OS10 Introduction to MTU Configuration

Key Topics Covered 

Default MTU Configuration

• The default MTU value is 9216 bytes starting from Release 10.5.4.4. Previously, it was 1532 bytes.
• Commands to configure and verify MTU values:
  • default mtu <value>: Sets a custom MTU value for all interfaces without a user configured MTU
  • no default mtu: Resets interfaces to the system default MTU value
  • show default mtu: Displays the system-level default MTU
  • show interface: Shows the current MTU for a specific interface 

MTU Configuration Examples

• Examples of how to display and modify the default MTU by using CLI commands
• Configuration scenarios for Virtual LANs (VLAN) and virtual networks, highlighting the importance of consistent MTU settings across all interfaces to avoid L2 and L3 VPN flaps

Considerations and Best Practices

• Ensure uniform MTU settings across all interfaces, including logical and physical interfaces between Border Gateway Protocol (BGP) peers
• Fragmentation is not supported on most Dell Power Switches, except for specific models like the S4200-ON Series and S5148-ON platforms
• OS10 selects the lowest MTU value that is configured on the VLAN or VLAN members to be the VLAN MTU

 MTU

• After upgrading to 10.5.4.4, the MTU defaults to 9216 on all VLANs and Ethernet Interfaces which do not already have a user configured MTU

Potential Issues with Incorrect MTU Configuration 

Packet Loss

• If the MTU is set too low, larger packets are dropped, leading to significant data loss and network performance degradation 

Network Flaps

• Inconsistent MTU settings between interfaces can cause L2 and L3 VPN flaps, disrupting network stability and connectivity 

Increased Latency

• Incorrect MTU settings can lead to increased fragmentation, causing higher latency and reduced throughput

Display the default MTU.

OS10# show default mtu
Default MTU 9216 bytes

System default with no user configuration

OS10# show interface ethernet 1/1/1
Ethernet 1/1/1 is up, line protocol is down
Hardware is Eth, address is 90:b1:00:00:00:a0
 Current address is 90:b1:00:00:00:a0
Pluggable media present, QSFP+ type is QSFP+ 40GBASE-LR4
 Wavelength is 1311
 Receive power reading is no power
Interface index is 11
Internet address is not set
Mode of IPv4 Address Assignment: not set
Interface IPv6 oper status: Disabled
MTU 9216 bytes, IP MTU 1500 bytes
LineSpeed 0, Auto-Negotiation off
Flowcontrol rx off tx off
ARP type: ARPA, ARP Timeout: 60
Last clearing of "show interface" counters: 20:45:24
Queuing strategy: fifo
Input statistics:
 0 packets, 0 octets
 0 64-byte pkts, 0 over 64-byte pkts, 0 over 127-byte pkts
 0 over 255-byte pkts, 0 over 511-byte pkts, 0 over 1023-byte pkts
 0 Multicasts, 0 Broadcasts, 0 Unicasts
 0 runts, 0 giants, 0 throttles
 0 CRC, 0 overrun, 0 discarded
Output statistics:
 0 packets, 0 octets
 0 64-byte pkts, 0 over 64-byte pkts, 0 over 127-byte pkts
 0 over 255-byte pkts, 0 over 511-byte pkts, 0 over 1023-byte pkts
 0 Multicasts, 0 Broadcasts, 0 Unicasts
 0 throttles, 0 discarded, 0 Collisions, wred drops
Rate Info(interval 30 seconds):
 Input 0 Mbits/sec, 0 packets/sec, 0% of line rate
Output 0 Mbits/sec, 0 packets/sec, 0% of line rate
Time since last interface status change: 20:45:25
OS10# configure terminal
OS10(config)# default mtu 9000
OS10(config)#

• Example 

OS10# default mtu 9216
OS10# no default mtu

Table 54. Use cases of VLAN MTU configuration.

Configuration	Ethernet Interface  Configuration	show interface output
interface vlan300 no shutdown !	interface ethernet1/1/6 no shutdown switchport mode trunk switchport trunk allowed vlan300 ! interface ethernet1/1/9 no shutdown switchport mode trunk switchport trunk allowed vlan300	OS10# show interface vlan300 Vlan300 is up, line protocol is up Address is 14:10:10:0d:00:e9, Current address is 14:10:10:0d:00:e9 Interface index is 84 Internet address is not set Mode of IPv4 Address Assignment: not set Interface IPv6 oper status: Enabled Link local IPv6 address: fe80::1010:10ff:fe0d:e9/10 MTU 1532 bytes, IP MTU 1500 bytes
interface vlan300 no shutdown !	interface ethernet1/1/6 no shutdown switchport mode trunk switchport trunk allowed vlan300 ! interface ethernet1/1/9 no shutdown switchport mode trunk switchport trunk allowed vlan300 mtu 1312	OS10# show interface vlan300 Vlan300 is up, line protocol is up Address is 14:10:10:0d:00:e9, Current address is 14:10:10:0d:00:e9 Interface index is 84 Internet address is not set Mode of IPv4 Address Assignment: not set Interface IPv6 oper status: Enabled Link local IPv6 address: fe80::1010:10ff:fe0d:e9/10 MTU 1312 bytes, IP MTU 1280 bytes
virtual-network 3333 ! interface vlan300 no shutdown virtual-network 3333 !	interface ethernet1/1/6 no shutdown switchport mode trunk switchport trunk allowed vlan300 ! interface ethernet1/1/9 no shutdown switchport mode trunk switchport trunk allowed vlan300 mtu 1312	OS10# # show interface vlan300 Vlan300 is up, line protocol is up Address is 14:18:77:0d:00:e9, Current address is 14:18:77:0d:00:e9 Interface index is 84 Internet address is not set Mode of IPv4 Address Assignment: not set Interface IPv6 oper status: Enabled Link local IPv6 address: fe80::1618:77ff:fe0d:e9/64 MTU 1532 bytes, IP MTU 1500 bytes
virtual-network 3333 ! interface vlan300 no shutdown virtual-network 3333 ! interface virtual-network 3333	interface ethernet1/1/6 no shutdown switchport mode trunk switchport trunk allowed vlan300 ! interface ethernet1/1/9 no shutdown switchport mode trunk switchport trunk allowed vlan300 mtu 1312	show interface vlan300 Vlan300 is up, line protocol is up Address is 14:18:77:0d:00:e9, Current address is 14:18:77:0d:00:e9 Interface index is 84 Internet address is not set Mode of IPv4 Address Assignment: not set Interface IPv6 oper status: Enabled Link local IPv6 address: fe80::1618:77ff:fe0d:e9/64 MTU 1532 bytes, IP MTU 1500 bytes OS10# show interface virtual-network 3333 Virtual-network 3333 is up, line protocol is up Address is 14:18:77:0d:00:ea, Current address is 14:18:77:0d:00:ea Interface index is 87 Internet address is not set Mode of IPv4 Address Assignment: not set Interface IPv6 oper status: Enabled MTU 1312 bytes, IP MTU 1280 bytes

• Example

OS10(conf-if-eth1/1/7)# mtu 3000

• Example

OS10# show default mtu
Default MTU 9216 bytes