4.2. Configuring the MTU in an existing NetworkManager connection profile

Procedure

1. Optional: Display the current MTU:

   # ip link show
   ...
   3: enp1s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
       link/ether 52:54:00:74:79:56 brd ff:ff:ff:ff:ff:ff
   ...

2. Optional: Display the NetworkManager connection profiles:

   # nmcli connection show
   NAME     UUID                                  TYPE      DEVICE
   Example  f2f33f29-bb5c-3a07-9069-be72eaec3ecf  ethernet  enp1s0
   ...

3. Set the MTU in the profile that manages the connection to the network with the divergent MTU:

   # nmcli connection modify Example mtu 9000

4. Reactivate the connection:

   # nmcli connection up Example

Verification

1. Display the MTU setting:

   # ip link show
   ...
   3: enp1s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9000 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
       link/ether 52:54:00:74:79:56 brd ff:ff:ff:ff:ff:ff
   ...

2. Verify that no host on the transmission paths fragments the packets:

   • On the receiver side, display the IP reassembly statistics of the kernel:

     # nstat -az IpReasm*
     #kernel
     IpReasmTimeout 0 0.0
     IpReasmReqds 0 0.0
     IpReasmOKs 0 0.0
     IpReasmFails 0 0.0

     If the counters return 0, packets were not reassembled.

   • On the sender side, transmit an ICMP request with the prohibit-fragmentation-bit:

     # ping -c1 -Mdo -s 8972 destination_host

     If the command succeeds, the packet was not fragmented.

     Calculate the value for the -s packet size option as follows: MTU size - 8 bytes ICMP header - 20 bytes IPv4 header = packet size