Cisco Routing For The CCNA And CCNP Administrative Distance
Below is a MRR and PLR article in category Computers Technology -> subcategory Computer Certification.
Cisco Routing for the CCNA and CCNP: Understanding Administrative Distance
Understanding administrative distance (AD) is crucial for Cisco certification candidates, yet it often causes confusion. In this article, we’ll clarify AD with examples to enhance your grasp of this essential topic.
What Is Administrative Distance?
Simply put, administrative distance measures a protocol's trustworthiness. Knowing AD's definition is not enough; you must also understand when it plays a role in routing decisions.
Routing Decisions and Longest Match
When a router routes a packet, it consults its routing table for the next-hop IP address needed to reach the destination. If multiple paths match, the router selects the "longest match"?"the route with the most bits set to "1" in the subnet mask.
Consider these examples from a fictional Cisco router:
- I 172.17.0.0 /24 via 172.1.1.1
- O 172.17.0.0 /25 via 173.1.1.1
- R 172.17.0.0 /26 via 174.1.1.1
Here, the router has three potential next-hop addresses for the network 172.17.0.0. The RIP route will be used because its mask of /26 provides the longest match. In this scenario, administrative distance does not influence the decision.
When Administrative Distance Matters
AD becomes relevant when masks are identical:
- I 172.17.0.0 /24 via 172.1.1.1
- O 172.17.0.0 /24 via 173.1.1.1
- R 172.17.0.0 /24 via 174.1.1.1
With matching masks of /24, the router needs AD to break the tie. AD measures a protocol’s credibility?"the lower the AD, the more trustworthy the route.
Know Your AD Values
Familiarize yourself with these AD values before attempting the CCNA or CCNP exams:
- Connected route: 0
- Static Route: 1
- EIGRP Summary: 5
- External BGP: 20
- EIGRP (Internal): 90
- IGRP: 100
- OSPF: 110
- ISIS: 115
- RIP: 120
- EIGRP (External): 170
- Unknown: 255 (Routes with an AD of 255 are not trusted and will not be used.)
In our comparison, the IGRP route wins because of its lower AD, despite the common perception that OSPF is more reliable.
Practical Implications
While the exam might explore scenarios where IGRP is preferred over OSPF, remember that IGRP doesn’t support variable-length subnet masking, unlike OSPF. This situation is rare but understanding it can be crucial for exams and real-world application.
By mastering AD and its role in routing, you'll be better prepared to tackle exam questions and real network challenges.
You can find the original non-AI version of this article here: Cisco Routing For The CCNA And CCNP Administrative Distance.
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