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FTOS Configuration Guide for the C-Series - Force10 Networks

FTOS Configuration Guide for the C-Series - Force10 Networks

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Configuring MTU Size on an Interface<br />

You must compensate <strong>for</strong> Layer-2 header when configuring IP MTU. If <strong>the</strong> packet includes a Layer-2<br />

header, <strong>the</strong> difference between <strong>the</strong> link MTU and IP MTU must be enough bytes to include <strong>for</strong> <strong>the</strong> Layer-2<br />

header. For example, <strong>for</strong> VLAN packets, if <strong>the</strong> IP MTU is 1400, <strong>the</strong> Link MTU must be no less than 1422:<br />

1400 IP MTU + 22 VLAN Tag = 1422 bytes Link MTU<br />

Table 21 lists <strong>the</strong> various Layer 2 overheads found in <strong>FTOS</strong> and <strong>the</strong> number of bytes.<br />

Table 21 Difference between Link MTU and IP MTU<br />

Layer-2 Overhead Difference between Link MTU and IP MTU<br />

E<strong>the</strong>rnet (untagged) 18 bytes<br />

VLAN Tag 22 bytes<br />

Untagged Packet with VLAN-Stack Header 22 bytes<br />

Tagged Packet with VLAN-Stack Header 26 bytes<br />

Link MTU and IP MTU considerations <strong>for</strong> port channels and VLANs are as follows.<br />

Port Channels:<br />

• All members must have <strong>the</strong> same link MTU value and <strong>the</strong> same IP MTU value.<br />

• The port channel link MTU and IP MTU must be less than or equal to <strong>the</strong> link MTU and IP MTU<br />

values configured on <strong>the</strong> channel members.<br />

Example: if <strong>the</strong> members have a link MTU of 2100 and an IP MTU 2000, <strong>the</strong> port channel’s MTU values<br />

cannot be higher than 2100 <strong>for</strong> link MTU or 2000 bytes <strong>for</strong> IP MTU.<br />

VLANs:<br />

• All members of a VLAN must have same IP MTU value.<br />

• Members can have different Link MTU values. Tagged members must have a link MTU 4 bytes higher<br />

than untagged members to account <strong>for</strong> <strong>the</strong> packet tag.<br />

• The VLAN link MTU and IP MTU must be less than or equal to <strong>the</strong> link MTU and IP MTU values<br />

configured on <strong>the</strong> VLAN members.<br />

Example: The VLAN contains tagged members with Link MTU of 1522 and IP MTU of 1500 and<br />

untagged members with Link MTU of 1518 and IP MTU of 1500. The VLAN’s Link MTU cannot be<br />

higher than 1518 bytes and its IP MTU cannot be higher than 1500 bytes.<br />

<strong>FTOS</strong> <strong>Configuration</strong> <strong>Guide</strong>, version 7.7.1.0 281

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