I recently had to install a Cisco Wireless LAN Controller (2112, if you’re interested), and had the usual fun and games with getting it to properly understand DHCP Option 43. For the uninitiated, option 43 is a vendor specific option, which, in the case of Cisco WLCs, is/are the manager IP address(es) of controllers that LWAPP access points should attempt to join when they boot up.

Different model APs require this option to be in different formats. For example, Aironet 1000 units require the option response to be type 0×66, and a comma-separated ASCII list of controller IP addresses, whereas Aironet 1130, 1200, 1230 and 1240 units require the response to be type 0xF1, followed by the length (number of addresses x four), then the hexadecimal representation of the controller IP address(es).

Cisco documentation exists for this, however their documentation for ISC’s dhcpd is incorrect. Unlike most corporate customers I run into, who run Microsoft DHCP server (for better or worse), this particular customer was running ISC’s dhcpd.

The first step is setting the option 43 type. I’m going to concentrate on the 1130, 1200, 1230 and 1240 units here, since this is the area where Cisco’s documentation is incorrect. I’m going to follow Cisco’s documentation here.

option space LWAPP;
option LWAPP.controller code 43 = string;

Then we have a vendor class, for the 1200 series units:

class "Cisco AP c1200" {
  match if option vendor−class−identifier = "Cisco AP c1200";
  option vendor−class−identifier "Cisco AP c1200";
  vendor−option−space LWAPP;
  option LWAPP.controller f1:04:c0:a8:f7:05;
}

Note the “f1:04″ at the start of the string. This means type 0xF1, followed by four bytes of vendor specific data. The “c0:a8:f7:05″ is the hexadecimal representation of the IP address 192.168.247.5. This results in dhcpd transmitting “2b 08 2b 06 f1 04 c0 a8 f7 05″ for option 43.

Ok, let’s take a look at this string. The “2b” indicates this is a vendor encapsulated options field (type 43), and “08″ means it’s eight bytes long. The next “2b” is where things start to go wrong. This is because the Cisco documentation told us to define LWAPP.controller as type 43 also, which is incorrect. The “06″ indicates that six bytes follow for this sub-code, and then we have our “f1 04 c0 a8 f7 05″ string verbatim. This causes the WLC to report an error parsing the option 43, saying that it cannot parse “2b 06 f1 04 c0 a8 f7 05″.

What we should have configured in dhcpd.conf is actually:

option space LWAPP;
option LWAPP.controller code 241 = string;


class "Cisco AP c1200" {
  match if option vendor−class−identifier = "Cisco AP c1200";
  option vendor−class−identifier "Cisco AP c1200";
  vendor−option−space LWAPP;
  option LWAPP.controller c0:a8:f7:05;
}

Note that we also dropped the “f1:04″ from the hex string, since we are now correctly specifying LWAPP.controller as code 241 (0xF1), and dhcpd automatically populates the “04″ for us, after counting the length of our hex string (four bytes = one IP address). This results in dhcpd sending “2b 06 f1 04 c0 a8 f7 05″.

Again we have our “2b”, indicating vendor encapsulated options, but this time the field is only six bytes long. Then we have “f1 04″, indicating our LWAPP.controller code, with four bytes of data – our controller IP address. This time around, the AP will correctly see the option 43 “payload” of just “f1 04 c0 a8 f7 05″, and correctly parse the sub-option 0xF1.

Of course, what this field really is (and this is more clearly detailed in Cisco’s instructions for configuring Microsoft DHCP server), is an array of IP addresses. You can eliminate the need to specify the addresses in hexadecimal by defining the LWAPP.controller as:

option LWAPP.controller code 241 = array of ip-address;

and then simply listing your controller IP addresses:

option LWAPP.controller 192.168.247.5, 192.168.247.6;

This would result in dhcp server sending “2b 0a f1 08 c0 a8 f7 05 c0 a8 f7 06″. Note the “f1 04″ changed to “f1 08″, since the array length is now eight bytes (two IP addresses).

Why Cisco didn’t simply publish this, is beyond me. They’ve made it very confusing for users who don’t understand DHCP vendor specific information. I suspect the person who wrote the dhcpd section of the Cisco documentation didn’t fully understand how ISC dhcpd handles vendor specific options.

In any case, our configuration can be made somewhat clearer, and consistent with dhcpd’s documentation, as follows:

option space LWAPP;
option LWAPP.controller code 241 = array of ip-address;

class "LWAPP" {
  match option vendor-class-identifier;
}

subclass "LWAPP" "Cisco AP c1200" {
  vendor-option-space LWAPP;
  option LWAPP.controller 192.168.247.5;
}

For each additional type of AP you have to support, just add another subclass, using the appropriate vendor class identifier string.

Have you ever needed to set up a bunch of equipment on a boardroom table or some other temporary location, and needed both native and 802.1q tagged VLANs, but only had one available switchport?

A quick n’ dirty solution is to use an unmanaged switch, such as one of the numerous 8-port desktop switches from manufacturers such as D-Link, Netgear, Linksys etc. Configure its upstream switchport as a trunk port, thus allowing your required VLANs to pass tagged frames to your unmanaged desktop switch.

Wait a second, you say…. unmanaged switches can’t do trunk ports. How can an unmanaged switch understand VLAN frames?

It doesn’t need to. What is an 802.1q tagged frame, other than a standard 802.3 ethernet frame with four additional bytes inserted? These four additional bytes are the 802.1q VLAN ID field and 802.1p CoS field. As long as the unmanaged switch does not truncate frames to the 802.3 standard 1518 bytes, it will happily forward the 1522-byte 802.1q tagged frames just like any other. The last time I encountered a switch that would not forward these slightly “oversized” frames, was about four years ago… and it was a very cheap and nasty brand (name withheld to protect the innocent guilty).

This trick also comes in handy when you have a user with a two-port VoIP phone (such as most Cisco, Snom, Polycom etc phones), using a voice-VLAN, and the user requires more switchports than are currently available at his/her desk. Simply connect the 8-port unmanaged switch before the IP phone (ie. to the upstream port), and connect the IP phone to the unmanaged switch. The phone still gets it’s tagged voice-VLAN frames, the PC gets its untagged data-VLAN frames (tag-stripped if necessary by the IP phone), and the user has 6 other ports available to connect whatever… including, if necessary, other VLANs (so long as they’re tagged, and the end device can work with tagged frames, since the unmanaged switch won’t strip the 802.1q tag).

Beware though, this should only ever be used as a temporary measure, since it does open a few security holes. If the “allowed VLANs” is not carefully configured on the upstream port, the opportunity exists to VLAN-hop, or flood traffic into other VLANs. And of course, since the unmanaged switch is, well, unmanaged, there is no individual “allowed VLANs” security on those 8 ports. All ports are effectively the same as that one upstream trunk port.

Have you used this method before? What brand/model unmanaged switch did you use? What were your experiences with it, and did you encounter any problems?