MTU values

Brushing up on my ICND1 topics for this Saturday when I'll be sitting my exam. My goal has switched from passing to getting a perfect score

Odom ICND1 book states that MTU's need to be 1500 or less for ethernet and most other data link protocols, but leaves a vague non-answer hanging there.

My question is, do I need to know of the values beyond 1500 because I can't find them. I'm assuming right now that if I see a value over 1500 in a show interface command then that would be wrong, but anything below would be correct or ok. Is this flawed logic and if so, why?

Find more posts tagged with

Save $250 on 2025 certification boot camps from Infosec!

Book now with code EOY2025

Button

Comments

hiddenknight821

SdotLow wrote: »

Brushing up on my ICND1 topics for this Saturday when I'll be sitting my exam. My goal has switched from passing to getting a perfect score

Odom ICND1 book states that MTU's need to be 1500 or less for ethernet and most other data link protocols, but leaves a vague non-answer hanging there.

My question is, do I need to know of the values beyond 1500 because I can't find them. I'm assuming right now that if I see a value over 1500 in a show interface command then that would be wrong, but anything below would be correct or ok. Is this flawed logic and if so, why?

I hope I am not violating the NDA in any way, but let me put it this way. Just stick with the most common value for MTU, and that's all you will ever need to know. You know the packet will be fragmented if it's beyond the maximum limit, right? When you take the test and you are having second guess, you should always stick with your first answer.

SdotLow

hiddenknight821 wrote: »

I hope I am not violating the NDA in any way, but let me put it this way. Just stick with the most common value for MTU, and that's all you will ever need to know. You know the packet will be fragmented if it's beyond the maximum limit, right? When you take the test and you are having second guess, you should always stick with your first answer.

I'm not asking for what's on the test, or what could be on the test. I'm asking how the MTU value works, and to be more specific, what is required of us to know for the ICND1 in it's relation.

All Odom brings up is a very brief paragraph stating what I initially said. You're not breaking the NDA.

I didn't know about it being fragmented if it's beyond the value, because he doesn't mention it in the book lol. If you wouldn't mind elaborating a bit, I'd appreciate it.

johnnyarks

SdotLow wrote: »

Brushing up on my ICND1 topics for this Saturday when I'll be sitting my exam. My goal has switched from passing to getting a perfect score

Odom ICND1 book states that MTU's need to be 1500 or less for ethernet and most other data link protocols, but leaves a vague non-answer hanging there.

My question is, do I need to know of the values beyond 1500 because I can't find them. I'm assuming right now that if I see a value over 1500 in a show interface command then that would be wrong, but anything below would be correct or ok. Is this flawed logic and if so, why?

Maybe if you understand why its 1500, then it will help you understand why it needs to be at 1500...(if thats makes sense =D) .... use other sources, I always learn one topic from one source say CBT nuggets vidz, then I reinforce it by reading the same topic in my Sybex book, then I do the labs by Chris Bryant, that way you get more than one perspective...youtube is very good for simple small things that need a quick explanation like this.

What is MTU? - A Word Definition From the Webopedia Computer Dictionary <--I like this site, very easy to understand definitions, enjoy.

from my understanding an MTU is the size of the packet, and 1500 is the max size, you want your MTU to match from say CLIENT > DESTINATION, if you have the CLIENT set to 1500 and the DESTINATION set to say "500" (just an example), then your packets gets broken up 3 times before the DESTINATION is able to understand your packet.

SdotLow

johnnyarks wrote: »

Maybe if you understand why its 1500, that helps understand why you need it at 1500...if that makes sense =D .... use other sources, I always learn one topic from one source say CBT nuggets vidz, then I reinforce it by reading the same topic in my Sybex book, then I do the labs by Chris Bryant, that way you get more than one perspective...youtube is very good for simple small things that need a quick explanation like this.

What is MTU? - A Word Definition From the Webopedia Computer Dictionary <--I like this site, very easy to understand definitions, enjoy.

Yeah, I have the CBT nuggets, Sybex book and the Cisco press book. I also have the Exam cram for ICND1 just so I can power through facts these last couple days. I'm sure I'll pass, I'd just like to see 1000/1000

.

Thanks for that link, that actually explained it quite well (well, good enough for what I wanted). I tried googling and got a bunch of useless links. I'll have to bookmark that site

johnnyarks

SdotLow wrote: »

Yeah, I have the CBT nuggets, Sybex book and the Cisco press book. I also have the Exam cram for ICND1 just so I can power through facts these last couple days. I'm sure I'll pass, I'd just like to see 1000/1000 .

Thanks for that link, that actually explained it quite well (well, good enough for what I wanted). I tried googling and got a bunch of useless links. I'll have to bookmark that site

yes, I love that site, they explain things so that anyone can understand the meanings.

hiddenknight821

SdotLow wrote: »

I'm not asking for what's on the test, or what could be on the test. I'm asking how the MTU value works, and to be more specific, what is required of us to know for the ICND1 in it's relation.

All Odom brings up is a very brief paragraph stating what I initially said. You're not breaking the NDA.

I didn't know about it being fragmented if it's beyond the value, because he doesn't mention it in the book lol. If you wouldn't mind elaborating a bit, I'd appreciate it.

Oh. I assumed you knew the MTU stuff. From what I understood, I thought you were saying you saw many different values in the book, and were not sure which one will be primarly used on the test, which would be a violation to the NDA if I told you which one. Sorry if I misunderstood you.

SdotLow

Nope. Just making sure I understand every little thing and minimize the need to rely on "which answer makes the most sense" type answering.

Just two more things on my list to brush up on. Thank again!

mark_s0

I'd recommend reading the TCP/IPguide.com website for stuff on MTU. Someone here recommended it to me a little while back and it really helped me understand it, especially the fragmentation part. It's very detailed and a lot of it is passed CCENT level so I wouldn't spend hours reading now but it's definately worth reading at some point to fully understand it.