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Basic Rate Interface 137
NOTE
The following paragraphs are a partial explanation of the output listed in Example 6-1. You
should take the time to understand this section because the output gives a great deal of
troubleshooting information. You may need more than one reading to get it all straight.
The ri is a reference indicator. It provides the router and the switch a way to keep straight all
the calls they may be processing. Notice in the IDREQ and the IDASSN, the ri value is the
same. If the router sends an IDREQ and receives no response, it retries every two seconds. Each
time the ri is different. The ai is an association indicator. ai = 127 is the router's way of
requesting a TEI from the switch. The switch reply is ai = 64. Therefore, 64 is the assigned TEI.
Notice that all remaining correspondence has tei = 64 referenced. Once the router has a TEI, it
sends a SABME (Set Asynchronous Balanced Mode Extended) message with sapi = 0. This
means that this is a signalling connection (that is, this is all taking place over the D channel).
If no TEI is assigned, Layer 2 does activate and the output from the debug isdn q921 command
renders only TX->IDREQ lines. If all the Layer 2 processes are successful, you will see
MULTIPLE_FRAME_ESTABLISHED under the Layer 2 Status section in the output of the
show isdn status command. See Example 6-3 in the "ISDN Call Setup" section for a
demonstration of the show isdn status command output.
Service Profile Identifiers (SPIDs)
Another key part of the ISDN BRI Layer 2 is the SPID. SPIDs are used only in BRI
implementations. PRI implementations do not require the use of SPIDs. The SPID specifies the
services to which you are entitled from the switch and defines the feature set that you ordered
when the ISDN service was provisioned.
The SPID is a series of characters manually entered into the router's configuration to identify
the router to the switch. This is different from the TEI discussed earlier. The TEI address is
dynamically assigned. The SPID is statically assigned to the router based on information
provided by the service provider. If needed, two SPIDs are configured, one for each channel of
the BRI. Usually, the SPID includes the ten-digit phone number of each B channel followed by
four additional digits (sometimes 0101) assigned by the telco.
BRI0: RX <- INFOc sapi = 0 tei = 64 ns = 1 nr = 1 i = 0x8007B962201013201013B0110
BRI0: TX -> RRr sapi = 0 tei = 64 nr = 2
BRI0: TX -> RRp sapi = 0 tei = 64 nr = 2
BRI0: RX <- RRf sapi = 0 tei = 64 nr = 1
BRI0: TX -> RRp sapi = 0 tei = 64 nr = 2
BRI0: RX <- RRf sapi = 0 tei = 64 nr = 1
Example 6-1
debug isdn q921 Output (Continued)