Support Model and Firmware version:
ECS4100 Series (Firmware version 1.2.30.183 and above)
ECS4120 Series (Firmware version 1.2.2.5 and above)
 
Summary:

*1 In order to set circuit-id to maximum the length of 246 characters, you have to configure remote-id to 1 character string. If you want to keep the default setting of DHCPSNP option82 Remote-id with Mac-address information (10 bytes = 4+6). The maximum length of Circuit-id that you can configure is 241 characters (255-10-4)

*2 In order to set remote-id to maximum the length of 246 characters, you have to configure circuit-id to 1 character string. If you want to keep the default setting of DHCPSNP option82 Circuit-id with Vlan-id, module, port number information (8 bytes = 4+2+1+1), the maximum length of Remote-id that you can configure is 243 characters (255-8-4)

1. PPPoE IA - Circuit ID and Remote ID

The Access-Node MUST encode and send the Circuit ID and Remote ID as a TAG in PPPoE discovery
Packet in the format described as below:


According to TR101, we append the capability of setting PPPoE IA sub-tags as following.
PPPoEIA Circuit-id string length to 57
PPPoEIA Remote-id string to 63
 
At the maximum length setting string with Circuit-id, the sub-tag of the packet will be
Type: 01
Length: 3f (63)
Value: node id (1 byte minimum) + " eth " (occupy 5 bytes) + string (57 bytes remain)

At the maximum length setting string with Remote-id, the sub-tag of the packet will be
Type: 02
Length: 3f (63)
Value: string (63 bytes remain)
 

2. DHCP Relay Agent Information Option

This document defines a new DHCP Option called the Relay Agent Information Option.  It is a "container" option for specific agent-supplied sub-options.  The format of the Relay Agent Information option is:

The length N gives the total number of octets in the Agent Information Field.  The Agent Information field consists of a sequence of SubOpt/Length/Value tuples for each sub-option, encoded in the following manner:

No "pad" sub-option is defined, and the Information field shall NOT be terminated with a 255 sub-option. The length N of the DHCP Agent Information Option shall include all bytes of the sub-option code/length/value tuples. Since at least one sub-option must be defined, the minimum Relay Agent Information length is two (2). The   length N of the sub-options shall be the number of octets in only that sub-option's value field. A sub-option length may be zero. The sub-options need not appear in sub-option code order.

  
The initial assignment of DHCP Relay Agent Sub-options is as follows:
                
DHCP Agent              Sub-Option Description
Sub-option Code
---------------               ---------------------------------
          1                        Agent Circuit ID Sub-option

          2                        Agent Remote ID Sub-option
 

 
According to RFC3046 and TR101, we append the capability of setting DHCPSNP option82 as following.
DHCPSNP option82 Circuit-id string length to 246.
DHCPSNP option82 Remote-id string to 246.
 
Note: DHCPSNP option82 total length is 255 bytes, both Circuit-id and Remote-id share this space.
 
At the max length setting string with Circuit-id while the Remote-id manually configured 1 byte string.
The space for Circuit-id string is 255-4-4-1=246 bytes.

Type: 01 (sub-option 1 circuit-id)
Length: f8 (248)
Type: 01 (string)
Length: f6 (246)
Value: string (246 bytes remain)
 
At the max length setting string with Remote-id while the Circuit-id manually configured 1 byte string.
The space for Remote-id string is 255-4-4-1=246 bytes

Type: 02 (sub-option 2 remote-id)
Length: f8 (248)
Type: 04 (string)
Length: f6 (246)
Value: string (246 bytes remain)
 
Note: When DHCPSNP option82 function enabled on an Edge-Core switch, the default setting of Circuit-id and Remote-id will have a format as following.
 

 

So, at default settings, the Circuit-id will have 8 bytes length, and the Remote-id will have 10 bytes length. In order to reach the maximum setting of 246 bytes string, users must manually configure either Circuit-id or Remote-id with 1 byte string.