Library /sys$common/syshlp/nclhelp.hlb set, routing, circuit, reachable_address, block_size  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Support: End, type outbound
    Default: 0			Value: 0-65536

    The data link block size to be used for this prefix.
    For an end system, the data link block size to be used for this
    prefix.  If the block size is set to the default, the manual
    block size of the circuit will be used instead.  This attribute
    is supported only if the type characteristic is set to outbound.

    set [node node-id] routing circuit circuit-name -
        reachable address simplename block size integer

    Example:

    set routing circuit csmacd-1 -
         reachable address to-area4 block size 500