Output Meaning
ISDN BR0: This is the interface.
TX -> This router is sending this information.
RX <- This router is receiving this information.
SABME Indicates the Set Asynchronous Balanced Mode Extended command.
This command places the recipient into modulo 128 multiple
frame acknowledged operation. This command also indicates
that all exception conditions have been cleared.
sapi Identifies the service access point at which the Data-Link layer
entity provides services to layer 3 or to the management layer. A
SAPI with the value 0 indicates it is a call control procedure.
IDCKRQ ri = 0 ai = 127 Indicates the Identity Check Request message type sent from the
ISDN service provider on the network to the local router during
the TEI check procedure. This message is sent in a UI command
frame. The ri field is always 0. The ai field for this message contains
either a specific TEI value for the local router to check or 127,
which indicates that the local router should check all TEI values.
IDREM This indicates the Identity Remove message type sent from the
network to the user-side layer management entity during the TEI
removal procedure. This message is sent in a UI command frame.
The message includes a reference number that is always 0, because
it is not responding to a request from the local router. It is sent twice
by the network to prevent a lost message.
IDCKRP Indicates the Identity Check Response message type sent from
the local router to the ISDN service provider on the network during
the TEI check procedure. This message is sent in a UI command
frame in response to the IDCKRQ message
IDREQ This indicates an Identity Request message sent from the local
router to the network during the automatic TEI assignment.
UAf This confirms that the network side has accepted the SABME command
previously sent by the local router. The final bit is set to 1.
INFOc This is an information command. It is used to transfer sequentially
numbered frames containing Information Fields cap provided
by layer 3.
IDASSN This indicates an Identity Assigned message type sent from the
network’s ISDN service provider to the local router during the
automatic TEI assignment procedure.
RRx This indicates Receive Ready. If x = r, it is responding to an
INFOc. If x = p, the router is polling the network side. And x = f
means the network side has responded to the poll and the final
bit is set.
Now what does everything in Table 26.7 mean? According to the output, the router attempts
to establish a connection with the switch, using legacy TEI information that it has left over:
ISDN BR0: TX -> SABMEp sapi = 0 tei = 77
The service provider’s switch disapproves of this and orders a check of the router’s current TEIs
with the IDCKRQ message. The ai of 127 (broadcast) simply tells the router that the switch
would like for it to check all TEIs it currently has registered:
ISDN BR0: RX <- IDCKRQ ri = 0 ai = 127
The router promptly returns an IDCKRP message for each TEI it finds within itself. In this case,
these are 77 and 78:
ISDN BR0: TX -> IDCKRP ri = 44602 ai = 77
ISDN BR0: TX -> IDCKRP ri = 37339 ai = 78
The switch does not want the router to continue using these TEIs, so it issues an IDREM message
for each offending TEI. This tells the router to forget about these TEIs:
ISDN BR0: RX <- IDREM ri = 0 ai = 77
The router quickly throws itself at the mercy of the switch by sending the IDREQ message with
an ai of 127. Notice that the router is looking for two TEIs, one for each logical B channel interface
within BRI0, but it has to issue four IDREQs to overcome the timeouts:
ISDN BR0: TX -> IDREQ ri = 43085 ai = 127
ISDN BR0: TX -> IDREQ ri = 11550 ai = 127
As soon as an IDASSN returns that matches the ri of one of the IDREQs, as follows:
ISDN BR0: RX <- IDASSN ri = 11550 ai = 79
the router turns around and establishes service with a new SABME message, using the new TEI:
ISDN BR0: TX -> SABMEp sapi = 0 tei = 79
ISDN BR0: TX -> IDREQ ri = 65279 ai = 127
Because the switch obviously approves of this TEI, it responds with the UA message the router
was originally looking for.
ISDN BR0: RX <- UAf sapi = 0 tei = 79
After the UAs come in, the whole INFO/RR exchange for layer 3 information begins for each
TEI assigned:
ISDN BR0: TX -> INFOc sapi = 0 tei = 79 ns = 0 nr = 0 i =
➥0x08007B3A0A30383335383636313031
ISDN BR0: TX -> RRr sapi = 0 tei = 79 nr = 1
This occurs for both the 79 and 80 TEIs.