[AGENT++] build_vb and too many variables

[Jochen Katz]

Hi Jacek,

> According to RFC (if I understand it correctly) one should put part of
> variables into PDU, not return error.

agent++ uses Pdu::get_asn1_length() / Vb::get_asn1_length() functions to
 calculate the max possible Vb count for replies. snmp++ does not check
itself, as snmp++ is for the manager side and does not have the problem
of too big Pdus.

Regards,
  Jochen