Interfaces | 453Configure MTU Size on an InterfaceIf a packet includes a Layer 2 header, the difference in bytes between the link MTU and IP MTU must belarge enough to include the Layer 2 header. For example, for VLAN packets, if the IP MTU is 1400 bytes,the Link MTU must 1422 bytes or greater to accommodate the 22-byte VLAN header:1400-byte IP MTU + 22-byte VLAN Tag = 1422-byte link MTUOn the E-Series and C-Series, you configure the Link MTU size on an interface by entering the mtucommand.On the E-Series, you must manually configure the IP MTU size on an interface by entering the ip mtucommand. On the C-Series and S-Series, the IP MTU size is automatically configured on an interface bytaking into account the Layer 2 overheads and difference in the number of bytes as shown in Table 20-9.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 MTUvalues configured on the channel members.Example: If the members have a link MTU of 2100 and an IP MTU 2000, the port channel’s MTU valuescannot be higher than 2100 for link MTU or 2000 bytes for IP MTU.VLANs:• All members of a VLAN must have the same IP MTU value.• Members can have different Link MTU values. Tagged members must have a link MTU 4 bytes higherthan 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 valuesconfigured on the VLAN members.Example: The VLAN contains tagged members with Link MTU of 1522 and IP MTU of 1500 anduntagged members with Link MTU of 1518 and IP MTU of 1500. The VLAN’s Link MTU cannot behigher than 1518 bytes and its IP MTU cannot be higher than 1500 bytes.Table 20-9. Difference between Link MTU and IP MTULayer 2 Overhead Difference between link MTU and IP MTUEthernet (untagged) 18 bytesVLAN Tag 22 bytesUntagged Packet with VLAN-Stack Header 22 bytesTagged Packet with VLAN-Stack Header 26 bytes
