Ch 21: Troubleshooting Wireless Connectivity Flashcards
Which of the following is considered to be the best first step in troubleshooting a wireless problem?
Reboot the wireless LAN controller
Gather more information to find the scope of the problem
Access the WLC and search for clients in the error logs
Access the WLC and look for alarms; if none are found, close the incident ticket
2.
The first course of action should always be to gather as much information as possible so that you can reduce the scope of the problem. Then you can investigate the few potential causes that remain.
To troubleshoot a single wireless client, which one of the following bits of information would be most helpful in finding the client device in a wireless LAN controller?
- The Ethernet MAC address of the client device
- The end user’s name
- The wireless MAC address of the client device
- The name of the application having issues
3.
The wireless MAC address is always an important parameter because you can enter it into the search bar of a WLC to find the client device.
Suppose you have accessed a WLC to search for a client’s MAC address. Information about the client is displayed, along with a sequence of dots indicating connectivity. The last green dot in the sequence is labeled Online. Which one of the following statements is the most correct?
- The client device is powered up and online but has not begun to join the wireless network.
- The client device has successfully joined the wireless network.
- The client device has associated with an AP to get online but has not authenticated successfully.
- None of the above
2.
The status Online means that the client has passed through each phase and policy that the WLC required and has successfully joined the wireless network.
According to the Connectivity chart on the WLC’s Client View screen, which one of the following states indicates that a wireless client has met all of the requirements to begin using a wireless network?
a. Association
b. Start
c. Authentication
d. DHCP
e. Online
E.
The status Online means that the client has successfully joined the network. The other states occur earlier in the connection sequence.
Suppose an end user tried to join a wireless network several minutes ago. The WLC Client View screen shows the client to be in the Association state but not the Authentication state. Which one of the following best describes the client’s current condition?
- The client does not support any of the 802.11 amendments.
- The client has an incorrect pre-shared key.
- Spanning Tree Protocol is blocking the AP’s uplink.
- The client failed to receive an IP address.
2.
The client has not yet passed the Authentication stage, so it must have failed to authenticate itself correctly. If the WLAN uses WPA2-Personal, then the client’s pre-shared key could be incorrect.
Suppose that you have a large wireless network with several controllers, many APs, a RADIUS server, and a syslog server. A user has reported connectivity problems in a specific building location but has provided no details about the AP or controller he tried to join. Which one of the following is the most efficient troubleshooting method you can use to find information about the client?
- Go to the client’s location and use your own computer to associate with the network and then find out which AP and controller you are using
- Access each WLC and check the status of every AP that is joined to it
- Search for the client’s MAC address on each controller
- Search for the client’s MAC address on the RADIUS server
3.
Out of the possible answers, the most efficient method would be to access each controller and search for the user’s MAC address. That would give you important information specific to that user.You could also leverage Prime Infrastructure or DNA Center to search for the client across all managed controllers at once.
- If you choose to use your own computer, you may never be able to duplicate the conditions the user had when he experienced the problem.
- Checking each AP is not an efficient approach because you have not narrowed the scope of the problem.
- Checking the RADIUS server might reveal some authentication problems, but only if the user’s problem involved failed authentication.
Suppose you search a WLC for a client device’s MAC address. The results show a Connection Score value of 10%. Which one of the following correctly describes the scenario?
- The client is using its connection only 10% of the time.
- The client is currently in the bottom 10% of all wireless clients in data usage.
- The client has a received signal strength of 10% at the AP.
- The client is currently using a data rate that is 10% of its maximum capability.
4.
The Connection Score indicates the client’s actual data rate as a percentage of its maximum supported data rate, assuming that the AP’s maximum data rate is higher.
Suppose that you have just received news that no users can connect with a newly installed AP. Which one of the following bits of information would be important when you search for the AP’s name from the WLC? (Choose all that apply.)
- The AP has a valid IP address.
- The AP is not found.
- The AP has no channel numbers listed for the 2.4 and 5 GHz bands.
- The AP has a valid MAC address.
1, 2, and 3.
The first three choices are important facts in troubleshooting the connectivity issues. For example:
- If you see a valid IP address listed for the AP, then it must be properly connected to the wired network, have appropriate power, and have discovered and joined the WLC. As a result, you can probably rule out wired connectivity problems at the AP.
- If the AP is not found in the WLC search, then the AP might not be powered on, might not have an IP address, or might not have discovered the WLC. Therefore, users would not be able to use the AP at all.
- If the AP has no channel numbers shown, then perhaps the wireless bands have not been enabled on the WLC, so the users have no BSS to discover and join.
- Knowing that the AP has a valid MAC address probably has no relevance at all because all APs are preconfigured with valid MAC addresses at the factory.
Suppose you search for an AP on a WLC and notice that Noise is −20 on the 2.4 GHz band. Which of the following statements is correct?
- The noise is at a very low level, which is good for wireless performance.
- The noise is at a very high level, which is good for wireless performance.
- The noise is at a very low level, which is bad for wireless performance.
- The noise is at a very high level, which is bad for wireless performance.
D.
The noise level is measured in dBm, from 0 dBm to −100 dBm or more.
For the best wireless performance, you want the noise level to be as minimal as possible, so −100 would be best. Because the actual level is −20, performance is probably very bad around the AP.
Suppose you access a WLC and search for the name of a specific AP for which users have complained about problems. When you look at the 5 GHz information about the AP, you notice that it is using channel 60 and has 5 dBm transmit power, 65 clients, −90 noise level, 1% channel utilization, and the Air Quality value 10. Which of the following conclusions would be most accurate?
- The AP has too many clients using the 5 GHz channel, which is causing poor performance.
- The noise level is too low, which is causing poor performance.
- The channel utilization is too low, which is keeping clients from using the channel.
- The Air Quality value indicates a severe problem with interference on the channel.
4.
The Air Quality level of 10 is very low, considering that 100 is the highest and best value. Therefore, something must be interfering with the AP and client operation on that channel.
It might be tempting to see the large number of clients on the AP and assume that there are too many to share the same channel. However, the channel utilization is very low, indicating that the 65 clients are mostly idle or quiet, leaving plenty of air time available for use.
Likewise, a noise level of −90 dBm is very low and does not indicate a problem.
When one or more network users report that they are having problems, your first course of action should be to _______________________.
When one or more network users report that they are having problems, your first course of action should be to gather more information.
Begin with a broad perspective and then ask pointed questions to narrow the scope of possible causes. You do not want to panic or waste time chasing irrelevant things. Instead, ask questions and try to notice patterns or similarities in the answers you receive.
Put the following crucial steps of a wireless client attempting to join the wireless network in order.
- Authentication
- DHCP
- Association
- Start
- Online
Perhaps the most important information about the client is shown as the sequence of large dots under the Connectivity heading (refer to Figure 21-4). Before a controller will permit a client to fully associate with a basic service set (BSS), the client must progress through a sequence of states.
Each state refers to a policy that the client must meet before moving on to the next state. The dots represent the client’s status at each of the following crucial steps as it attempts to join the wireless network:
- Start: Client activity has just begun.
- Association: The client has requested 802.11 authentication and association with an AP.
- Authentication: The client must pass a Layer 2 Pre-Shared Key (PSK) or 802.1x authentication policy.
- DHCP: The WLC is waiting to learn the client’s IP address from a Dynamic Host Configuration Protocol (DHCP) server.
- Online: The client has passed Layer 2 and Layer 3 policies, successfully associated, and can pass traffic.
If a step was successful, a green dot is displayed. If not, a black dot appears.
A probing client always begins in the Start state and then moves into Layer 2 policy states and Layer 3 policy states as required. For example, if a client is attempting to associate with a WLAN that is configured for some form of 802.1x authentication, the client must pass through the Authen- tication state. If it successfully authenticates, it can move further down the list of states.
What is the significance of the “Connection Score” calculated by the WLC for a client? How is this calculated and interpreted?
The WLC calculates a connection score, which is a percentage value represented by the client’s current data rate divided by the lower maximum supported rate of either the client or the AP.
For the client shown, 130 Mbps divided by 144 Mbps (the maximum rate supported by the client—but not shown) is 0.903, or 90%.
With a high Connection Score percentage, we may assume that the client is enjoying good performance over the wireless connection.
