Which DHCP options do you configure most, and what are options 43, 66 and 150 for?
Show the answer
Say this first
The everyday set is option 1 subnet mask, 3 default gateway, 6 DNS servers, 15 domain name and 51 lease time. For devices that boot from the network, 66 is the TFTP server name and 67 the boot file (PXE and many IP phones), 150 is Cisco's list of TFTP server addresses for Cisco IP phones, and 43 is vendor-specific data, which Cisco access points use to learn their wireless controller addresses. Option 82 is added by relays and switches, not set in a pool.
Go deeper
The options worth knowing by number
| Option | Name | Notes |
|---|---|---|
| 1 | Subnet mask | |
| 3 | Router | default gateway list |
| 6 | DNS servers | list of addresses |
| 15 | Domain name | DNS suffix for the client |
| 42 | NTP servers | |
| 43 | Vendor-specific information | format defined by the vendor |
| 50 | Requested IP address | client to server |
| 51 | Lease time | seconds on the wire |
| 53 | Message type | DISCOVER, OFFER and so on |
| 54 | Server identifier | which server owns the lease |
| 60 | Vendor class identifier | the client says what kind of device it is |
| 61 | Client identifier | IOS bindings show this, for example 01 plus the MAC |
| 66 | TFTP server name | one name or address, as a string |
| 67 | Boot file name | PXE |
| 82 | Relay agent information | inserted by relays or snooping switches |
| 121 | Classless static routes | |
| 150 | TFTP server addresses | Cisco, list of IPs |
Option 43 for Cisco APs
A Cisco lightweight AP expects a type-length-value: type f1, length 4 bytes per controller, then
each controller address in hex. Two controllers at 10.0.50.10 and 10.0.50.11:
f1 + 08 + 0a00320a + 0a00320b.
ip dhcp excluded-address 10.50.0.1 10.50.0.10
ip dhcp pool AP-MGMT
network 10.50.0.0 255.255.255.0
default-router 10.50.0.1
dns-server 10.0.0.53
option 43 hex f1080a00320a0a00320b
!
ip dhcp excluded-address 10.40.0.1 10.40.0.10
ip dhcp pool VOICE
network 10.40.0.0 255.255.255.0
default-router 10.40.0.1
dns-server 10.0.0.53
domain-name corp.example.com
option 150 ip 10.0.0.15 10.0.0.16
lease 0 8lease 0 8 is 0 days and 8 hours. The IOS default is 1 day. Other vendors' APs and phones use 43
with their own sub-option formats, usually matched by option 60.
They will ask next
How does the server know what kind of device is asking?
Option 60 (vendor class identifier) and option 55 (the list of options the client wants). Servers can match on option 60 and hand phones, APs and PCs different options from the same scope.
A Cisco phone gets an address but never registers. Which option do you check?
Option 150 (or 66) must point at the call manager's TFTP server, and UDP 69 must be allowed from the voice VLAN to it. The phone downloads its config file over TFTP before it registers.
What happens if the server sends both option 121 and option 3?
RFC 3442 says the client must ignore option 3. If you use classless static routes, include the default route (0.0.0.0/0) inside option 121 itself.
Traps
- Typing the WLC address for option 43 as plain text. Cisco APs expect the hex TLV.
- Treating options 66 and 150 as the same thing. 66 is a single standard string, 150 is Cisco's list of IP addresses.