You need to have an understanding of the LAPD frame before you understand how layer 2
negotiates. This will help you identify where a potential or existing problem is occurring. One
useful feature of Cisco equipment is that it includes good diagnostic tools for finding ISDN
problems. Knowing which side of the ISDN connection does what will help you identify a problem
and start corrective action.
The first part of the process is TEI assignment, which is accomplished by using this process:
1. The terminal endpoint (TE) and the network initially exchange Receive Ready (RR) frames,
listening for an initiated connection.
2. The TE sends an Unnumbered Information (UI) frame with a SAPI of 63 (management procedure,
query network) and TEI of 127 (broadcast).
3. The network assigns an available TEI (in the range 64–126).
4. The TE sends a Set Asynchronous Balanced Mode Extended (SABME) frame with a SAPI
of 0 (call control, used to initiate a SETUP) and a TEI of the value assigned by the network.
5. The network responds with an Unnumbered Acknowledgment (UA); SAPI = 0, TEI = assigned.
As you examine this partial output from the command debug isdn q921, please refer to
Table 26.7, which explains the meaning of the output.
ISDN BR0: TX -> SABMEp sapi = 0 tei = 77
ISDN BR0: RX <- IDCKRQ ri = 0 ai = 127
ISDN BR0: TX -> IDCKRP ri = 44602 ai = 77
ISDN BR0: TX -> IDCKRP ri = 37339 ai = 78
ISDN BR0: RX <- IDREM ri = 0 ai = 77
ISDN BR0: TX -> IDREQ ri = 44940 ai = 127
ISDN BR0: RX <- IDREM ri = 0 ai = 78
ISDN BR0: TX -> IDREQ ri = 43085 ai = 127
ISDN BR0: TX -> IDREQ ri = 11550 ai = 127
ISDN BR0: RX <- IDASSN ri = 11550 ai = 79
ISDN BR0: TX -> SABMEp sapi = 0 tei = 79
ISDN BR0: TX -> IDREQ ri = 65279 ai = 127
ISDN BR0: RX <- UAf sapi = 0 tei = 79
ISDN BR0: TX -> INFOc sapi = 0 tei = 79 ns = 0
➥nr = 0 i = 0x08007B3A0A30383335383636313031
ISDN BR0: RX <- IDASSN ri = 65279 ai = 80
ISDN BR0: TX -> SABMEp sapi = 0 tei = 80
ISDN BR0: RX <- INFOc sapi = 0 tei = 79 ns = 0 nr = 1 i =
➥0x08007B3B028181
ISDN BR0: TX -> RRr sapi = 0 tei = 79 nr = 1
ISDN BR0: RX <- UAf sapi = 0 tei = 80
ISDN BR0: TX -> INFOc sapi = 0 tei = 80 ns = 0 _
nr = 0 i = 0x08007B3A0A30383335383636333031
ISDN BR0: RX <- INFOc sapi = 0 tei = 80 ns = 0 nr = 1 i =
➥0x08007B3B028381
ISDN BR0: TX -> RRr sapi = 0 tei = 80 nr = 1