Jumbo frames

The maximum transmission unit(MTU) is the maximum size IP frame the switch can receive for Layer 2 frames inbound on a port. The switch drops any inbound frames larger than the MTU allowed on the port. Ports operating at a minimum of 1 Gbps can accept forward frames of up to 9220 bytes (including four bytes for a VLAN tag) when configured for jumbo traffic. You can enable inbound jumbo frames on a per-VLAN basis. That is, on a VLAN configured for jumbo traffic, all ports belonging to that VLAN and operating at a minimum of 1 Gbps allow inbound jumbo frames of up to 9220 bytes.

Operating rules

  • Required port speed: This feature allows inbound and outbound jumbo frames on ports operating at a minimum of 1 Gbps.

  • GVRP operation: A VLAN enabled for jumbo traffic cannot be used to create a dynamic VLAN. A port belonging to a statically configured, jumbo-enabled VLAN cannot join a dynamic VLAN.

  • Port adds and moves: If you add a port to a VLAN that is already configured for jumbo traffic, the switch enables that port to receive jumbo traffic. If you remove a port from a jumbo-enabled VLAN, the switch disables jumbo traffic capability on the port only if the port is not currently a member of another jumbo-enabled VLAN. This same operation applies to port trunks.

  • Jumbo traffic sources: A port belonging to a jumbo-enabled VLAN can receive inbound jumbo frames through any VLAN to which it belongs, including non-jumbo VLANs. For example, if VLAN 10 (without jumbos enabled) and VLAN 20 (with jumbos enabled) are both configured on a switch, and port 1 belongs to both VLANs, port 1 can receive jumbo traffic from devices on either VLAN. For a method to allow only some ports in a VLAN to receive jumbo traffic, see Configuring a maximum frame size.

Configuring jumbo frame operation

For detailed information about jumbo frames, see Jumbo frames.

Overview

  1. Determine the VLAN membership of the ports or trunks through which you want the switch to accept inbound jumbo traffic. For operation with GVRP enabled, refer to the GVRP topic under “Operating Rules”, above.

  2. Ensure that the ports through which you want the switch to receive jumbo frames are operating at least at gigabit speed. (Check the Mode field in the output for the show interfaces brief <port-list> command.)

  3. Use the jumbo command to enable jumbo frames on one or more VLANs statically configured in the switch. (All ports belonging to a jumbo-enabled VLAN can receive jumbo frames.

  4. Execute write memory to save your configuration changes to the startupconfig file.

Viewing the current jumbo configuration

Syntax:

show vlans

Lists the static VLANs configured on the switch and includes a Jumbo column to indicate which VLANs are configured to support inbound jumbo traffic. All ports belonging to a jumbo-enabled VLAN can receive jumbo traffic. (For more information, see Configuring a maximum frame size.) See Figure Example: listing of static VLANs to show jumbo status per VLAN.

Example: listing of static VLANs to show jumbo status per VLAN

Example: listing of static VLANs to show jumbo status per VLAN
Syntax:

show vlans ports <port-list>

Lists the static VLANs to which the specified ports belong, including the Jumbo column to indicate which VLANs are configured to support jumbo traffic.

Entering only one port in <port-list> results in a list of all VLANs to which that port belongs.

Entering multiple ports in <port-list> results in a superset list that includes the VLAN memberships of all ports in the list, even though the individual ports in the list may belong to different subsets of the complete VLAN listing.

Example:

If port 1 belongs to VLAN 1, port 2 belongs to VLAN 10, and port 3 belongs to VLAN 15, executing this command with a port-list of 1 - 3 results in a listing of all three VLANs, even though none of the ports belong to all three VLANS. (See Listing the VLAN memberships for a range of ports.)

Listing the VLAN memberships for a range of ports

Listing the VLAN memberships for a range of ports
Syntax:

show vlans <vid>

Shows port membership and jumbo configuration for the specified vid . (See Example: of listing the port membership and jumbo status for a VLAN.)

Example: of listing the port membership and jumbo status for a VLAN

Example: of listing the port membership and jumbo status for a VLAN

Enabling or disabling jumbo traffic on a VLAN

Syntax:

vlan <vid> jumbo

[no] vlan <vid> jumbo

Configures the specified VLAN to allow jumbo frames on all ports on the switch that belong to that VLAN. If the VLAN is not already configured on the switch, vlan <vid> jumbo also creates the VLAN.

A port belonging to one jumbo VLAN can receive jumbo frames through any other VLAN statically configured on the switch, regardless of whether the other VLAN is enabled for jumbo frames.

The [no] form of the command disables inbound jumbo traffic on all ports in the specified VLAN that do not also belong to another VLAN that is enabled for jumbo traffic. In a VLAN context, the command forms are jumbo and no jumbo.

(Default: Jumbos disabled on the specified VLAN.)

Configuring a maximum frame size

You can globally set a maximum frame size for jumbo frames that will support values from 1518 bytes to 9216 bytes for untagged frames.

Syntax:

jumbo max-frame-size <size>

Sets the maximum frame size for jumbo frames. The range is from 1518 bytes to 9216 bytes. (Default: 9216 bytes)


[NOTE: ]

NOTE: The jumbo max-frame-size is set on a GLOBAL level.

Default: 9216 bytes


Configuring IP MTU


[NOTE: ]

NOTE: The following feature is available on the switches covered in this guide. jumbos support is required for this feature. On switches that do not support this command, the IP MTU value is derived from the maximum frame size and is not configurable.


You can set the IP MTU globally by entering this command. The value of max-frame-size must be greater than or equal to 18 bytes more than the value selected for ip-mtu. For example, if ip-mtu is set to 8964, the max-frame-size is configured as 8982.

Syntax:

jumbo ip-mtu <size>

Globally sets the IP MTU size. Values range between 1500 and 9198 bytes. This value must be 18 bytes less than the value of max-frame-size.

(Default: 9198 bytes)

SNMP implementation

Jumbo maximum frame size

The maximum frame size for jumbos is supported with the following proprietary MIB object:

hpSwitchMaxFrameSize OBJECT-TYPE

This is the value of the global max-frame-size supported by the switch. The default value is set to 9216 bytes.

Jumbo IP MTU

The IP MTU for jumbos is supported with the following proprietary MIB object:

hpSwitchIpMTU OBJECT-TYPE

This is the value of the global jumbos IP MTU (or L3 MTU) supported by the switch. The default value is set to 9198 bytes (a value that is 18 bytes less than the largest possible maximum frame size of 9216 bytes). This object can be used only in switches that support max-frame-size and ip-mtu configuration.

Displaying the maximum frame size

Use the show jumbos command to display the globally configured untagged maximum frame size for the switch, as shown in the following Example:.

HP Switch(config)# show jumbos

 Jumbos Global Values

  Configured : MaxFrameSize : 9216   Ip-MTU : 9198
  In Use     : MaxFrameSize : 9216   Ip-MTU : 9198

For more information about frame size, see Jumbo frames.

Operating notes for maximum frame size

  • When you set a maximum frame size for jumbo frames, it must be on a global level. You cannot use the jumbo max-frame-size command on a per-port or per-VLAN basis.

  • The original way to configure jumbo frames remains the same, which is per-VLAN, but you cannot set a maximum frame size per-VLAN.

  • Jumbo support must be enabled for a VLAN from the CLI or through SNMP.

  • Setting the maximum frame size does not require a reboot.

  • When you upgrade to a version of software that supports setting the maximum frame size from a version that did not, the max-frame-size value is set automatically to 9216 bytes.

  • Configuring a jumbo maximum frame size on a VLAN allows frames up to max-frame-size even though other VLANs of which the port is a member are not enabled for jumbo support.

Operating notes for jumbo traffic-handling

  • HP Switch does not recommend configuring avoice VLAN to accept jumbo frames. Voice VLAN frames are typically small, and allowing a voice VLAN to accept jumbo frame traffic can degrade the voice transmission performance.

  • You can configure the default, primary, and/or (if configured) the management VLAN to accept jumbo frames on all ports belonging to the VLAN.

  • When the switch applies the default MTU (1522-bytes including 4 bytes for the VLAN tag) to a VLAN, all ports in the VLAN can receive incoming frames of up to 1522 bytes. When the switch applies the jumbo MTU (9220 bytes including 4 bytes for the VLAN tag) to a VLAN, all ports in that VLAN can receive incoming frames of up to 9220 bytes. A port receiving frames exceeding the applicable MTU drops such frames, causing the switch to generate an Event Log message and increment the "Giant Rx" counter (displayed by show interfaces <port-list>).

  • The switch allows flow control and jumbo frame capability to co-exist on a port.

  • The default MTU is 1522 bytes (including 4 bytes for the VLAN tag). The jumbo MTU is 9220 bytes (including 4 bytes for the VLAN tag).

  • When a port is not a member of any jumbo-enabled VLAN, it drops all jumbo traffic. If the port is receiving "excessive"inbound jumbo traffic, the port generates an Event Log message to notify you of this condition. This same condition also increments the switch's "Giant Rx" counter.

  • If you do not want all ports in a given VLAN to accept jumbo frames, you can consider creating one or more jumbo VLANs with a membership comprising only the ports you want to receive jumbo traffic. Because a port belonging to one jumbo-enabled VLAN can receive jumbo frames through any VLAN to which it belongs, this method enables you to include both jumbo-enabled and non-jumbo ports within the same VLAN.

    For example, suppose you want to allow inbound jumbo frames only on ports 6, 7, 12, and 13. However, these ports are spread across VLAN 100 and VLAN 200 and also share these VLANs with other ports you want excluded from jumbo traffic. A solution is to create a third VLAN with the sole purpose of enabling jumbo traffic on the desired ports, while leaving the other ports on the switch disabled for jumbo traffic. That is:

      VLAN 100 VLAN 200 VLAN 300
    Ports 6-10 11-15 6, 7, 12, and 13
    Jumbo-enabled? No No Yes

    If there are security concerns with grouping the ports as shown for VLAN 300, you can either use source-port filtering to block unwanted traffic paths or create separate jumbo VLANs, one for ports 6 and 7, and another for ports 12 and 13.

  • Outbound jumbo traffic. Any port operating at 1 Gbps or higher can transmit outbound jumbo frames through any VLAN, regardless of the jumbo configuration. The VLAN is not required to be jumbo-enabled, and the port is not required to belong to any other, jumbo-enabled VLANs. This can occur in situations where a non-jumbo VLAN includes some ports that do not belong to another, jumbo-enabled VLAN and some ports that do belong to another, jumbo-enabled VLAN. In this case, ports capable of receiving jumbo frames can forward them to the ports in the VLAN that do not have jumbo capability, as shown in Forwarding jumbo frames through non-jumbo ports.

    Forwarding jumbo frames through non-jumbo ports

    Forwarding jumbo frames through non-jumbo ports

    Jumbo frames can also be forwarded out non-jumbo ports when the jumbo frames received inbound on a jumbo-enabled VLAN are routed to another, non-jumbo VLAN for outbound transmission on ports that have no memberships in other, jumbo-capable VLANs. Where either of the above scenarios is a possibility, the downstream device must be configured to accept the jumbo traffic. Otherwise, this traffic will be dropped by the downstream device.

Troubleshooting

A VLAN is configured to allow jumbo frames, but one or more ports drops all inbound jumbo frames

The port may not be operating at a minimum of 1 Gbps on the other switches covered in this guide. Regardless of a port's configuration, if it is actually operating at a speed lower than 1 Gbps for the other switches, it drops inbound jumbo frames. For example, if a port is configured for Auto mode (speed-duplex auto), but has negotiated a 7 Mbps speed with the device at the other end of the link, the port cannot receive inbound jumbo frames. To determine the actual operating speed of one or more ports, view the Mode field in the output for the following command:

show interfaces brief <port-list>

A non-jumbo port is generating "Excessive undersize/giant frames" messages in the Event Log

The switches can transmit outbound jumbo traffic on any port, regardless of whether the port belongs to a jumbo VLAN. In this case, another port in the same VLAN on the switch may be jumbo-enabled through membership in a different, jumbo-enabled VLAN, and may be forwarding jumbo frames received on the jumbo VLAN to non-jumbo ports.