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hubs and switches
CCNA Hubs, Repeaters, and Switches
| Question | Answer |
|---|---|
| What is attenuation? | Attenuation is the gradual weakening of an electric sign al over distance. |
| What is a hub? | A multiport repeater. It is an OSI L1 device. |
| What is a repeater? | An L1 device that is used to boost a signal across a long distance |
| Briefly describe the universal image for a hub | A square with a double arrow on it [↔] |
| Describe the problem with connecting two or more hosts to a hub? | With a hub, only one host can communicate at a time. If two or more hosts connected to a hub send data at the same time, the signals will collide and all data will be unusable |
| What is a collision domain? | A collision domain is a segment of cable on which two stations can't transmit at the same time without causing a collision. |
| What is CSMA/CD | Carrier Sense Multiple Access with Collision Detection |
| How does Carrier Sense Multiple Access with Collision Detection work? | Before sending data, a host checks to see if any other host is already sending data. If the check reveals another host sending data, it waits before listening to the wire again. If no one is sending data, the host sends the signal. |
| What problem did hubs and repeaters solve? | They helped us deal with the problem of attenuation |
| What 2 major problems occur from a network using hubs and repeaters? | 1. All connected hosts are in one big collision domain 2. All those same hosts are in one big broadcast domain as well |
| What is “listening to the wire”? | Listening to the wire refers to a host checking to see if any other host is currently trying to send data. |
| What happens if two PCs within a collision domain send data at the exact same time after listening to the wire? | The voltage on the wire itself changes, signaling to the hosts that a data collision has occurred. This will generate a jam signal, indicating to other other hosts that data should not be transmitted at this time. |
| What happens to the host if it receives a jam signal? | A backoff timer is invoked. Backoff timers are set to milliseconds and are random values, which greatly decreases the odds of another collision. |
| What is the biggest problem with CSMA/CD (Carrier Sense Multiple Access with Collision Detection)? | It necessarily introduces delays that we’d rather not have |
| True or false: In a network with hubs and repeaters, it is impossible for a host to send a broadcast without every other host receiving that broadcast. | TRUE – Every other host on the network will receive all broadcasts whether they want/need it or not. |
| Can hubs limit the scope of broadcasts? | No |
| True or false: The more collision domains you have, the more likely you are to have a data collision? | FALSE – The more collision domains that exist, the less likely you are to have data collisions. |
| What is a bridge, how was it used? | A bridge is an L2 device that are generally placed between two hubs to create smaller collision domains. |
| What is the major benefit to using bridges? | Smaller collision domains, resulting in less collisions |
| What is the biggest problem with bridges? | The do not solve the issue of having a large broadcast domain. |
| What is microsegmentation? | Microsegmentation refers to a host-to-one port topology, and this design eliminates any chance of collisions |
| Describe the universal symbol for a switch | A box, with two arrows pointing left and two arrows pointing right. _____________ | ← | | →| | ← | | → | _____________ |
| What L2 device replaces hubs, repeaters, and bridges? | the Switch! |
| What is the biggest benefit to using a switch? (as opposed to using hubs/repeaters/bridges) | Assuming each host is connected to its own switch port, each host will get its own collision domain. This eliminates the possibility of data collisions. |
| In terms of broadcasts, what is the default behavior of a network of hosts connected by a switch? | They will all be in the same broadcast domain |
| What allows us to segment broadcast networks? | VLANs (Virtual Local Area Networks) |
| Describe hubs | Hubs give us one collision domain consisting of all connected hosts, one broadcast domain containing those same hosts, and allow only one host to transmit at a time. |
| Describe bridges | Bridges allow us to logically segment the network and to lessen the size of collision domains, but do nothing to segment broadcast domains. |
| Describe switches | Switches bring one-host collision domains and the capability to logically segment the broadcast domain, although that’s not done by default. |
| What 3 actions can a switch take when it receives a frame? | 1. Forward it 2. Filter it (fancy talk for “drop it”) 3. Flood it (fancy talk for “send it everywhere, except back to where it came from) |
| How many MAC addresses are contained in every frame? | Two. A source MAC address and a destination MAC address. |
| List the drawbacks to creating a MAC address table consisting only of static entries. | 1. You would have to make a new static entry every time you connect a new host (easy to forget and easier to mistype) 2. Makes switching a machine to a different port VERY cumbersome |
| When a switch first builds its MAC table, what happens when 1 host sends a frame to a different host? | The switch first checks to see if the source host is in the MAC table, adding it if not found. It then checks for the destination MAC address. If the destination is found in the table it will forward the frame, otherwise it will flood the frame. |
| What would happen if a switch notices that the source MAC address and Destination MAC address are at the same port? | The frame would be filtered (dropped) |
| In relation to switches, when does Forwarding happen? | When the switch has an entry for the frame’s destination MAC. That frame will be forwarded only out the port indicated by the MAC table. |
| In relation to switches, when does Flooding occur? | When the switch has no entry for the frame’s destination MAC in the MAC table. With flooding, the frame is sent out every port except the port the frame came in on. |
| In relation to switches, when does Filtering happen? | When the source and destination MAC addresses are located off the same port. The frame is simply dropped. |
| What are the two types of frames that are sent out to all hosts? | Broadcast Frames and Flooded Frames |
| What is the destination MAC address of a Broadcast Frame? | Ff-ff-ff-ff-ff-ff (case insensitive) |
| How long to static MAC address stay in a switch’s MAC table? | Until they are manually removed |
| How long do dynamically learned addresses stay in a switch’s MAC table? | 5 minutes (300 seconds). The 5 minute timer is reset every time a frame comes in with that particular source MAC address. |
| What are the three processing methods used by a switch to process a frame during frame forwarding? | 1. Store-and-forward 2. cut-through 3. Fragment-free |
| Which is the most common frame-forwarding method on cisco switches? | Store-and-forward |
| Describe store-and-forward | With store-and-forward, the entire frame is stored by the switch before transmission begins. The switch analyzes the Frame Check Sequence for data corruption. |
| What is the main Pro and Con of the store-and-forward method, in relation to frame forwarding? | PRO – offers the greatest level of error detection of the three methods CON – due to the level of error detection it is the slowest method |
| Describe the cut-through method of frame forwarding | With cut-through forwarding, the switch automatically begins to forward the frame before the entire frame has actually been received. |
| What is the main Pro and Con of the cut-through method, in relation to frame forwarding? | PRO – cut through is the fastest of the the three methods CON – offers zero error detection |
| Describe fragment-free forwarding | The switch assumes that any corruption will take place in the first 64 bytes of a frame. If no corruption is found in the first 64 bytes, the frame is assumed to be corruption free and the forwarding process begins |
| What is the main Pro and Con of the fragment-free method, in relation to frame forwarding? | PRO – middle ground method; it offers better speed that store-and-forward and more error protection than cut-through CON – Slower than cut through and less error protection than store-and-forward |
| Which frame forwarding method is the fastest? | Cut-through |
| Which frame forwarding method is the slowest? | Store-and-forward |
| Which frame forwarding method offers the most error protection? | Store-and-forward |
| Which frame forwarding method offers the least error protection? | Cut-through |
| Which frame forwarding method is the “middle ground” for both speed and data integrity? | fragment free |
| How are MAC addresses organized? | The first half of the address (first six hex characters) is the Organizationally Unique Identifier (OUI). The second half of the address is a combination of hex characters not used by this vendor on any other device with a similar OUI |
| True or false: Mac addresses are case insensitive? | TRUE |
| True or false: the following are all acceptable ways to express a MAC address: Aaaa.aaaa.aaaa Aa-aa-aa-aa-aa-aa aa.aa.aa.aa.aa.aa Aa:aa:aa:aa:aa:aa | TRUE |
| Convert to the following Hex value to Decimal: A7 | 167 A = 10; 10 * 16 = 160 160 + 7 = 167 |
| Convert to the following Hex value to Decimal: 4B | 75 4 * 16 = 64 B = 11 64 + 11 = 75 |
| Convert to the following Hex value to Decimal: A52 | 2642 A = 10 10 * 256 = 2560 5 * 16 = 80 2560 + 80 + 2 = 2642 |
| What are the units for 4 digit hex to decimal conversion? | 4096, 256, 16, 1 LTR |
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