Introduction
This document provides a brief overview of the Path Maximum Transmission Unit (MTU) Discovery algorithm for Version 6.0+ of the Wireless LAN Controller (WLC) and the Next Generation Wiring Closet (NGWC).
Network Diagram
Sender (Access Point) ----- (Multiprotocol Label Switching (MPLS)/Router) ----- Reciever (WLC/NGWC)
Initial Setting
The sender detects the default MTU and sends a discovery request (Access Point (AP) to WLC). The request is 1500 bytes in size with a Don't Fragment (DF) bit set. If the reciever receives it, the reciever replies with another 1500 bytes packet. When the sender receives the reply from the reciever, the MTU path is set at 1500 MTU.
In cases where the reciever does not receive the discovery request (or vice versa), the MTU is set to 576 and the dynamic discovery process starts.
Dynamic Path Maximum Transmission Unit (PMTU) Discovery
If allowed, MTU information is in the Internet Control Message Protocol (ICMP) message, then the sender/reciever sets that as the next MTU to try. If no information is available, the sender/reciever gets an ICMP error (similiar to the one here), the initial MTU size is set at 576, and dynamic discovery is in play.
Every 30 seconds, the sender attempts to increase its MTU in order to check whether the path has improved. The sender increases the MTU size to the next level of 576, 1006, 1492, and 1500. Based on the configuration of the router, the customer might see these ICMP errors every 30 seconds per AP.
ICMP Error over MPLS:
MPLS: ICMP: dst (10.10.10.10) frag. needed and DF set unreachable sent to 192.168.1.5
Verify
Enter this command in order to verify that everything works properly.
3850#show ap config general | b <APname>
.
CAPWAP Path MTU : 1500
Controller) >show ap config general <APname>
.
.
CAPWAP Path MTU.................................. 1500
Troubleshoot
Perform a packet capture on the AP switchport in order to troubleshoot issues.
(Requires a Cisco Service Contract.) 
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