Changing MTU size live! (no lan interruption)

[PatRoy]

Hi. Ive got the following routing table:

[root@NCRCI]:/etc/rc.config.d# netstat -rn
Routing tables
Destination Gateway Flags Refs Interface Pmtu
127.0.0.1 127.0.0.1 UH 0 lo0 32808
10.1.0.0 10.1.1.47 U 2 lan900 9000
127.0.0.0 127.0.0.1 U 0 lo0 32808
default 10.1.1.254 UG 0 lan900 1500

I'd like to bring down the MTU size of 10.1.0.0 down to 1500 instead of the jumbo frames of 9000. Is it possible to do this live without network interuptions? Running HPUX 11.31. Thanks. Pat

[PatRoy]

The 9000 is currently configured through /etc/rc.config.d/hpietherconf for all NICS. I'd need to bring this down to 1500 without fear of affecting the server (prod server heavily used)

[kevin_m]

I've found it's simple to use SMH for changing MTU's on an individual adapter basis. However modifying lan900 always required stopping APA and updating each component interface individually. SMH wouldn't allow changes to the aggregate interface, and each NIC would stop responding for a few seconds. I doubt that helps for your situation but wanted to mention it in case.
- Kevin

[PatRoy]

Okay thanks.

[rick jones]

Beyond issues with APA, what is your definition of "no lan interruption?" IIRC changing the MTU (often) means a reset of the interface hardware, which will be a rather small interruption.

It _may_ be possible to leave the MTU as it is for the HW and alter the PMTU in the route. Certainly if the goal is to use a smaller MTU. I don't know if a route(1m) command can modify the PMTU of an existing routing entry or not, but the manpages might be worth a look.

[Sagar Sirdesai]

Below are the list of steps to change to mtu size to 1476 0n lan50.

nwmgr -s -f -c lan50 -A mtu=1476 --cu
lan50 current values:
New MTU = 1476

lanadmin -m 50
MTU Size = 1476


Now to make the changes available across reboot use the below command


nwmgr -s -A all --sa --fr cu -c lan50.
lan50 saved values:
New Speed = auto_on
New MTU = 1476
New MAC Address = 0x001e0bfdb0a6
New Receive Flow Control = On
New Transmit Checksum Offload = Off
New Receive Checksum Offload = Off
New Virtual MTU = 0
TCP Segmentation Offload is now disabled.
New Max Send Buffers = 1
New Max Send Coalesce Ticks = 150
New Max Recv Buffers = 1
New Max Recv Coalesce Ticks = 0
New Interrupt Throttle Mode = -1
New Diagnostics Threshold = 0

nwmgr -s -c lan52 -A mtu=1476 --cu
lan52 current values:
New MTU = 1476
nwmgr -s -c lan52 -A all --saved --from cu
lan52 saved values:
New Speed = auto_on
New MTU = 1476
New MAC Address = 0x001e0bfdc060
New Receive Flow Control = On
New Transmit Checksum Offload = Off
New Receive Checksum Offload = Off
New Virtual MTU = 0
TCP Segmentation Offload is now disabled.
New Max Send Buffers = 1
New Max Send Coalesce Ticks = 150
New Max Recv Buffers = 1
New Max Recv Coalesce Ticks = 0
New Interrupt Throttle Mode = -1
New Diagnostics Threshold = 0