The circuit ID consists of five fields, where the fifth field (Relay Custom Info) is optional and not used in this scenario. Note that the circuit ID in the switch is encoded in binary, but is expected by the router in hexadecimal notation. This can be seen in an Ethernet trace (filtered for BootP). For this reason the circuit ID must be prefixed with the string 0x. Furthermore, the VID and the port number must be converted if necessary. - Circuit ID type: The 1 stands for circuit ID. The field has two digits, so the entry has to be 01.
- Length: Unless relay custom info is transmitted, the length is always 4 bytes (i.e. 2 bytes each for the VLAN ID and the port). The field has two digits, so the entry has to be 04.
- VID: Enter the VLAN ID of the switch port. The field has four digits, so the entry could be 0010 for example.
- Port number: Enter the port number of the switch port. The field has four digits, so the entry could be 0009 for example.
- Relay custom info: Additional information can optionally be entered into this field.
Example: A network member in VLAN 10 and connected to port 9 should be given a specific IP address. The VID 10 is a in hexadecimal notation. The resulting circuit ID appears as follows: | Hexadecimal notation | Circuit ID type | Length | VID | port number |
|---|
| 0x | 01 | 04 | 000a | 0009 |
| 