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

ISDN Operation and Troubleshooting
Objective:
This paper lab explains the fundamentals of ISDN operation. Here we will start with the
theory of ISDN operation, then break it down a little more in-depth layer by layer,
discuss troubleshooting commands for ISDN, and then finish by looking at how to
decipher the debug and show command outputs of working and non-working ISDN lines.
ISDN Theory of Operation:
ISDN, as a WAN technology, is fairly simple: once you know how to set it up and use it.
It is a technology that has been around for a while now and is used for main WAN
connections or, more likely, as backup connections for other WAN technologies. Once
you understand how ISDN operates you should be more likely to understand what you
need to set up on your routers and how to troubleshoot it.
ISDN operation is a simple (I think it is…) three-step operation that correlates nicely with
the lower three layers of the OSI model:
1. ISDN DDR generates “interesting” traffic PHYSICAL
2. ISDN call is made DATA LINK
3. PPP handshaking NETWORK
Then you are ready to go! Let’s look at each step a bit more in-depth.
Layer-By-Layer ISDN Operation:
Physical Layer
As we discuss the steps they will be numbered and correlated to the router configuration.
Use this to correlate the discussion (“the theory”) with the implementation (“learning by
doing”). (1) Of course no traffic will pass through a physical interface if it is physically
“shut down” so we must also configure our interfaces to be up during this phase. (2)
ISDN uses Dial on Demand Routing (DDR) to establish the first phase of connection at
the physical layer. We set up access control lists in our configuration that determine
“what is” and “what is not interesting traffic.” This will decide whether or not we move
on to the second phase. Finally you may see the term “spoofing” used during
troubleshooting or checking the status of an ISDN connection. The router “spoofs”
(fakes) a connection during the set up phases to imitate an active state, otherwise the next
steps could not take place. Some commands that must be used to set up a basic ISDN
connection include:
router(config)#int bri0/0 (1)
router(config-if)#ip address 192.168.1.1 255.255.255.0 (1)
router(config-if)#no shut (1)
router(config-if)#dialer-group 1 (2)
router(config-if)dialer map ip 192.168.1.2 name routerB 5552000 (2)
router(config)#dialer-list 1 protocol ip permit (2)
420