Terminal Endpoint Identifier TEIs

A terminal endpoint can be any ISDN-capable device attached to an ISDN network. The TEI is a number between 0 and 127, where 0-63 are used for static TEI assignment, 64-126 are used for dynamic assignment, and 127 is used for group assignments. (0 is used only for PRI and is discussed later.) The TEI provides the physical identifier, and the service access point identifier (SAPI) carries the logical identifier.

The process of assigning TEIs differs slightly between North America and Europe. In North America, Layer 1 and Layer 2 are activated at all times. In Europe, the activation does not occur until the call setup is sent (known as "first call"). This delay conserves switch resources. In Germany or Italy, as well as in other parts of the world, the procedure for TEI assignment can change according to local practices.

In other countries, another key piece of information to obtain is the bus type. Supported types are point-to-point or point-to-multipoint connection styles. In Europe, if you are not sure, specify a point-to-multipoint connection, which will enable dynamic TEI addressing. This is important if BRI connections are necessary because Cisco does not support BRI using TEI 0, because it is reserved for PRI TEI address 0. If you see a TEI of 0 on a BRI, it means that a dynamic assignment has not yet occurred, and the BRI may not be talking to the switch. In the U.S., a BRI data line is implemented only in a point-to-point configuration.

Example 6-1 shows a typical ISDN Layer 2 negotiation. Example 6-1 debug isdn q921 Output

RouterA#debug isdn q921

Example 6-1 shows a typical ISDN Layer 2 negotiation. Example 6-1 debug isdn q921 Output

RouterA#debug isdn q921

BRI0

TX

->

IDREQ ri = 65279 ai = 127

BRI0

RX

<-

UI sapi = 0 tei = 127 i = 0X0801FF0504038090A218018896250101

BRI0

TX

->

IDREQ ri = 61168 ai = 127

BRI0

RX

<-

IDASSN ri = 61168 ai =64

BRI0

TX

->

SABMEp sapi = 0 tei = 64

BRI0

RX

<-

UAf sapi = 0 tei = 64

BRI0

TX

->

INFOc sapi = 0 tei = 64 ns = 0

nr

= 0

i = X08017F5A080280D1

BRI0

RX

<-

RRr sapi = 0 tei = 64 nr = 1

BRI0

RX

<-

INFOc sapi = 0 tei = 64 ns = 0

nr

= 1

i = X08007B963902EF01

BRI0

TX

->

RRr sapi = 0 tei = 64 nr = 1

Example 6-1 debug isdn q921 Output (Continued)

BRI0

RX

<-

INFOc sapi

= 0

tei

= 64 ns

= 1

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

0X8007B962201013201013B0110

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.

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