A blog with tips, tricks and tutorials to help you prepare your CCIE Wireless lab exam.

Tuesday, September 7, 2010

Workgroup Bridge (WGB) CLI commands

Workgroup bridges (WGB) are IOS access points configured as clients to the wireless infrastructure. They can associate to Aironet IOS APs or LWAPP/CAPWAP APs. They usually are used to relay traffic from non-802.11 clients, acting as a sort of common wireless NIC for those clients.

Beyond the simple WGB configuration, there are several "optional" parameters that you may need to configure.

- Preamble: 3 or 4 MAC addresses

To understand most of these settings, keep in mind some 802.11 basics:
 This is an 802.11 header. It has up to 4 MAC addresses. The logic is that if you send a frame from a wireless client to another wireless client via an AP, you need 3 MAC addresses: the source (the original emitter), the final destination, and the AP. Then we use the terms:

DA (destination address) for the final destination,
SA (source address) for the original source,
RA (receiver address) for the MAC address of the device supposed to receive that frame,
TA (transmitter address) for the MAC address of the device sending that frame.

So for example, if you send from a station A to a station C via an AP B, the frame goes from A to B (during which SA=TA=A, DA=C, RA=B, then from B to C (during which SA=A, DA=RA=C, but this time TA=B). You see that we do not use the fourth address, because it would always be A, B or C repeated. Instead, we just position the addresses where they make sense.
Address 1 is always the device receiving the frame, Address 2 is always the device transmitting the frame, and Address 3 gives additional information about who the original sender was or who the final destination is supposed to be.

The only case where you use 4 addresses is when you have 2 relays... for example a wireless bridge: you packet goes from station A, through AP B, is relayed to another AP C in bridge mode over a wireless link, then goes to your final client D. Between these 2 bridges, you need to tell:
Address 1 = RA = C
Address 2 = TA = B
Address 3 = DA = D
Address 4 = SA = A.
This is a case of "4 MAC address header". In all other cases, you use 3 MAC addresses and the fourth MAC address field is not used.

Why do we care for WGB? Because the big question is: is your WGB a normal client, just like a laptop, or is-it more a sort of infrastructure device?
It all depends of course on what type of non-802.11 clients (and how many) connect through that WGB, and what you try to achieve.

- Universal vs Cisco mode:

During the association request phase, the WGB signals to the AP that it is a special client through 2 special vendor specific tags by the end of the frame and IAPP (inter AP messaging). So the AP knows that this is not a normal client... and this is good! Because as soon as that WGB will complete the association phase, it will start behaving very strangely: imagine, it is going to relay traffic to and from several wired clients. This means that your WGB, MAC address aaaa.bbbb.cccc authenticates, negotiates the keys and everything, and as soon as you say "OK, client aaaa.bbbb.cccc, your are in my cell", you start seeing packets coming from other MAC addresses, 1111.2222.3333 for example, using the same keys as your aaaa.bbbb.cccc client... and you are supposed to find all this perfectly fine and normal! Cisco access points are perfectly fine with that, as long as those packets still have the special WGB tag indicating that it is all coming through the same WGB... but many other vendor APs turn to panic mode and block the traffic (actually exactly as they should, as this is not a normal behavior).

If you want your WGB to associate to a non-Cisco AP, then you may have to moderate its WGB erratic behavior. This quieter mode is called the universal mode (as opposed to the "normal" mode, which is the Cisco mode). In this mode, the AP only displays one single MAC address, exactly like a normal client. No IAPP in this mode, but you can still see the Cisco vendor specific information in the WGB messages, just in case the AP would be a Cisco. The downside of this mode of course is that you can only bridge one non-802.11 client, the one having the MAC address displayed by the WGB.

When you configure the WGB, you define the Cisco or universal mode from the radio configuration section. In the CLI, use:

interface d0 (or d1)
station-role workgroup-bridge
station-role workgroup-bridge universal .

The has to be the wired MAC of the non-802.11 client talking through the WGB. The WGB will become transparent, and the only MAC address you'll see in the wireless space will be the MAC of the wired client

Few things to keep in mind:
- This wired client must exist. If the WGB does not detect that MAC address on its Ethernet interface, it associates to the WLAN using its own MAC address, which means that you see in the wireless space the WGB BVI1 MAC address instead of the client behind.
- When the non-802.11 client appears on the wired interface, the WGB disassociates then re-associates using this time the wired client MAC address.
- Are you really limited to one device in universal mode? Well not really, you are limited to one MAC address in the wireless space. If that address is the MAC address of a router connected to the WGB, and if you have 10 devices behind that router, fine! Only 1 MAC address is seen and all is well.

- Infrastructure-client:

By default, WGB are normal clients, okay we saw that. You can configure the AP to see the WGB as an infrastructure client. This is done from the radio configuration section. In the Web interface, in the radio page, this is called "reliable multicast delivery to the WGB". In the CLI, use:

interface d0 (or d1)

What does-it do? Well, two things:
- the first one, which is the most publicised, is that if the WGB is seen as an infrastructure client (which is NOT the default), then multicast (and broadcast) packets to the WGB are acknowledged (by the WGB). The advantage is that if you have several clients behind the WGB, you send one broadcast (or one multicast), then a second copy of that broadcast/multicast encapsulated into a unicast frame to the WGB and the ACK from the WGB confirms that the frame was received and is going to be relayed to the wired clients. This increases reliability (usually, broadcasts and multicasts are never acknowledged).. The downside is that it also increases traffic on the radio, as packets that are usually not acknowledged are now acknowledged (and you send as many unicast copies as you have WGBs). So the limitation of this mode is that the AP will not allow more than 20 WGB clients.
- The second one is that if your WGB is an infrastructure client, it can associate to an infrastructure SSID. Infrastructure SSIDs are SSIDs created only to authenticate other APs (bridges, repeaters, etc.). A WGB in standard mode (Cisco or Universal) is by default a "client", not an "infrastructure client" and therefore cannot associate to an infrastructure SSID. Make your WGB an infrastructure client and voila, it can magically associate now to infrastructure SSIDs.

Last detail:

- From a pure wireless standpoint, a question I often get: who is acknowledging those frames, when you have several clients behind the WGB? One of the clients? The WGB itself? The answer is: no-one! An ACK frame does not contain any SA or TA, just a RA...

- Multicast-mode client / infrastructure:

This one is usually happily confused with the previous one, as infrastructure client talks about multicast. But they do not do the same thing.
When you configure your WGB in Cisco mode (so NOT in universal mode, which is incompatible with this feature... well, you can always try, but you won't make them work together), in infrastructure client or not, the WGB will by default use 3 MAC addresses, pretending to be whatever non-802.11 it is relaying traffic for. In the scheme shown at the beginning of this post, you will see those various clients as if they each were wireless clients. The WGB relays their frames, pretending to be each of them in turn.

One downside of course is that the WGB is just a client, and a relay. In a worst case scenario configuration, suppose your WGB has 8 wired clients, and is set as infrastructure client. Bad news, you have 15 other WGBs with the same number of clients in your cell, each with 8 clients, so 128 clients in total... one of them is a phone, that needs music on hold, sent as a multicast frame... you see where this is going? The AP sends one frame, all 8 WGBs receive it, all 16 WGBs acknowledge that frame (nice collisions in your cell!), and as it is a multicast frame, all 16 WGBs relay that frame to all your wired clients, 127 of which don't care... is that efficient? Not really.
Yeah, that's also what they thought at Cisco. Although the fact that the WGB acknowledges or not is based on its infrastructure-client configuration, you still have 127 clients getting a frame they don't care about in any case.
So they built the multicast infrastructure mode. In that mode, multicast and broadcasts sent to a WGB use 4 MAC addresses, instead of 3 MAC addresses when using the multicast client mode.

What difference does that make? Well, if you configure your WGBs as multicast infrastructure, this is what happens:
The AP sends one multicast frame. The DA is the multicast address, but this time Address 1 is RA, the WGB behind which the phone sits (Address 2 is the AP or TA, Address 3 is DA, the multicast address, and Address 4 is SA, the original sender of the multicast frame). All 16 WGB receive that frame (coz they have no choice), 15 of them drop it because RA is not their MAC address, and only one WGB takes the frame, acknowledges it or not depending on its infrastructure-client configuration, then relays it to its wired interface. 8 clients receive it, 7 that do not care, plus our phone.
This mode is more efficient... in that case.

You configure that mode from the CLI with the command:

interface d0 (or d1)
station-role workgroup-bridge multicast mode {infrastructure | client}

Notice, again, that this mode is not compatible with Universal. Using station-role workgroup-bridge  multicast mode blahblah meansh that you say "station-role workgroup-bridge", which is the default, and not "station-role workgroup bridge unversal ".
There is no mode configured by default, which means that the WGB would accept any frames, with 3 or 4 addresses. You can force the mode one way or the other (3 addresses or 4) depending on the special case you try to solve.

- Mobile-station

WGB are not good roamers. They connect to an AP and stay there until they disconnect. If you WGB is mobile (on a cart in an hospital for example), you may need it to roam better. You can imrpove its roaming behavior with the CLI global config command:


When you enable this setting, the workgroup bridge scans for a new parent association when the RSSI to its AP gets too poor or when it has to many retransmits. This makes that the WGB will roam when its signal to the AP degrades. When the mobile station setting is disabled (the default setting) the workgroup bridge does not search for a new AP until it loses its current association. 

Additional tricks:

These two are borderline I would say, in the sense that you would not use them everyday... but there are cases when they can be useful:

- Scanned channels:

Just like a normal client, the WGB will scan all channels to discover an AP with the right SSID. This may take time, and be a waste of time if you use the classical channels 1/6/11 scheme. In this case, you can configure your WGB to only scan the channels on which you know that you have APs, instead of scanning all channels. This is done at the radio level, with the CLI command:

int d0 (or d1)
mobile station scan 1 6 11

- CCX neighbors

This setting is related to the channel scan. When the WGB scans, it actualizes its list of available APs. This is a CCX mechanism by which the WGB can transmit to its AP the details of the others APs the WGB heard. When you statically configure which channels to scan, the logic is that you know on which channels you have APs, so you don't need the WGB to tell you. You can therefore disable the CCX neighbor list building with the CLI command:

int d0 (or d1) 
mobile station ignore neighbor-list 

Wowoo, that was a long post, sorry for that. I hope it clarifies things... comment if you have any addition or question!

Saturday, July 31, 2010

Autonomous APs: Network EAP vs. Open with EAP, the right combination

When configuring an SSID with EAP/802.1X on an autonomous AP, you are given the choice between Network EAP and Open with EAP (or both).

On the CLI, you would say:
dot11 ssid whatever
   authentication open eap eap_methods
   authentication network-eap eap_methods
As Cisco documentation (for example here) is... er... not completely clear (thanks Seth for pointing me to it!), if not completely wrong, here is a quick summary of which one to choose and when.

First, background information on why this is here (skip this part if you don't care about the whys and just care about the hows).
All this started at the time when we had "nothing (Authentication set to 0 in the AP beacons)" or WEP PSK (Authentication set to 1 in the AP beacons). WEP was weak, so everybody needed a replacement for it. Cisco implemented LEAP, which implies both au authenticaion mechainsm and some encryption. So Cisco set the Authentication value to 1 in the AP beacons using LEAP, not to indicate PSK but to indicate "authentication required" (and this is also why you cannot use LEAP with no encryption). But this does not really conform to the (future, when LEAP was created) 802.11i (and WPA) specifications, so this is Cisco specific...
Later on, when WPA and 802.11i appeared, the protocol detailed that, for compatibility with the 802.1X protocol, the authentication would occur at the association phase. In other words, with 802.1X you plug your PC to a switch, and it is only when you do that that authentication occurs. Similarly in the wireless space, you go through the 802.11 authentication phase (request/response) in an open manner, and it is only when you go to the association phase, which is the "hey, plug me to your cell" message, that the AP says "wait, I need to do 802.1X authentication first", and the EAP process starts. So, Authentication is set to 0 (for its 802.11 part), and EAP/802.1X starts at the association phase.
At the same time, in 2004, ASLEAP was released, and so was 802.11i, following WPA the year before. So when Cisco replaced LEAP with EAP-FAST, the information I have is that they conformed to the WPA/802.11i specifications, and Authentication is set to 0.

So, (whatever the documentation above says) Network EAP = LEAP. Open with EAP = Any other EAP. This is something you see when checking Network EAP: the popup window clearly states that if you use EAP-FAST or any other EAP (than LEAP), you should check Open with EAP. You can of course check both when you want to allow LEAP and another EAP, but you will not be able to authenticate using EAP-FAST if you choose Network EAP only...

There is one exception though... for long, LEAP used to be the default "secure" authentication method. This makes that some old Cisco clients (for example access points!) need to be offered LEAP to get started (or turned on, name it the way you like)!
In other words, if you build a wireless link between 2 APs, for a repeater, bridge or Workgroup Bridge configuration, and if you use 802.1X on that link, you need to offer LEAP (i.e Network EAP) for the secure authentication to be used. So you can offer Network EAP, or Network EAP and Open with EAP, but you should not offer just Open with EAP.
Which one is going to be used if you offer both? Well, it all depends on how you configure your client side (non root Bridge, Workgroup Bridge, etc). If you use the "old" EAP Client (optional feature), in the SSID page:

Which is in the CLI:
dot11 ssid whatever
   authentication client username jerome password 7 104D000A0618
This is going to use LEAP only.
If you want to use another EAP, for example EAP-FAST, you need to empty that EAP Client field (it cannot be used in combination with another method), then use the AP Authentication feature.

In this page, you define credentials and method. You can pick up several methods if you want.
Then from the SSID page, you can call these methods:

In the CLI, this is:
dot11 ssid whatever
   dot1x credentials jerome
   dot1x eap profile Myfast
eap profile Myfast
 method fast
dot1x credentials jerome
 username cisco
 password cisco
So you offer both LEAP and Open with EAP, and using the newer AP Authentication method allows you to to use the credentials you defined, and use the most secure method selected. In this example, we use EAP FAST only, so that's the one we'll use. Of course, the RADIUS to which you main AP points (local RADIUS on the main APor external RADIUS) needs to allow that method.
Careful when testing, it is only from the non-root AP / WGB /  repeater CLI that you will use, at authentication time, which method was used. The main AP CLI will just tell you "WPA", or "Open", etc., but not the details of the authentication method used.
If you offer just Open with EAP, as the AP expects LEAP (Network LEAP) among the possibilities, then the EAP part is discarded. Although your link may come up, if you look carefully at your non-root AP CLI, you will see that the authentication is going to be "Open" (NOT with EAP), so there is no real authentication there. As soon as you also offer Network EAP, the non-root AP is happy, being offered LEAP, btu also something stronger, and is going to use the stronger method (EAP-FAST in our example)>
Give it a try in your lab!

Friday, July 30, 2010

13 reasons to reboot your access points or your controller

Do they hire people fr Microsoft at Cisco? I don't know, but I know for sure that there are more and more features that require you to reboot your controller or your access points. It is quite difficult to keep track of them in the lab, as you do not want to waste time rebooting too often. On the other hand, if you do not reboot, some changes will not take effect, and you will not always be able to test those changes. So here is a list of the changes I found that require a reboot. If you find more, add them to the list!

Access Points: well, those are obvious.
  • AP mode: every time you change the AP mode (local to monitor for example), the AP reboots to reload the firmware matching its new mode. Good thing is the AP will popup to tell you it has to reboot.
  • Static IP: whenever you switch your AP IP address from static to DHCP or back, from the controller interface, a popup asks you to reboot the AP. Funny thing is that if you change the IP address from the AP CLI (using clear lwapp ap ip address, to remove a static IP address, or lwapp ap ip address a.b.c.d , you just need a shut/no shut.
  • Primary, etc.: if you change the AP primary/secondary/tertiary controller and want the AP to use this information, you need to wait for the AP to unicast a discovery message to that controller. Supposedly, this happens every 30 seconds, so whithin half a minute your AP should send an LWAPP keepalive message to that controller, then join it... if AP Fallback is set to Enable (Controller > General) on the current controller to which the AP is associated... well, you may want to reboot the AP to speed up the process and make the discovery more reliable, at least on code 4.2 (on 5.2 and up, seems to work a lot better).
  • Webauth WLAN: when you create your first WebAuth WLAN, you need to reboot your APs for the certificate to feature to be enabled on them. This does not seem to be consistent between code versions, on 4.2.130, failure to reboot the APs seems to cause clients to fail to get the certificate warning page, unless you refresh many times. Good practices say "reboot your APs".
  • Mesh AP and MAC filter list: no mesh in the lab yet... but for the sake of listing them all... if you have Mesh APs, and check or uncheck the MAC Filter List box in the Wireless > Mesh page (this parameter forces the AP to be authenticated through a MAC address list before being allowed to join the controller), all your mesh APs have to reboot to get applied this parameter. Supposedly they are supposed to reboot automatically... well, more than often, you may want to reboot those APs that forgot that they had to reboot automatically...

  • SNMP user/communities: on the 2106 and code 4.2.130 and 4.2.207 at least, the SNMP communities and v3 users seem to be loaded at boot time. You may discover this painfully when trying to add your WLC to you WCS and get "No response from device, check SNMP communities, version or network for issues". Reboot the controller, things should be better. An interesting detail is that this "feature" (that's how we also call the Blue Screen of Death, right?) seems to be platform and code dependant. So you may have or not have this issue... but keep it in mind if you have an SNMP issue with a newly added SNMPv1/2c community or v3 user...
  • Virtual Gateway interface: whenever you make any change to the Virtual Gateway Interface (changing its address or associated DNS name), you need to reboot the controller. The Virtual Gateway parameters are loaded at boot time and not refreshed (cannot change dynamically) during the active life of you controller.
  • Certificates: whenever you change a certificate on the controller, you need to reboot the controller for this certificate to be loaded in RAM and used. This is true for any certificate (https access to your controller, WebAuth WLAN certificate, local certificate for EAP/802.1X authentication, etc.).
  • LAG: if you set LAG to enable or disable, the change only happens next time you reboot.
  • NTP server: controllers query NTP servers at boot time, and at intervals defined by the CLI command config time ntp (or on the Controller > NTP page, Polling Interval). The default interval is 24 hours (86400 seconds), a bit too long a wait most of the time. A quick reboot forces the controller to check the NTP server (I am of course assuming a lab environment, not a production environment).
  • Max user database: when you change the maximum number of users in the database (from Security > General), this parameter will only be applied upon next reboot.
  • Symmetric Mobility Tunnelling: when you enable or disable this feature (from Controller > Mobility Management > Mobility Anchor Config), you need to reboot the controller for this feature to take effect.
  • Code and Config download: this is obvious, but whenever you download a new code to the controller or a previously saved config, you need to reboot the controller for those to be applied.
Don't reboot:
  • Short/long preambles? Cisco documentation states that you need to reboot the controller if you enable or disable short preambles (in Wireless > 802.11b/g/n > Network). This is a myth (or a leftover from ooooold times). When you change this parameter, you clearly see in the AP beacons capability field the Short Preamble Allowed parameter switching from 1 to 0, and the "long -preamble only" stations failing or succeeding to associate accordingly (tried on an old handheld scanner).
So, in the lab, should you reboot every time you change one of those, or reboot once sometime during the day (for example at lunch break)? I am not sure there is an absolute rule. I find many people saying "reboot once, this will save you time". I like to multi-task. So I would rather reboot when I change one of these parameters, do another task then come back to finish this one. This will ensure that I can test the feature if needed, and that I do not leave too many things to verify when coming back from lunch...

Wednesday, July 21, 2010

7921: can you do WPA2?

You may have read different stories about the 7921 phone and its support for "strong encryption", that is AES-CCMP in a WPA2 logic. This is what the issue is:
The 7921 phone, until firmware 1.3(4), does support CCKM, WPA and WPA2, but not in any combination of those!
- WPA (TKIP) and CCKM work fine
- WPA (AES) and CCKM work fine, but this is bad practice (so that's a no no, unless you are specifically asked for this awful combination, coz after all, it's a lab, not real life)
- WPA2 (TKIP) and CCKM work fine, but this too is ugly, so another no no, unless they really insist.
- WPA2 (AES) and CCKM do NOT work together. What happens is that the phone does not take the roaming process well for the AES encrypted keys, and forces a full reauthentication.
In other words, you usually want to enable CCKM with Voice, because CCKM caches the key and allows for faster roaming (which is good for voice). But this does not work with the 7921 and a firmware older than 1.3(4).

So what should you do? Use WPA (TKIP) and CCKM, or WPA2 (AES) without CCKM? Well, all pushes you to use WPA (TKIP) with CCKM. The reason is that there is no reasonably usable  attack against WPA/TKIP encryption (provided you use 802.1X authentication, and not PSK), so TKIP is rather secure, and CCKM is definitely a plus for voice.

But all this is is you use a firmware older than 1.3(4). On 1.3(4) and later, you would prefer WPA2(AES) with CCKM, which is a supported combination on 1.3(4) and later. This firmware was released in its stable form at the end of May 2010. The CCIE W lab started a year before. So unless the hardware was updated in the lab (the lab blueprint does not specify a7921 specific firmware version), you will get firmware 1.2.1 and will have to go for the WPA/CCKM combination, if you have voice, 7921s and a requirement for strong encryption... but it may be worth checking!
On your phone, press the down arrow to access the tool box, then go to "6, status", then "4.Firmware version" to check the App Load ID, which will display the firmware with a value in the form CP7921G-1.2.1.LOADS.

Thursday, July 1, 2010

CCIE Wireless lab: more seats to come

If you ever tried to book a seat for the CCIEW lab, you probably were shocked to see that the waiting list was at best 3 months, and in many cases 6 months.
So far, there are 3 racks for the CCIE W lab: 2 in San Jose, 1 in Brussels. You can take the lab from 3 locations: San Jose (on San Jose pod 1 or 2, but only one pod is used at a time for San Jose candidates), Brussels (on Brussels pod), and Sydney (using one of the San Jose pods).
The lab team is adding a third physical rack in San Jose, that will be dedicated to RTP candidates. So, from August 1st, you will be able to take the lab from RTP (for those who may not know, RTP is in North Carolina, USA), having a local AP and VoWLAN phone in RTP, and the rest of the pod as rack 3 in San Jose. This is great news, because it will not only add new seats on the East coast, but also partially unload San Jose...
This is huge effort for the CCIE W lab team. Although it looks very simple from outside ("hey, you guys are Cisco, just get to the warehouse, pick up hardware, and set up a new rack!"), it is in fact far more complex. Just like for you and me, getting a lab gear is an investment on the team budget, that has to be justified in terms of utilization... and as you know, each pod is expensive (yeah, of course they have a discount, but it still makes each pod a serious investment), so ROI on such an investment is probably not obvious for most managers, including at Cisco. So they probably have been struggling to get that new pod going so that us, candidates, get more seats. Thanks guys! This will make our attempts easier and the waiting time between attempts shorter.

Tuesday, January 19, 2010

Coverage hole algorithm thresholds

You know the coverage hole algorithm, triggered when some clients get below a certain SNR, and configured here:

 Now here is a question: I have wireless clients that do not react well when their AP signal gets too low... could you configure the Coverage Hole algorithm to be triggered if any client signal on AP1 gets below 19 dBm SNR? Oh, AP1 power level is statically set to 2  (and level 2, as you could see if you ran the show run-config command on my EUROPEAN controller, is 14 dBm).
Can you find what the threshold should be?

Ok, here is the solution. First, Coverage Minimum Exception Level is to be set to 1 (as the algorithm as to be triggered if "any client"...).
The harder part is the Coverage value. The client SNR cutoff triggering the coverage hole algorithm is calculated as follows:
Client SNR Cutoff Value (|dB|) = [AP Transmit Power (dBm) – Constant (17 dBm) – Coverage Profile (dB)]
My AP power level is set to 14 dBm, and as you can see on the above picture, the default Coverage Profile value is set to 12. So we have:
Client SNR Cutoff Value (|dB|) = 14 -17 -12 = -15
Although this number is negative, we are looking at an SNR cutoff of 15 dBm.
You want 19 instead (so result must be -19)?
The only value we can change is the Coverage Profile, setting it to 16 solves the problem...
Easy he? Yeah, I know...
In code 5.2, it gets a little better, you can also set RSSI values etc...

Wednesday, January 6, 2010

Do you need DHCP Option 60?

When configuring a DHCP scope for your LWAPP access points, you can return, along with an IP address, the controller Management IP address. This is called option 43, Vendor Specific Option. It is vendor specific in the sense that, in our case, only an access point would know what to do with Controller Management IP address information.
If you have read older papers, you may also have seen the option 60, Vendor Class Identifier. Do you need it, or not?
This is what it is about: when you AP boots, it sends a DHCP discover message. This message contains, among other things, a section that identifies what kind of device is requesting that IP address:

As you can see from this packet capture, in this case the AP says "I am a Cisco AP c1250. Can I get an IP address please?"
When you define an option 60 in your DHCP scope in combination with the option 43, you say "IF client is a Cisco AP c1250, then return the Vendor Specific Option defined in option 43". In other words, the content of option 43 is returned only to those clients that present the right option 60.

When using Windows DHCP server, you do not have much of a choice. The right process is to
- first define the Vendor Class Identifier
- then for this vendor class identifier define what kind of Vendor Specific Option you want to return. In our case, the option is always "option 241: controller Management IP Address" for Aironet APs and "option 102 (also controller Management IP address" for 1000 series Airespace APs
- Last, define the value of this option 241 or 102 (that is, the controller Management Interface IP address you want to send back).
So, with Windows DHCP server, you use option 60 without any real good way to avoid it.

When using an IOS DHCP scope, you do not need to specify an option 60. You can specify only the option 43, which will be option 102 if returned in ASCII and option 241 if returned in Hex.
If you specify an option 60 for that scope, the content of option 43 is returned only to those clients that present the right option 60. If you do not specify an option 60 for that scope, the content of option 43 is returned to any DHCP client asking for an IP address in that subnet.

What is the recommended practice? Or: why do some papers describing option 43 for the IOS mention the option 60, while some (many) others don't?

Well, the answer is quite simple: in the lab, I would try to put my option 60 whenever I can...
In real life, well, option 60 is cleaner: you do not send the content of option 43 to clients that do not need it. On the other hand, you should have a infrastructure addressing space, and a client addressing space. This means that your APs should be in one subnet/VLAN (the AP VLAN on that switch), and clients should be in other subnets. This makes that only APs should be in the subnet for which you provide this option 43. So there should NOT be any client NOT needing this option 43 and still getting an IP address in that scope: all clients of that scope are supposed to be APs! This is why you find many papers without this option... not to mention the fact that this option may be cumbersome to use when you have different APs of different models in the same subnet... but the lab is of course not about what is nice and simple, but what can technically be done, right?