CCNP Switch Chapter 15
What is an intracontroller roam?
A client moves around the office and changes APs, but the same controller provides the association and the LWAPP or CAPWAP tunnel. This type of roam is straightforward because controller WLC1 simply updates its tables to begin using the LWAPP or CAPWAP tunnel to AP2 to find the client. Any leftover data that was buffered from the old association is easily shifted over to the new association within the controller.
What is a BSS and what does it do?
An 802.11 Basic Service Set (BSS) centralizes access and control over a group of wireless devices by placing an access point (AP) as the hub of the service set.
What is an autonomous mode AP?
An AP that is configured individually... one that handles its own use of RF channels and is standalone
What is an AP's primary function?
An AP's primary function is to bridge wireless data from the air to a normal wired net- work.
How many WLCs can an LAP be joined or bound to at a time?
An LAP is always joined or bound to one WLC at any time. However, the LAP can maintain a list of up to three WLCs (primary, secondary, and tertiary). As the LAP boots, it tries to contact each WLC address in sequential order. If it cannot find a responding WLC at all, the LAP tries an IP subnet broadcast to find any available WLC.
How can a wireless station find out if its frame was received correctly?
An acknowledgement has to be sent back by the recipient confirming that the frame was sent error-free
What is an ad hoc wireless network (or IBSS)?
An ad hoc wireless network, or Independent Basic Service Set (IBSS), is a system in which two or more wireless clients to communicate directly with each other, with no other means of network connectivity. It's a part of 802.11.
What is association?
Association is membership with an AP. A client must send an association request message, and the AP grants or denies the request by sending an association reply message. When associated, all communications to and from the client must pass through the AP
What does the WLC "Flexible client roaming" function mean to clients?
Clients can roam at either Layer 2 or Layer 3 with very fast roaming times.
In addition to the DIFS period of waiting between transmissions, what other wait timer must a station abide by?
Every station must implement a random back off timer. Before transmitting a frame, a station must select a random number of timeslots to wait. This number lies between zero and a maximum contention window value. The idea here is that stations ready to transmit will each wait a random amount of time, minimizing the number of stations that will try to transmit immediately.
What is an ESS?
Extended Service Set (ESS) is created when geographically-separated APs are interconnected by a switched infrastructure
What does the WLC "Self-healing wireless coverage" function mean it does?
If an LAP radio dies, the coverage hole is "healed" by turning up the transmit power of surrounding LAPs automatically.
What does the WLC "Dynamic client load balancing" function mean to clients?
If two or more LAPs are positioned to cover the same geographic area, the WLC can associate clients with the least used LAP. This distributes the client load across the LAPs.
With what type(s) of wireless infrastructure is roaming possible (BSS, IBSS, ESS)?
Roaming is only possible with Extended Service Sets (ESS)
What is roaming?
Roaming occurs when a client is passed from one AP to another as its location changes.
What is the best approach to designing an AP's location and range or coverage area?
Perform a site survey! Duh...
What is difficult about having a bunch of autonomous APs?
- managing security, because each AP has its own security policies, and there is no central point of entry between the wireless and wired networks... in this case you can't have one IDS or IPS in its way - managing RF operation... you have to separately configure channels and power levels so the signals don't overlap - it's difficult to prevent coverage holes if an AP's radio fails
What is an SSID?
Service Set Identifier (SSID): a text string included in every frame sent. If the SSIDs match across the sender and receiver, the two devices can communicate.
What does an intercontroller roam look like when the WLCs are on different subnets?
The two controllers bring up an Ether-IP tunnel between them to carry some of the client's traffic. The Ether-IP tunnel encapsulates MAC-layer data inside an IP packet (port 97) and sends them to the other controller. Packets received over the tunnel are unencapsulated by the other controller, where they reappear in their original form.
What must a station do if it waits until its random backoff timer expires and is ready to transmit, only to find that someone else is already transmitting?
The waiting station must now wait the duration of the newly transmitted frame, followed by the DIFS time, and then the random backoff time.
Why can't wireless LANs use CSMA/CD like Ethernet links?
There is no clear-cut way to determine whether a collision has occurred. Even the transmitting stations won't realize it because their receivers must be turned off while they are transmitting
Do LAPs and their WLCs have to be located on the same VLAN or IP subnet?
They can be, but they don't have to
What must clients do with an AP to make sure that their WLAN communication operations are tightly coupled?
They must perform a handshake. Handshaking removes the possibility of one-way communication, where the client can hear the AP but the AP can't hear the client.
How do LAPs and WLCs authenticate to each other?
They use X.509 digital certificates that are preinstalled in each device when it is purchased. By using certificates behind the scenes, every device is properly authenticated before becoming part of the Cisco Unified Wireless Network. This process helps ensure that no rogue LAP or WLC (or devices posing as an LAP or WLC) can be introduced into the network.
What does the duration value accomplish when it is placed inside a wireless frame?
Transmitting stations can provide an estimate of the amount of time needed to send a frame by including a duration value within the 802.11 header. The duration contains the number of timeslots (typically in microseconds) needed for the size of frame being sent. Other wireless stations must look at the duration value and wait that length of time before considering their own transmissions.
Why is it important that client association requests are sent by the LAP up to the WLC?
Two reasons: - All client associations can be managed in a central location. - Client roaming becomes faster and easier; associations can be maintained or handed off at the controller level. Through the WLCs, LAPs can support both Layer 2 and Layer 3 roaming. The client's IP address can remain the same while roaming, no matter which tunnel the client passes through to reach the controllers. Also any buffered data from the client does not need to be passed along when the AP changes.
How do you get your LAP to find the WLC IP address to use during its bootstrap process?
Use either of these methods: - A DHCP server that adds option 43 to its reply containing a list of WLC addresses. - With the IP subnet broadcast option, the LAP broadcasts a join request message, hoping that a WLC is also connected to the local subnet or VLAN. This method works only if the LAP and WLC are Layer 2-adjacent.
What does an LAP do if its WLC fails for some reason?
When the LAP realizes its WLC is no longer responding, it reboots and begins the process of searching for live WLCs again. This means any client associations will be dropped while the LAP reboots and joins a different controller.
What does an intercontroller roam look like when the WLCs are on the same subnet?
When the client moves into AP2's cell, the same SSID is found, and the client can move its association to WLC2 (no IP change). The WLCs can easily hand off the client's association through a mobility message exchange where information about the client is transferred from one WLC to the other
On a shared Ethernet segment, what duplex mode do PCs communicate with?
half-duplex
On a shared Wireless LAN, what duplex mode do PCs communicate with?
half-duplex... why? because WLANs have to transmit and receive using the same frequency
In a split-MAC architecture, or Cisco Unified Wireless Network, where are the management functions performed? What are those management functions?
on the Wireless LAN Controller (WLC) Functions: - RF Management - Association and Roaming Management - Client Authentication - Security Management - QoS
Is data encryption on 802.11 handled by stream or per packet?
per packet
What are the two types of messages LAPs and WLCs use?
- Control messages: Exchanges that are used to configure the LAP and manage its operation. The control messages are authenticated and encrypted so that the LAP is securely controlled by only the WLC. - Data: Packets to and from wireless clients associated with the LAP. The data is encapsulated within the LWAPP or CAPWAP protocol but is not encrypted or otherwise secured between the LAP and WLC.
What are the functions of a WLC?
- Dynamic channel assignment - Transmit power optimization - Self-healing wireless coverage - Flexible client roaming - Dynamic client load balancing - RF monitoring - Security management
What ports do LWAPP and CAPWAP use?
- LWAPP uses UDP destination ports 12222 and 12223 on the WLC end - CAPWAP uses UDP destination ports 5246 and 5247
What are the two tunneling protocols that a LAP and WLC can use to communicate between each other?
- Lightweight Access Point Protocol (LWAPP) (Cisco) - Control and Provisioning Wireless Access Points protocol (CAPWAP)
When a station is ready to send a frame, what are the two conditions it can be in? What can the station do in each of those two conditions?
- No other device is transmitting: The station can transmit its frame immediately. - Another device is already transmitting a frame: The station must wait until the frame in progress has completed; then it must wait a random amount of time before transmitting its own frame.
What are the potential criteria an AP can require before allowing a client membership to the BSS?
- a matching SSID - a compatible wireless data rate - authentication credentials
What IEEE standard is wireless?
802.11
What IEEE standard is Ethernet?
802.3
How do you perform a site survey and what is the point of it?
A test AP is placed in a desirable spot while a test client moves about, taking live measurements of the signal strength and quality. The idea is to plot the AP's range using the actual environment into which it will be placed, with the actual obstacles that might interfere with the client's operation.
What layer 2 protocol does wired transmission use to transmit and receive frames?
Carrier Sense Multiple Access/Collection Detect (CSMA/CD)
So what do wireless LANs use as a layer 2 transmission and receipt protocol?
Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA)
Given how tightly-coupled these LAPs and WLCs are, it could be pretty damaging if the WLC is cut off from the LAP over a WAN link... How does Cisco handle situations like this?
Cisco Hybrid Remote Edge Access Point (HREAP) is a special case for remote sites where the LAPs are separated from the WLC by a WAN link. With HREAP, the remote LAPs can keep operating even while the WAN link is down and their WLC is not available, much like an autonomous AP would do. This allows wireless users to keep communicating within the remote site until the link (and WLC) is restored.
What is DIFS?
DCF Interframe Space (DIFS): The short amount of time that 802.11 standards require all stations to wait before transmitting
What is the DCF?
Distributed Coordination Function (DCF) is the process used to stagger frame transmission on a wireless network so that CSMA/CA can occur correctly
What does an LAP "binding" to a WLC mean?
It means that the two devices bring up a tunnel between them to carry 802.11-related messages and also client data.
What must a wireless client attempting to use the network do first before it sends frames to a recipient in a BSS?
It must first arrange a membership with the Access Point (AP)
After an intercontroller roam from a WLC in one subnet to a WLC in another subnet, where does the client's traffic go?
It starts by going to the anchor WLC (because that's the subnet the client's IP is in) and then it's forwarded through the tunnel to the foreign (closest) WLC.
If a client maintains its same IP address as it roams between APs, what process is it undergoing?
Layer 2 roaming
If a client roams between APs located in different IP subnets, what process is it undergoing?
Layer 3 roaming
Why would you want multiple WLCs, and why would you want a WCS to use with them?
Multiple WLCs offer some redundancy so that LAPs can recover from a WLC failure. Managing several WLCs can require a significant effort, due to the number of LAPs and clients to be managed and monitored. The Cisco Wireless Control System (WCS) is an optional server platform that can be used as a single GUI front-end to all the WLCs in a network. From the WCS, you can perform any WLAN management or configuration task, as well as RF planning and wireless user tracking.
What would the network diagram look like if an AP was mapping VLANs to SSIDs?
NOTE: You've got to have a trunk link going to the switch to transport both VLANs (if you're not using a WLC)
What if a client moves between mobility groups? Can the IP address stay the same?
No it's going to get a new IP and new session information, but the association can be transferred to the new mobility group.
Can a LAP boot up and support wireless clients without a WLC?
No, it needs the WLC for these functions
Is an AP a passive device like an Ethernet hub?
No. An AP manages its wireless network, advertises its own existence so that clients can associate, and controls the communication process. For example, recall that every data frame sent successfully (without a collision) over a wireless medium must be acknowledged. The AP is responsible for sending the acknowledgment frames back to the sending stations.
Do LAPs need trunk links to their upstream switches?
No. They use only one VLAN to connect the LAPs to the WLC. There are still individual VLANs for each SSID, but those VLANs are sent through the LWAPP or CAPWAP tunnel... so you're basically hijacking the administrative VLAN to transport your other VLANs
Within a BSS, can two clients still communicate with each other directly?
No. Within a BSS, all traffic has to first go through the AP
If wireless encryption is used, is the traffic encrypted all the way to the WLC?
Not necessarily. Packets are encrypted as they leave the wireless client and unencrypted when they arrive on the LAP. The same is true for packet authentication, if it is used. So the encrypted data does not pass through the LWAPP or CAPWAP tunnel at all.
What do you need to feed your WCS in order for it to triangulate the location of a client? What other food helps?
The WCS uses building floor plans to display dynamic representations of wireless coverage. It can also be fed information about the building construction to improve its concept of RF signal propagation. Once this is done, the WCS can locate a wireless client to within a few meters by triangulating the client's signal as received by multiple LAPs.
What does the WLC "Security management" function mean it does?
The WLC can require wireless clients to obtain an IP address from a trusted DHCP server before allowing them to associate and access the WLAN.
What does the WLC "Dynamic channel assignment" function mean it does?
The WLC chooses and configures the RF channel used by each LAP based on other active access points in the area.
What is an "anchor" WLC?
The WLC in an intercontroller roam situation that is linked to the client by similar subnet
What does the WLC "RF monitoring" function mean it does?
The WLC manages each LAP so that it scans channels to monitor the RF usage. By listening to a channel, the WLC can remotely gather information about RF interference, noise, signals from surrounding LAPs, and signals from rogue APs or ad-hoc clients.
What does the WLC "Transmit power optimization" function mean it does?
The WLC sets the transmit power of each LAP based on the coverage area needed. Transmit power is also automatically adjusted periodically.
If you have 802.11 RFID tags on the objects in your organization, what can you do with a WCS and a Cisco Wireless Location Appliance?
With 802.11 RFID tags, you can track objects as they move around in the wireless coverage area. Tracking objects by their MAC addresses can be handy when you need to locate a rogue or malicious wireless client, or when you need to track corporate assets that tend to move around within a building or complex.
Can an AP control how many clients it is associated with at a time?
Yes
Can PoE be used with LAPs?
Yes it can!
Can an AP support multiple SSIDs?
Yes it can, as long as multiple VLANs are extended to it over a trunk link (for autonomous APs)... see this pic
Does traffic between two wireless clients have to go through a WLC?
Yes that traffic must go from Client A through the LAP, through the LWAPP or CAPWAP tunnel, into the WLC, back through the tunnel, through the LAP and on to Client B.
Can you enable PortFast with LAPs? If so, where?
Yes you can! You can enable spanning-tree portfast on the access mode ports where LAPs connect. The LAP VLAN terminates on the LAPs and does not get extended any further. Therefore, no danger exists of that VLAN forming a loop somewhere in the wireless cloud.
Is it possible that stations will still transmit at the same time regardless of 802.11's attempts to prevent collisions?
Yes, because the backoff timer is random, a chance still exists that two or more stations will choose the same value. Nothing else will prevent these stations from transmitting at the same time and causing a collision. This will simply be seen as an error over the wireless network; no acknowledgments will be returned, and the stations will have to reconsider sending their frames again.
Is long-distance communication between APs possible without wires?
Yes, it's called either AP-to-AP (line-of-sight) or mesh. AP-to-AP is used for inter-building communication when there is a line of sight. Mesh is used for metropolitan environments where multiple APs (interconnected wirelessly) along the way provide access to clients.
If you're setting up a bunch of APs to use in an ESS, should their cells overlap?
Yes, the cells should overlap by a small percentage. Keep in mind that when AP cells overlap, adjacent APs cannot use identical frequencies. If two neighboring APs did use the same frequency, they would only interfere with each other.
If your PC is not associated with the AP, can you still listen and receive frames going over the air?
Yes. Regardless of the association status, any PC is capable of listening to or receiving the frames that are sent over a wireless medium. Frames are freely available over the air to anyone who is within range to receive them.
Say you've got a large coverage area and you're trying to reduce the number of APs you have to buy... what can you do, and what do you have to keep in mind before doing it?
You can always run an AP at its maximum transmit power to make the most of its range. Doing this would reduce the number of APs you need. However, when an AP is configured to provide a large coverage area, it also opens the potential for overcrowding. Remember that an AP cell is essentially a half-duplex shared medium that all clients must share. As the number of clients goes up, the amount of available bandwidth and airtime goes down.
In what way is an AP like a bridge?
You can think of an AP as a translational bridge, where frames from two dissimilar media are translated and then bridged at Layer 2. In simple terms, the AP is in charge of mapping a VLAN to an SSID.
How do you get SSID VLANs to communicate with each other in a Cisco Unified Wireless Network architecture?
You have to connect a layer 3 switch/router to the WLC
What is a LAP?
a Lightweight Access Point (LAP) performs only real-time layer 1 and 2 802.11 operations like the following: - RF transmission and receipt - MAC management - Encryption The LAP gets its name because the code image and the local intelligence are stripped down, or lightweight, compared to the traditional autonomous AP.
What is a cell?
a cell is an AP's coverage area
What is a picocell?
a cell that has had its size even more reduced than a microcell... picocells are normally used in extremely controlled environments like stock exchanges
What is a microcell?
a cell that has had its size reduced
In an ESS environment, how many APs is a client connected to at any one time?
a client is connected to just one AP at a time. This minimizes the chance that any data being sent or received while roaming is lost.
What is a logical mobility group?
a group of WLCs that are linked together to allow clients to roam between them. you can have up to 24 WLCs in a mobility group
What is Cisco Unified Wireless Network?
an architecture that offers the following centralized capabilities so that all wireless LAN devices are connected anywhere in the network: - WLAN security - WLAN deployment - WLAN management - WLAN control It creates a split-MAC architecture so some functions are controlled by the AP and some are controlled by a Wireless LAN Controller (WLC)
What is a service set?
any group of wireless devices
What layer should APs be located?
the access layer
What layer should you place WLCs?
the distribution layer
What is a supplicant?
the software that uses a wireless network adapter to interact with the wireless protocols
Do collisions still occur with wireless LANs?
yes
