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TCC_2
Advocate
Advocate

Core issue

The ability of various Cisco Catalyst switches to support different frame sizes depends on many factors, one of which is the hardware and software involved. Certain modules can support larger frame sizes than others, even within the same platform. Additionally, maximum frame size support can also change based on the software version used.

Jumbo frames are frames larger than the standard Ethernet frame size of 1518 bytes, which includes the Layer 2 header and Frame Check Sequence (FCS). In other words, jumbo frames refer to Ethernet packets of up to 9000 bytes in size. In Catalyst 4500 series switches, the Supervisor III and IV can handle packets up to a maximum size of 9198 bytes.

Jumbo frame support can be enabled on these types of interfaces:

  • Port-channel interface
  • Switched Virtual Interface (SVI)
  • Physical interface (Layer 2 (L2) / (L3))

When the jumbo frame feature is enabled on a port, the port can switch large or jumbo frames. This feature optimizes server-to-server performance. The default Maximum Transmission Unit (MTU) frame size is 1548 bytes for all Ethernet ports. The MTU size is increased to 9216 bytes when the jumbo frame feature is enabled on a port.

Resolution

On a Cisco Catalyst 3750 series switch, the jumbo frames are supported on the Gigabit Ethernet ports. The MTU cannot be changed on an individual interface. It must be set globally. The switch must be reset afterward in order for the MTU change to take effect.

Consider this example:

3750(config)#system mtu jumbo 9000

Note: The default MTU size is 1500 bytes. If a value is entered that is outside the allowed range for the specific type of interface, the value is not accepted.

Refer to the Catalyst 3750 Series section of Jumbo/Giant Frame Support on Catalyst Switches Configuration Example for more information.

Jumbo frame support is available in Cisco IOS  Software Release 12.1(13)EW and later for the Supervisor III and IV. On the 4000/4500 platform, jumbo frames are supported only on non-blocking Gigabit Ethernet ports on certain blade models. These are the Gigabit modules and specific ports that support jumbo frames:

  • Both supervisor uplink ports on Supervisor III (WS-X4013) and Supervisor IV (WS-X4014)
  • WS-X4306-GB
  • WS-X4232-GB-RJ, ports 1-2 only
  • WS-X4418-GB, ports 1-2 only
  • WS-X4412-2GB-TX, ports 13-14 only

The configuration of this feature is performed on each port, as shown:

sup3(config-if)#mtu 9198

Refer to these documents:

Comments
SgtMike70
Beginner
Beginner

"The default Maximum Transmission Unit (MTU) frame size is 1548 bytes for all Ethernet ports."

or

"Note: The default MTU size is 1500 bytes"

 

which is it?

douglasparsons
Beginner
Beginner

MTU and MTU Frame size are two different things. The Frame contains the MTU and thus is bigger. So with a MTU of 1500 the Frame size then becomes 1548 since the Frame contains all the data and header information as the packet transverses the switches and router. Where the MTU is the data contained within the Frame along side the header information that is generated by the switch itself as the packet is formed and handed off from the host to the switch.

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