Solved

Count to Infinity Problem With Distance-Vector Routing Protocols

Posted on 2011-03-08
5
808 Views
Last Modified: 2012-06-27
Hi,

I'm revising for my Cisco CCNA. I understand the general concept of count-to-infinity, but I have a problem with it: It was my understanding that routing tables only updated themselves if they were sent updates with routes that were shorter than that already stored. With the count-to-infinity scenario, the routing metric will always be bigger than that currently stored in the table, so the router shouldn't update the routing table to include it. How then, does the metric ever rise to infinity? And if this isn't the case (i.e. routers update regardless of metric values), surely the routing table can't be described as "a table containing the shortest known paths to different networks"?

Chris
0
Comment
Question by:chris_smith_51
5 Comments
 
LVL 50

Expert Comment

by:Don Johnston
ID: 35075285
>How then, does the metric ever rise to infinity?

Because the route with the shorter metric ages out. Once that happens the route with the higher metric is now accepted.

0
 
LVL 2

Expert Comment

by:Paul_McClure
ID: 35079516
Think of the scenario with routers A B and C connected as A-B-C. If router A goes down then router B does not receive any packets. Router C on the other hand will tell router B that router A is only 2 hops away. Router B will assume that router A is available from router C and it will slowly propagate through the network. The split-horizon method can reduce the probability of a count-to-infinity problem but in nearly all cases a hold timer within a routing protocol can remove the possibility of this happening.
0
 
LVL 2

Author Comment

by:chris_smith_51
ID: 35080348
donjohnston> That ties in with something I briefly read. So packets enter a routing loop until the shorter metric gets aged out, at which point it is replaced by the higher metric and so on...? What determines the age-out time? Is it configurable or dependent on network management protocols? Typically, what would this time be?

Cheers
0
 
LVL 7

Expert Comment

by:hau_it
ID: 35080914
0
 
LVL 50

Accepted Solution

by:
Don Johnston earned 250 total points
ID: 35082127
>What determines the age-out time?

The routing protocol. For example, in RIP it's 180 seconds. And like everything, it's adjustable.



0

Featured Post

Flexible connectivity for any environment

The KE6900 series can extend and deploy computers with high definition displays across multiple stations in a variety of applications that suit any environment. Expand computer use to stations across multiple rooms with dynamic access.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

Title # Comments Views Activity
Hit router interface limit 7 38
Cisco EIGRP Network 6 25
creating SVI on layer 3 switch 1 24
Configure BGP 22 12
Creating an OSPF network that automatically (dynamically) reroutes network traffic over other connections to prevent network downtime.
Shadow IT is coming out of the shadows as more businesses are choosing cloud-based applications. It is now a multi-cloud world for most organizations. Simultaneously, most businesses have yet to consolidate with one cloud provider or define an offic…
After creating this article (http://www.experts-exchange.com/articles/23699/Setup-Mikrotik-routers-with-OSPF.html), I decided to make a video (no audio) to show you how to configure the routers and run some trace routes and pings between the 7 sites…
After creating this article (http://www.experts-exchange.com/articles/23699/Setup-Mikrotik-routers-with-OSPF.html), I decided to make a video (no audio) to show you how to configure the routers and run some trace routes and pings between the 7 sites…

839 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question