MTU question

[Zach8585]

Quick question about MTU...

Having some point-to-point issue with voice over IP phones and the vendor is asking me to try changing the MTU.  My question is;

Can I have a different MTU setting from the core switch to the remote location (via point-to-point ISP connection) than I do with the rest of the network?  Or does the whole network have to match?  I am just trying to prove to the vendor that it isn't an MTU issue, but I don't want to mess up the rest of the network if I do increase the MTU to a higher level on the link between the two locations.

Thanks in advance

[M02@rt37]

Hello @Zach8585,

Yes, it's posible to have a different MTU setting between the core switch and the remote location via a point-to-point ISP connection than the rest of the network. The MTU setting can be configured on a per-interface basis, so you can adjust it for the specific link between the two locations without affecting the rest of the network. However, it is important to ensure that the MTU setting is consistent across all devices within the same network segment to avoid fragmentation and performance issues. If you do increase the MTU on the link, it's a good idea to test the impact on the network and ensure that all devices are properly configured.

[MHM Cisco World]

this is why we select high end SW for Core, 

SP usually run default 1500 so from Core-SP is 1500
inside your network sometims you need to run jumbo or baby giant frame, here the Core do hard work of fragment and defragment.

[Joseph W. Doherty]

". . . the vendor is asking me to try changing the MTU."

For VoIP?

Hmm, "interesting" suggestion by your vendor, as VoIP packets tend to be small, so small, hard to image them bumping into MTU issues.

It's possible, the MTU issues can cause issues for many kinds of traffic, but again, usually not VoIP.

However, it's also possible, some MTU issues can indirectly cause VoIP issues, by overloading the device or the link, but as most MTU issues are due to MTU being too small for much of the non-VoIP traffic, and that adjusting MTU is very seldom done, unclear what kind of MYU change they would want you to try.

Often VoIP issues are more due to congestion, when sharing a link or path with non-VoIP traffic.  This often especially true when working with ISP as they generally provide no QoS support, which is often commonly needed to insure VoIP traffic obtains the network resources to work correctly.

What do your interface stats look like for this ISP connection?

[Zach8585]

I think they're desperate to try and pin this issue on us and a network issue when I'm pretty confident that it is not a network issue, but an issue with their equipment on our network.

But I'm playing along with them for a little bit before I put my foot down.  I'm now testing MTU for them.

[bettygdurand]

Ideally, the Maximum Transmission Unit (MTU) should be consistent across your network infrastructure to ensure smooth and reliable communication between devices. Having a different MTU setting between the core switch and the remote location could potentially cause issues, especially if packets need to traverse different network segments with varying MTU sizes.

However, in certain scenarios, it is possible to have different MTU settings for specific network segments. This is known as path MTU discovery, where the MTU is dynamically adjusted based on the path between two communicating devices. In such cases, the network devices along the path negotiate the appropriate MTU size for the specific link.

If you want to test whether the MTU is causing the issue with your VoIP phones, you can temporarily adjust the MTU on the link between the core switch and the remote location. Keep in mind the following considerations:

1. Document the current MTU settings: Before making any changes, ensure you have a record of the existing MTU settings on all relevant network devices to revert them if necessary ehsaas program.

2. Make targeted changes: Adjust the MTU only on the specific devices or interfaces involved in the point-to-point connection between the core switch and the remote location. This helps contain the impact to that particular segment.

3. Test and monitor: After changing the MTU, test the VoIP phones to see if the issue persists. Monitor the network closely for any adverse effects or performance degradation.

4. Revert if necessary: If changing the MTU does not resolve the issue or causes other problems, revert the MTU settings back to their original values.

[Joseph W. Doherty]

However, in certain scenarios, it is possible to have different MTU settings for specific network segments. <Agreed> This is known as path MTU discovery <sorry, confused - what is known as PMTUD the former sentence or the following clause?  Former sentence is not PMTUD, following clause isn't quite fully correct, either, as sending host might choose to disable PMTUD, i.e. allow fragmentation.  However, adjusting sender's MTU would be the normal response.>, where the MTU is dynamically adjusted based on the path between two communicating devices. In such cases, the network devices along the path negotiate the appropriate MTU size for the specific link <sorry, disagree, network devices along path don't "negotiate" - details of PMTUD>.