To Cisco Routers, This Concept Is Number One!
If you were to ask a number of networking professionals how routers make forwarding decisions, you might hear one answer far more than any other – Administrative Distance. In simplest terms, Administrative Distance (AD) is a ranking of believability. That is, if I am a router with more than one routing protocol running, to whom do I listen if each protocol is telling me about some particular destination? Well that is not a bad answer, but let’s blow one common misconception out of the water.
I have built the following scenario:
As you can see from the diagram, R3 is running both RIP and EIGRP. RIP is announcing the network 192.168.50.0 as a classful Class C network. R3 has also been configured with a summary for EIGRP which is 192.168.0.0 /16. This means that R4, which is also running RIP and EIGRP, is hearing about the destination of 192.168.50.0 from RIP with a 24 bit mask, and it has an EIGRP entry with a 16 bit mask. Let’s take a look at the routing table for R4:
R4#sh ip route
Gateway of last resort is not set
I have highlighted the relevant portions of the output above. Here is where the common misconception rears its ugly head. Many networking people, if they were asked how R4 would select a path, would respond that since EIGRP has a better AD than RIP (EIGRP is 90, RIP is 120), R4 will select the path from R4 to R2 to R3. How to verify our choice? How about a traceroute from R4 to the target destination?
As you can readily observe from the results of the traceroute, the inclination to select best AD above all else can lead one astray. It turns out that there is another facet of router path selection which holds sway even above AD. What is it? Longest prefix match. In the diagrammed topology presented above, R4 knows about the destination address from RIP, and it matches to 24 bits. R4 also knows about the destination as 192.168.0.0, but it matches to only 16 bits. So R4 listens to RIP in this instance.
As mentioned, it is surprising to learn how many people go through the entire CCNA instructor-led training path and believe that the number one factor in path selection is AD. Let it not be said that anyone reading this blog would make the same mistake!
Until next time, happy path selecting…
Mark Jacob
Cisco Instructor – Interface Technical Training
Phoenix, AZ
A Simple Introduction to Cisco CML2
Mark Jacob, Cisco Instructor, presents an introduction to Cisco Modeling Labs 2.0 or CML2.0, an upgrade to Cisco’s VIRL Personal Edition. Mark demonstrates Terminal Emulator access to console, as well as console access from within the CML2.0 product. Hello, I’m Mark Jacob, a Cisco Instructor and Network Instructor at Interface Technical Training. I’ve been using … Continue reading A Simple Introduction to Cisco CML2
Cable Testers and How to Use them in Network Environments
This content is from our CompTIA Network + Video Certification Training Course. Start training today! In this video, CompTIA Network + instructor Rick Trader demonstrates how to use cable testers in network environments. Let’s look at some tools that we can use to test our different cables in our environment. Cable Testers Properly Wired Connectivity … Continue reading Cable Testers and How to Use them in Network Environments
Data Models in Business Analysis
This video is from our PMI-PBA Business Analysis for IT Analysts and Project Managers (PMI-PBA)® Certification now available at Interface Technical Training. Also see Steve’s PMP Project Management Certification Course: Project Management Professional (PMP®) Certification Video Training PMBOK® 6th Edition Video Transcription: Data Models are part of the elicitation analysis in PMI-PBA. This is the way … Continue reading Data Models in Business Analysis
Write a Comment
