Configuring MTU

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Depending on the value of MTU set in HiperSockets or external interfaces, HSCI might benefit from having a corresponding MTU value set.

Before you begin

This task assumes that you have already configured the MTU of HiperSockets and external interfaces appropriately, see Configuring and activating an interface and Finding out the maximum frame size.

By default, HSCI uses an MTU of 1500 bytes, regardless of the MTU values configured for HiperSockets and the external interface. If the external interface uses an MTU of 1500 and the HiperSockets interface has an MTU greater than or equal to 1500, HSCI will operate correctly without additional adjustments.

About this task

If either the HiperSockets interface or the external interface is configured with an MTU smaller than 1500, the MTU of HSCI must be set to the lower of those values to prevent network issues.

If both interfaces support MTU values greater than 1500, HSCI will still function correctly, but increasing its MTU to the lower of those values enables the use of jumbo frames, which can improve throughput.

Note: Jumbo frames can increase network latency and require end‑to‑end support in the network infrastructure.

Procedure

  1. Find the current MTU of HiperSockets and external interface.
    Issue a command of the following form: 
    # ip link show <interface>
    For example, assuming the interfaces are encb400 and enc8000, use: 
    # ip link show encb400
3: encb400: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9000 qdisc mq master hsci8000-br state UP mode DEFAULT group default qlen 1000 link/ether ce:b8:d3:01:ae:7a brd ff:ff:ff:ff:ff:ff
    and 
    # ip link show enc8000
9: enc8000: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 8192 qdisc mq master hsci8000-br state UP mode DEFAULT group default qlen 1000
link/ether fe:e7:f0:27:00:02 brd ff:ff:ff:ff:ff:ff altname enxfee7f0270002
  2. Set the HSCI MTU to the lower of the values found.
    Issue ip commands of the following form: 
    # ip link set dev hsci<hsi_bus_id> mtu <mtu_value>
# ip link set dev hsci<hsi_bus_id>-bp mtu <mtu_value>
    For example, assuming the interfaces from previous example form HSCI hsci8000, use: 
    # ip link set dev hsci8000 mtu 8192
# ip link set dev hsci8000-bp mtu 8192

Results

The HSCI setup can now use jumbo frames.