# MTU Troubleshooting on Cisco IOS

**URL:** https://forum.networklessons.com/t/mtu-troubleshooting-on-cisco-ios/1078
**Category:** Lessons Discussion
**Created:** [December 26, 2016, 10:25pm UTC](https://forum.networklessons.com/t/mtu-troubleshooting-on-cisco-ios/1078 "2016-12-26T22:25:30Z")
**Posts on this page:** 1
**Showing post:** 231

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### Author: ![davidilles](https://cdn-forum.networklessons.com/user_avatar/forum.networklessons.com/davidilles/32/9686_2.png) [@davidilles](https://forum.networklessons.com/u/davidilles)
#### Post date: [June 4, 2024, 11:55am UTC](https://forum.networklessons.com/t/mtu-troubleshooting-on-cisco-ios/1078/231 "2024-06-04T11:55:00Z")

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Hello, everyone.

About Path MTU Discovery. If the client notices that its 1500-byte packet was dropped because the MTU was lowered somewhere to 1476 (or just a lower value), what exactly will it do with the 1500-byte data? It won’t fragment them, right? Will it just break the packet down into smaller pieces, or? But that’s fragmentation, isn’t it? And that’s undesired. 😄

Or is it safe to say that fragmentation or reassembly is okay on the hosts as they have time to do all that but not on a networking device like a router whose job is to forward packets as fast as possible?

Also, when a packet fragment is dropped, why do we retransmit the entire packet and not just the fragment?

Thank you.

David

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