🚀 go-pugleaf

Between the Type 2.1 Node Reference and Network Product Formats, you
probably have all the available info for type 2.1 local channel
protocol.  In general, type 2.1 nodes "discover" each other by XID3
exchange.  VTAM system definition for the PU will say "TYPE=2,XID=YES". 
VTAM will send a null XID; if it sees in response XID0 or XID1, it
assumes the node is type 2.0; if it sees in response null XID or XID3,
it assumes the node is type 2.1.

>                        the IBM publication "Network Product Formats"
> rather clearly states that a channel 2.1 link uses XID format two (2).

I don't see this in my June '89 copy.  If you look near the end of the
description of XID3, you'll see that there are two DLC-dependent
sections defined--one for SDLC (also used for Token Ring and X.25), the
other for Channel DLC.  My copy says this CDLC section is for
communication "between T4 and T2.1 nodes", but that's not terribly
accurate.  In fact, when NCP or VTAM communicates with a T2.1 node, they
assume the identity of a T2.1 node.  There is no T4-T2.1 nor T5-T2.1
protocol defined, only T2.1-T2.1.
--
Dave Skeen                     IBM Internal: dls@achilleus.austin.ibm.com
D61/803  Zip 2603              IBM VNET:     SKEEN at AUSTIN
Austin, TX 78758               Internet:     dls@dce.austin.ibm.com