ArticlesReader.com Menu

Newest Articles
Most Viewed Articles
ArticlesReader.com RSS
Submit Article
Login
Signup

Search the articles

Articles Main Categories

Advice
Animals
Automobiles
Business
Career
Communications
Computer Programming
Computers
Entertainment
Environment
Family
Fashion
Finance
Food
Health & Medical
Home & Garden
Humor
Internet Business
Internet Marketing
Legal
Leisure & Recreation
Marketing
Other
Politics
Reference & Education
Religion
Self Improvement
Sports
Technology & Science
Travel
Writing

Subscribe

Receive alert message from us when new articles submitted to our site for free.

Syndicate

Related Products

Home::Computer Certification

Cisco Routing For The CCNA And CCNP: Administrative Distance

Author : Chris Bryant, CCIE #12933

The textbook definition of "administrative distance" is simple enough: "the measurement of a protocol's believability". It's not enough to know the definition, however you've got to know when AD comes into the picture and when it does not.

When a packet needs to be routed, the router looks in its routing table for the next-hop IP address the packet should take to get to the destination. There may be more than one matching path, in which case the router will look for the "longest match". The route that has the longest match - the route with the most bits in the mask set to "1" - will be the route that is used.

Consider the following three routes 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

This router has three possible next-hop IP addresses that it can send packets destined for the network 172.17.0.0. The masks are of different lengths, meaning that the route with the longest match (again, the route with the most bits set to "1") will be used. In this example, the RIP route will be used, since it has the longest match with a mask of /26. The administrative distances do not matter.

AD does matter when the masks are the same length, as shown here:

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

The longest match rule always precedes the use of AD, but here there is a three-way tie regarding the masks. They're all /24 (or 255.255.255.0 in dotted decimal). AD will be used to break this tie.

As mentioned, AD is a measurement of a protocol's believability. It is important to keep in mind that the lowest AD will be preferred. And while the routing table will show you the ADs of the respective protocols, it's a very good idea to know these ADs before taking the CCNA or CCNP:

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 (A router will not believe a source with an AD of 255, and such routes will not be placed into the routing table.)

The three protocols we looked at in the comparison were RIP, IGRP, and OSPF. While your first instinct may be that the OSPF route would be the most believable, IGRP actually has a lower AD than the other two and would be the route installed in the routing table.

Since IGRP does not support variable-length subnet masking and OSPF does, you may never see this scenario outside of an exam question. But if you do see it in the exam room or in a production network, you'll understand how an IGRP route could be preferred over an OSPF route.

Article Source: http://www.articledashboard.com

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com), home of free http://www.thebryantadvantage.com/">CCNA and http://www.thebryantadvantage.com/">CCNP tutorials, The Ultimate CCNA Study Package, and Ultimate CCNP Study Packages. Video courses and training, binary and subnetting help, and corporate training are also available.

For a FREE copy of his latest e-books, "How To Pass The http://www.thebryantadvantage.com/">CCNA" or "How To Pass The http://www.thebryantadvantage.com/">CCNP", send a request to chris@thebryantadvantage.com today !

Spam emails More free articles

More related feeds

Cisco Routing For The CCNA And CCNP: Administrative Distance
The textbook definition of administrative distance is simple enough: the measurement of a protocol’s believability. It’s not enough to know the definition, however you’ve got to know when AD comes into the picture and when it does not. ...

Cisco Routing For The CCNA And CCNP Administrative Distance
The book definition of “administrative distance” is ultimate enough: “the activity of a protocol’s believability”. It’s not sufficiency to undergo the definition, still you’ve got to undergo when AD comes into the represent and when it ...

Softwares download, IT and Certification Solutions: Cisco Routing ...
Cisco Routing For The CCNA And CCNP: Administrative Distance. The textbook definition of "administrative distance" is simple enough: "the measurement of a protocol's believability". It's not enough to know the definition, however you've ...

cisco routing for the ccna and ccnp administrative distance
and while the routing table will show you the ads of the respective protocols, it's a very good idea to know these ads before taking the ccna or ccnp:. connected route: 0. static route: 1. eigrp summary: 5. external bgp: 20 ...

let's go on our way: 10 Ccent, Ccna, Ccnp Interview Questions
10 Ccent, Ccna, Ccnp Interview Questions Q-1 What type of routing protocol maintains neighbors? Answer- Link State Q-2 what is the range of values for administrative distance? Answer: 0-255 Q-3 Describe the difference between unicast, ...

Actualtests ccnp 642-901 | Free Latest Cisco Certification ...
You Can Find Free Quality Exam At Pass4sure ，CCNA,CCNP,CCIE,CCSP,CCVP,CISCO. QUESTION 1: Certkiller uses EIGRP as the primary routing protocol in their network. How does EIGRP advertise subnet masks for each destination network? ...

Actualtests ccnp 642-892 | Free Latest Cisco Certification ...
C. The summary address is assigned an administrative distance of 10 by default. D. Manual summaries can be configured with the classful mask only. E. The summary address is entered into the routing table and is shown to be sourced from ...

Mostly Asked Question In CCNA Exam Part II - CCNA | The IT Stuff
The routing table shows the amount of time elapsed since a router adjacency was formed; The neighbor table shows all adjacent Cisco devices. Administrative distance is shown as a column in the neighbor table. ...

Pass4sure 350-020 CCIE 2.73 | Free Latest P4s Passforsure Cisco ...
The purpose of Administrative Distance, as used by Cisco routers, is:. A. To choose between routes from different routing protocols when receiving updates for the same network B. To identify which routing protocol forwarded the update ...

Pass4sure CCNP 642-901 2.95 | Free Latest Topcerts Pass4sure ...
What administrative distance is given to EIGRP summary routes? A. 0 B. 1 C. 5 D. 90 E. 95 F. 170. Pass4sure CCNP 642-901 Questions and Answers : 353 Q&As Updated: October 14th , 2008 Price: $129.99 $89.99. Free download: pass4sure CCNP ...

Cisco Routing For The CCNA And CCNP: Administrative Distance

Related articles