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EIGRP Metrics

Hi Experts,

In Eigrp I know that the following contributes the Metric

1. Bandwidth
2. Delay
3. Load
4. Reliability
5. MTU

I trying to understand how MTU influence the Metric. It labbed it. R1 (ser
0/0) ----(ser 0/0) R2

I changed the metric of R1 ser 0/0 to 1200 and noticed the Metric value
didn't change. How and when MTU comes into play.

Also I noticed the Eigrp messages donot contain MTU details.

Can you pl give your valuable time and help us

sairam

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

Re: EIGRP Metrics

Hello Sairam,

this is a good question.

MTU is not part of the composite metric. However, EIGRP routes both internal and external are carried within TLV data structures.

The TLV is oriented at providing in multiple fields data ready for EIGRP calculation of local node that is:

prefix

prefix len

minimum BW on path

cumulated delay on path

and also:

minimum MTU on path

number of router hops

you can see this information using sh ip eigrp topology

the local node to calculate the metric from its own point of view just needs to:

add the delay of its own interface where the update has been received

to verify the bandwidth parameter of its own interface where the update has been received if its higher of current minimum BW this field is left unchanged.

to add 1 to router hop field

to verify the MTU parameter of its own interface where the update has been received if its higher of current minimum MTU this field is left unchanged.

MTU and number of router hops are used as tie breakers in case of paths with equal EIGRP metric, the path with the highest minimum MTU is preferred as well as the path with the minimum router hop count.

to be noted that delay already provides a measure of hop count weighted with delay of each link in the path.

Hope to help

Giuseppe

8 REPLIES
Hall of Fame Super Silver

Re: EIGRP Metrics

Hello Sairam,

this is a good question.

MTU is not part of the composite metric. However, EIGRP routes both internal and external are carried within TLV data structures.

The TLV is oriented at providing in multiple fields data ready for EIGRP calculation of local node that is:

prefix

prefix len

minimum BW on path

cumulated delay on path

and also:

minimum MTU on path

number of router hops

you can see this information using sh ip eigrp topology

the local node to calculate the metric from its own point of view just needs to:

add the delay of its own interface where the update has been received

to verify the bandwidth parameter of its own interface where the update has been received if its higher of current minimum BW this field is left unchanged.

to add 1 to router hop field

to verify the MTU parameter of its own interface where the update has been received if its higher of current minimum MTU this field is left unchanged.

MTU and number of router hops are used as tie breakers in case of paths with equal EIGRP metric, the path with the highest minimum MTU is preferred as well as the path with the minimum router hop count.

to be noted that delay already provides a measure of hop count weighted with delay of each link in the path.

Hope to help

Giuseppe

Hall of Fame Super Blue

Re: EIGRP Metrics

Giuseppe

As usual an excellent detailed reply. Logged on this morning and see your 4 short and i couldn't resist

Congratulations, the forums are a much better place for you being involved in them.

Jon

Hall of Fame Super Silver

Re: EIGRP Metrics

Hello Jon,

I can only say thanks for your very kind words and for your help in reaching this new status!

Sometimes I wonder if we are too much involved in these forums, and I know that the answer is yes.

Some of my  job colleagues say that I have a second job here...

Best Regards

Giuseppe

Hall of Fame Super Blue

Re: EIGRP Metrics

Sometimes I wonder if we are too much involved in these forums, and I know that the answer is yes.

I think you may be right but then it beats watching tv or playing games in my opinion. I tend to think of it as one of my main hobbies.

With the added benefits of keeping your skills up to date and just as importantly helping people out when they are stuck for a solution. I'm sure we have all been in the position where we are banging our heads against the wall trying to understand how to do something and to be able to help people out makes it all worthwhile.

Plus it's nice to get a star or 2

Jon

Re: EIGRP Metrics

Hi all,

Giuseppe, you have my most sincere congratulations! I could see this coming yesterday. I wanted to help like Jon did, but in some of your latest posts I didn't have the necessary knowledge to judge your posts. In reading one of those posts I ended up studying 7600 datasheets at 4 a.m. in the morning!!! Before deciding to finally go to sleep I thought "hey, he's gonna make it anyway, doesn't need my help"! And also thought "I'll just write some code. When point threshold is crossed, an alarm will go off and I will know it's time to see the star changing in front of my eyes". I envy Jon for stealing my thunder!

You guys are more involved than I am. Still, I totally agree with Jon's view about this hobby and the rest. I am more impressed with how technology works rather than the end user services of the internet. If I have some free time, I really don't know what to do with the internet! Besides, in these forums there are many more opportunities for fun than it seems at a first glance!

Keep up the good work and have fun!

Best Regards,

Maria

Hall of Fame Super Blue

Re: EIGRP Metrics

Maria / Giuseppe

Before deciding to finally go to sleep I thought "hey, he's gonna make it anyway, doesn't need my help"!

Couldn't agree more. Giuseppe didn't need my help whatsoever, i just couldn't resist

I envy Jon for stealing my thunder!  

Jon

New Member

Re: EIGRP Metrics

Hi Giuseppe

Hope you are doing well. Merry Chirsmas and adv. New year wishes

After a short gap I am seeing you again.Thanks for sharing your valuable ideas

Infact I tested this in lab setup and using wireshark I captured the hello packet. But I couldnt trace any MTU parameter there. That is the seed of this post.

I googled enough to get the clue. But no use

However since you are said the reason for MTU involved in path decision, I will record it and go ahead.

I have rated it as extremely helpful and question answered

see you again

sairam

Hall of Fame Super Silver

Re: EIGRP Metrics

Hello Sairam,

just a little add:

the router hop count has also another use: in EIGRP there is (or there was?) a max hop count concept that has default values of 100 (or 224?) that I never see in action but were in some test questions when I was studying for CCNP.

These kind of protocol details are reported in books like:

Routing TCP/IP volume I by Jeff Doyle and Jennifer Dehaven Carroll that is a must

you need to capture the update packets to see the TLV of prefixes they are not sent in hellos

Edit:

I wish you an Happy New Year and peaceful

Hope to  help

Giuseppe

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