Dell Force10 Z9000 FTOS Command Line Reference Guide for the Z9000 System FTOS - Page 714
Link MTU and IP MTU Delta, Tagged Packet
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Command History Usage Information Version 8.3.11.1 Version 8.3.7.0 Version 8.1.1.0 Version 7.5.1.0 pre-Version 6.2.1.0 Introduced on the Z9000. Introduced on the S4810. Introduced on the E-Series ExaScale. Introduced on the C-Series. Introduced on the E-Series. If the packet includes a Layer 2 header, the difference between the link MTU and IP MTU (ip mtu command) must be enough bytes to include the Layer 2 header: • On the C-Series, the IP MTU gets adjusted automatically when the Layer 2 MTU is configured with the mtu command. • On the E-Series, you must compensate for a Layer 2 header when configuring IP MTU and link MTU on an Ethernet interface. Use the ip mtu command. When you enter the no mtu command, FTOS reduces the IP MTU value to 1536 bytes. On the E-Series, to return the IP MTU value to the default, enter no ip mtu. Link MTU and IP MTU considerations for port channels and VLANs are as follows. port channels: • All members must have the same link MTU value and the same IP MTU value. • The port channel link MTU and IP MTU must be less than or equal to the link MTU and IP MTU values configured on the channel members. For example, if the members have a link MTU of 2100 and an IP MTU 2000, the port channel's MTU values cannot be higher than 2100 for link MTU or 2000 bytes for IP MTU. VLANs: • All members of a VLAN must have same IP MTU value. • Members can have different Link MTU values. Tagged members must have a link MTU 4 bytes higher than untagged members to account for the packet tag. • The VLAN link MTU and IP MTU must be less than or equal to the link MTU and IP MTU values configured on the VLAN members. For example, the VLAN contains tagged members with Link MTU of 1522 and IP MTU of 1500 and untagged members with Link MTU of 1518 and IP MTU of 1500. The VLAN's Link MTU cannot be higher than 1518 bytes and its IP MTU cannot be higher than 1500 bytes. The following shows the difference between Link MTU and IP MTU. Layer 2 Overhead Ethernet (untagged) VLAN Tag Untagged Packet with VLAN-Stack Header Tagged Packet with VLAN-Stack Header Link MTU and IP MTU Delta 18 bytes 22 bytes 22 bytes 26 bytes 714
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