Administrative Distance, Metrics and Cost

I've been struggling with this for a while and would like to get my head around it. Here's what I understand (I hope I'm right!):

Administrative Distance has nothing to do with physical distance or number of hops between host A and host B. There are default ADs which can be changed. The lower the AD, the more "trustworthy" the route is. The default AD for IGRP is lower than RIP.

Some questions about AD:
1. What does "trustworthy" mean in this context? As a novice, I'd interpret it to mean that the cables and plugs were all secure, the router hardware and software are stable and not liable to crash etc.

2. If IGRP has a lower default AD than RIP, it's more "trustworthy", so why not use it in preference to RIP? I suspect it's more complicated to configure than RIP but is that the only reason? (We have configured RIP in the course but not IGRP)

Metrics are a composite value, derived from several components, depending upon the rouring protocol. They may use the number of hops between host A and host B, the available bandwidth over the path, delay etc.

Some questions about metrics:
1. How is this different from AD? If I have two paths and one has a lower metric, that would appear to be more efficient or reliable in getting traffic from host A to host B quickly so I could regard it as more trustworthy over a route which took twice as long to do the same job.

2. Are there formulae to calculate the actual metric from the various parameters which feed into it? I've seen an example of show ip eigrp topology which resulted in the following:

P 10.55.103.0/24, 1 successors, FD is 6049536
via 10.220.100.1 (6049536/5537536), Serial3/0
via 10.55.100.14 (52825600/281600), Tunnel55

What on earth do the 6/7/8 digit numbers mean and how have they been calculated?

I have no idea where Cost comes into all of this!

Sorry for having gone on for so long. I just thought it appropriate to let you know where I'm at currently so, hopefully, someone could clarify the final parts of my puzzle.

Thanks for your time.

I’m still learning this stuff myself, but here are my two cents on this…

I think trustworthy is defined by how reliable the route is in comparison to other routing protocols. An IGRP has a lower AD because it’s essentially "smarter", and can factor in different variables into its metric value (Bandwidth, Delay, etc). Whereas RIP, only uses hop count for it’s metric values. RIP wouldn’t too well if you had multiple routes to one network, one route being 10 hops away all on gigabit pipes, and the other 5 hops away on 10Mb pipes. RIP would use the latter route for the destination network thinking it’s the better route.


From what I understand, the difference between a metric value and administrative distance is that you can modify the administrative distance value. In the example above, you could set the administrative distance of the longer (faster) route to a smaller value, making it the preferred route to the destination network. You can also setup floating static routes by setting static routes with an AD higher than dynamic routes, causing the static route to “float” in memory as a backup route if all of the other routes fail.

My guess is that the folks who designed protocols like IGRP did so in such a way that they knew routes using this protocol would be more “trustworthy” than older protocols like RIP under most circumstances (because it can calculate different variables into its metric value), but at the same time, they gave us the ability to fine tune this setting to accomodate different network environments. In other words, it's a default/recommended value that can be adjustted.


I could be completely off the ball with this assessment though.

-JT