Reference Guide

394 | Interfaces
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The MTU range is 592-12000, with a default of 1500. The S-Series automatically configure the IP MTU.
Table 20-4 lists the various Layer 2 overheads found in FTOS and the number of bytes.
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.
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 the 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.
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.
:
Adjust the keepalive timer
Use the keepalive command to change the time interval between keepalive messages on the interfaces. The
interface sends keepalive messages to itself to test network connectivity on the interface.
To change the default time interval between keepalive messages, use the following command:
Table 20-4. Difference between Link MTU and IP MTU
Layer 2 Overhead Difference between Link MTU and IP MTU
Ethernet (untagged) 18 bytes
VLAN Tag 22 bytes
Untagged Packet with VLAN-Stack Header 22 bytes
Tagged Packet with VLAN-Stack Header 26 bytes
Command Syntax Command Mode Purpose
keepalive INTERFACE Change the default interval between keepalive
messages.