MTU size

We were getting a lot of errors on the input/CRC etc on T3 circuit setup on CE on MPLS site. Upon removing, MTU size of 1500 from serial int config, the errors went away. However, upon show int <serial int>, we see the MTU size of 4470. Where is this new MTU size coming from, is it the provider or some setting on PE?  Can someone explain the dynamics of the interface errors. Please let me know.
totaramAsked:
Who is Participating?
I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

giltjrCommented:
What is the hardware device type and model.

Some routers/L3 switches have MTU set at the "system" level and all interfaces use that as the default MTU.
0
totaramAuthor Commented:
In this case, hw is Cisco7204 VXR.... how does one find the defualt MTU Val set @ the system level??
0
giltjrCommented:
I will have to double check but try "show system mtu", "show system jumbo", "show jumbo".
0
Powerful Yet Easy-to-Use Network Monitoring

Identify excessive bandwidth utilization or unexpected application traffic with SolarWinds Bandwidth Analyzer Pack.

surbabu140977Commented:
serial/T3 links in 7204 will use default system MTU size which is 4470.

Depending on the adapter you are using, it can be configured for channelized T1 with mtu 1500.
(may not be your requirement)

You need to have a jumbo frame enabled switch at L2, else this large mtu size will cause issue due to excessive fragmentation.


Best,
0
giltjrCommented:
I have not worked with MPLS networks  before, but I would also check with your MPLS provider.  I would assume with the MPLS tagging that your MTU would need to be bigger than 1500.
0
surbabu140977Commented:
Just editing my reply because just forgot the pop function. The SP, before sending the MPLS frame back to customer strips off any MPLS tagging and hence the frame arrives(should arrive)  at 1500 bytes only.

4470 is the max supported size of the port adapter. There will be no fragmentation till that size. So although it's showing 4470, I am sure it's receiving MTU much lesser size than that.

Does not look like you will have an MTU issue here.

Best,
0
giltjrCommented:
Assuming that you are end point/last hop on the MPLS network, yes the MPLS tags are remove.

However, if you are a middle point routing you would still see them.

I would run a packet capture on the serial  interface and see what you are receiving.

Although I would not expect to see CRC errors because of a MTU mismatch.
0
totaramAuthor Commented:
Giltjr;
None of the above command work.. sh system or sh jumbo...

Is there a way to display what MTU size is received from service provider assuming it is being tagged?
0
surbabu140977Commented:
I agree with Giltjr  that MTU size won't give you interface errors. If interface errors are the one we are looking for, something else is there.  Packet capture can tell you the mtu size. I don't recall anything in router that shows mtu size of received packets.....

Best,
0
totaramAuthor Commented:
Yes, we are looking to find the source of errors on input.. could it be framing from the provider??
0
surbabu140977Commented:
It seems to be physical in nature. Probably one or more T1's are generating errors.

First test to be done is Hard plug loopback tests  to clear the router and the t3 port adapter of fault.

input errors typically are never alone. There will be other accompanying errors like crc or drops. Please clear the counters and post the "sh int serialXX"  output. If too many error counters are there in 1 hr, you should ask your SP to do a intrusive testing so that they can see those errors.

They can test individual T1's with BERT. If you are comfortable enough, you can yourself do loop testing on every T1 to determine which one is faulty.

Best,
0
Craig BeckCommented:
4470 is the default MTU for ATM interfaces.

Input/Output errors are usually always physical (bad cable, etc) or interface negotiation issues, however if you've set the MTU to be too small you might see input errors although generally the receiving interface should see them as Giants.
0

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
totaramAuthor Commented:
Does framing play any role in input error.. right now it is M23 for DS3 connection and think it is not right..
0
Craig BeckCommented:
I wouldn't expect it to.  If you do have a framing issue though there should be an LOF alarm on the physical interface if you look at the router.

But again, that usually comes back to a physical issue such as a bad cable, or the framing type is incorrect.
0
giltjrCommented:
Was it working at all?  I would expect it not to work at all if the framing was set incorrectly and changing MTU would not cause it to get better.
0
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today
Networking Hardware-Other

From novice to tech pro — start learning today.