol in the TCP/IP protocol stack. UDP is a simple protocol that exchanges datagrams without acknowledgements or guaranteed delivery, requiring that error processing and retransmission be handled by other protocols.
UPS - Uninterruptible Power Supply: | Top
A backup device designed to provide an uninterrupted power source in the event of a power failure. UPSs are commonly installed on file servers and wiring hubs.
Virtual Circuit: | Top
A logical circuit created to ensure reliable communication between two network devices. It is defined by a VPI/VCI pair and can be either permanent (PVC) or switched (SVC). Virtual circuits are used in Frame Relay and X.25.
VLAN (Virtual Local Area Network): | Top
A group of devices on a LAN that are configured (using management software) so that they can communicate as if they are attached to the same wire, when in fact they are located on a number of different LAN segments. Because VLANs are based on logical instead of physical connections, they are extremely flexible.
It is important to remember that the devices or users within a VLAN can be grouped by function, department, application, and so on, regardless of their physical segment location.
A VLAN creates a single broadcast domain that is not restricted to a physical segment and is treated like a subnet.
VTP (VLAN Trunking Protocol): | Top
VTP is a Layer 2 messaging protocol that maintains VLAN configuration consistency throughout a common administration domain. VTP manages the additions, deletions, and name changes of VLANs across multiple switches, minimizing misconfigurations and configuration inconsistencies that can cause problems, such as duplicate VLAN names or incorrect VLAN-type specifications.
VTP advertisements are flooded throughout the management domain every 5 minutes, or whenever there is a change in VLAN configurations. Included in a VTP advertisement is a configuration revision number, as well as VLAN names and numbers, and information about which switches have ports assigned to each VLAN. By configuring the details on one server and propagating the information through advertisements, all swithces know the names and numbers of all VLANs.
One of the most important components of the VTP advertisements is the configuration revision number. Each time a VTP server modifies its VLAN informaiton, it increments the configuration revision number by one. The VTP server then sends out a VTP advertisement that includes the new configuration revision number. When a switch receives a VTP advertisement with a larger configuration revision number, it updates its VLAN configuration.
VTP operates in one of three modes:
- Server mode
- Client mode
- Transparent mode
VTP servers can create, modify, and delete VLANs and other configuration parameters of the entire VTP domain; this information, in turn, is propagated to the VTP clients in the same domain. VTP servers save VLAN configurations in the Catalyst NVRAM, whereas, in clients, the VLAN configuration is not stored. VTP messages are transmitted by the server out all trunk connections.
A VTP client cannot create, change or delete VLANs, nor can it save VLAN configurations in nonvolatile memory.
VTP transparent mode provides an option so that some switches can use VTP but other swithces can ignore VTP, while not stopping other swithces from using it. A switch in transparent mode forwards VTP advertisements received from other switches that are part of the same management domain, while ignoring the information in the VTP message. A switch configured in VTP transparent mode can create, delete, and modify VLANs, but the changes are not transmitted to other switches in the domain. The changes affect the local switch only.
WAN - Wide Area Network: | Top
A data communications network that serves users across a broad geographic area and often uses transmission devices provided by common carriers*. Frame Relay, SMDS, and X.25 are examples of WANs.
* Qwest, Worldcom, AT&T, Sprint.
Window Size: | Top
The number of packets/messages that can be transmitted while waiting an acknowledgement.