BigIron RX Series Configuration Guide 18153-1002253-01Configuring packet parameters 7To configure the untagged max-frame-size on a VLAN, enter a command such as the following at heInterface Configuration level.BigIron RX(config-vlan-20)#BigIron RX(config-vlan-20)#max-frame-size 5000Please reload system!BigIron RX(config-vlan-20)#Syntax: max-frame-size The variable specifies the maximum frame size for each port that is connected thesame PPCR as described in Table 49. Values can be from 64 to 9212 bytes. The default is 1518bytes.Changing the MTUThe IP MTU is the maximum length of an IP packet that a Layer 2 packet can contain. If an IPpacket is larger than the IP MTU allowed by the Layer 2 packet, the device fragments the IP packetinto multiple parts that will fit into Layer 2 packets, and sends the parts of the fragmented IPpacket separately, in different Layer 2 packets. The device that receives the multiple fragments ofthe IP packet reassembles the fragments into the original packet.The default IP MTU is 1500 bytes for Ethernet II packets and 1492 for Ethernet SNAP packets. Youcan change the IP MTU globally or an individual ports. You can increase the IP MTU size toaccommodate large packet sizes, such as jumbo packets, globally or on individual physical ports.However, IP MTU cannot be set higher than the maximum frame size, minus 18.For jumbo packet, the device supports hardware forwarding of Layer 3 jumbo packets. Layer 3 IPunicast jumbo packets received on a port that supports the frame's IP MTU size and forwarded toanother port that also supports the frame's IP MTU size are forwarded in hardware.Configuration considerations for Increasing the IP MTUConsider the following before configuring the maximum value to increase the IP MTU:• The maximum value of an IP MTU cannot exceed the configured maximum frame size (jumboframe), minus 18. For example, global IP MTU cannot exceed the value ofdefault-max-frame-size, minus 18 bytes. IP MTU for an interface cannot exceed the value of themaximum frame size configured on a port, minus 18 bytes. The 18 bytes is used for IPoverhead, VLAN tagging, etc.• When you increase the IP MTU size of a port, the increase uses system resources. Increasethe IP MTU size only on the ports that need it. For example, if you have one port connected toa server that uses jumbo frames and two other ports connected to clients that can support thejumbo frames, increase the IP MTU only on those three ports. Leave the IP MTU size on theother ports at the default value (1500 bytes). Globally increase the IP MTU size only if needed.• Use the same IP MTU size on all ports that will be supporting jumbo frames. If the deviceneeds to fragment a jumbo frame (and the frame does not have the DF bit set), the devicefragments the frame into 1500-byte fragments, even if the outbound port has a larger IP MTU.For example, if a port has an IP MTU setting of 8000 and receives an 8000-byte frame, thenmust forward the frame onto a port with an IP MTU of 4000, the device does not fragment the8000-byte frame into two 4000-byte frames. Instead, the device fragments the 8000-byteframe into six fragments (five 1500-byte fragments and a final, smaller fragment.)
