Cisco Certification Introduction To ISDN
Below is a MRR and PLR article in category Computers Technology -> subcategory Computer Certification.
Cisco Certification: Introduction to ISDN
Overview
ISDN is a crucial and complex topic for CCNA aspirants to master. Chris Bryant, CCIE
12933, explains its foundational aspects, which are key to both certification success and practical application.
Understanding ISDN
ISDN, prevalent in the field, often serves as a backup when Frame Relay connections fail. It's essential to grasp ISDN basics for exams and career success.
ISDN operates between two Cisco routers with BRI or PRI interfaces. Essentially, it involves one router dialing the other. Understanding the triggers for dialing and disconnection is vital.
Cost Implications
Since ISDN resembles a phone call, costs accrue by the minute. If a router fails to disconnect, unforeseen expenses can escalate quickly, leading to hefty phone bills.
The Concept of "Interesting Traffic"
Cisco routers utilize "interesting traffic" to determine when a connection should be initiated. Without defining this traffic, routers won’t dial each other.
Defining Interesting Traffic:
- Use the `dialer-list` command to define what qualifies as interesting traffic.
- Specify protocols, sources, destinations, or port numbers to control link activation.
Maintaining and Terminating Connections
While interesting traffic initiates the link, any traffic can traverse it once it's active. However, the lack of interesting traffic prompts disconnection based on an idle timeout.
- Idle Timeout: If interesting traffic doesn’t cross the ISDN link within a specified period, it disconnects, preventing unnecessary charges.
Preventing Unnecessary Connections
ISDN is billed like a phone call, possibly incurring long-distance charges. If not managed properly, a backup ISDN link could stay active longer than needed, especially when used alongside Frame Relay.
Timeout and Routing Protocols
Cisco’s ISDN uses the idle timeout, defaulted to two minutes, to determine when to disconnect. Interesting traffic must reset this timer to keep the link active.
- Handling RIP and EIGRP: To prevent routing updates from maintaining the connection, restrict multicast routing update addresses in the access list defining interesting traffic. Ensure the protocol can still function properly.
- Configuring OSPF: Use the `ip ospf demand-circuit` command. This keeps the OSPF adjacency intact without regular Hello packets, allowing the link to drop without losing adjacency.
Importance for Cisco Certification
Knowledge of these commands is vital for Cisco certification at all levels, especially CCNA. Understand how adjacency remains stable even when Hellos are suppressed, and integrate these skills into your Cisco toolkit.
Myth Busting: CDP and ISDN Links
A common misconception is that Cisco Discovery Protocol (CDP) might inadvertently keep ISDN links active. However, CDP, a protocol between directly connected Cisco devices, doesn’t typically affect ISDN link status. Test this in a lab to deepen understanding.
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By mastering these ISDN concepts, CCNA candidates can enhance their expertise and be well-prepared for both exams and real-world scenarios.
Chris Bryant
CCIE #12933
You can find the original non-AI version of this article here: Cisco Certification Introduction To ISDN.
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