Learning by Doing: CISCO Certified Network ... - SCN Research

Data Link Layer
Two things happen here: The bearer channel (B-channel) is set up and the data channels
(D-channel) are set up. This, essentially, is how a call is made. There is a bit of overlap
with the physical layer (dialer strings/maps) much. (1) The D-channel is uses a protocol
called “Link Access Protocol-D” or LAPD. This uses Q.921 for establishment.
Therefore it makes sense for us to debug Q.921 during troubleshooting. (2) If a protocol
is used then it must hand-shake (establish, negotiate, and maintain of LCP-Link Control
Protocol). This is where service profile identifiers (SPID) may or may not be used (a.k.a
“TEI”)and username/password problems can be found. Also, certain manufacturers of
networking equipment do not require specific encapsulations. Nine times out of ten they
do require PPP for encapsulation. For example, MERGE boxes do not require PPP but
ADTRAN units do require PPP. Good stuff to know when setting them up.
router(config-if)dialer map ip 192.168.1.2 name routerB 5552000 (2)
router(config-if)isdn spid1 51055512340001 5551234 (2)
router(config-if)isdn spid2 51055512350001 5551235 (2)
router(config-if)#enc ppp (2)
router(config-if)#ppp authentication chap (2)
router(config)#username joe password cisco (2)
router(config)#ip host joe 192.168.1.1 (2)
Network Layer
(1) This is where our network layer implementation of PPP takes place (NCP-Network
Control Prtocol). This is where username and password problems can also be found. (Ok
so there is some overlap). Here we will also find the Q.931 protocol to finish our ISDN
connection.
router(config)#username joe password cisco (2)
router(config)#ip host joe 192.168.1.1 (2)
Troubleshooting ISDN:
Just like we have done before you will start at the physical layer and work your way up
the OSI model:
1. ISDN DDR generates “interesting” traffic PHYSICAL
2. ISDN call is made DATA LINK
3. PPP handshaking NETWORK
Physical layer:
Step 1: Since ISDN uses interesting traffic to initiate a call we must first generate
interesting traffic. The easiest way is to ping the other ISDN interface to see if the line
comes up. To check for interesting traffic beyond what happens we can use these
commands (use them in this order too):
sh controllers bri
sh int bri0/0
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