Dell SmartFabric OS10 User Guide Release 10.5.4

To return to the default MTU value, use the no mtu command. If an IP packet includes a L2 header, the IP MTU must be at least 32 bytes smaller than the L2 MTU.

Set the MTU value on all interfaces, including logical and physical interfaces between BGP peers (iBGP and eBGP), to be uniform to avoid L2 and L3 VPN flaps.

The following table shows the different use cases of VLAN MTU configuration. Use case 1 and 2 describe configuration of VLAN without attaching to virtual network. Use case 3 and 4 describe configuration of VLAN and attaching the VLAN to a virtual network.

interface vlan300 
no shutdown
!

interface ethernet1/1/6
no shutdown
switchport mode trunk 
switchport trunk allowed vlan 300 
!
interface ethernet1/1/9
no shutdown
switchport mode trunk
switchport trunk allowed vlan 300

OS10# show interface vlan 300

Vlan 300 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 vlan 300
!
interface ethernet1/1/9
no shutdown
switchport mode trunk
switchport trunk allowed vlan 300
mtu 1312

OS10# show interface vlan 300
Vlan 300 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 vlan 300
!
interface ethernet1/1/9
no shutdown
switchport mode trunk
switchport trunk allowed vlan 300
mtu 1312

OS10# # show interface vlan 300
Vlan 300 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 vlan 300
!
interface ethernet1/1/9
no shutdown
switchport mode trunk
switchport trunk allowed vlan 300
mtu 1312

show interface vlan 300
Vlan 300 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

For use case 1 and 2:

• Member interfaces inherit the MTU value that is configured on the port channel interface.
• Tagged members must have a link MTU 4 bytes higher than untagged members to account for the packet tag.
• OS10 selects the lowest MTU value that is configured on the VLAN or VLAN members to be the VLAN MTU.

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 Link MTU of VLAN cannot be higher than 1518 bytes and its IP MTU cannot be higher than 1500 bytes.

The show running-configuration command displays the VLAN MTU configuration. The show interface vlan command displays the VLAN MTU operational values.

For use case 3 and 4:

In the kernel and NPU, VLAN bridge is removed, and virtual network bridge is created. Data forwarding happens in the virtual-network context. The show interface vlan command displays the VLAN MTU configuration and not the VLAN MTU operational values.

In use case 3, when a VLAN is attached to a virtual network, the VLAN broadcast domain is removed, and only the associated virtual network broadcast domain becomes active. Hence, the show interface vlan command does not display the correct value of the active broadcast domain. Rather it displays the MTU for the removed broadcast domain. If a virtual network is not configured as L3 (the interface virtual network command is not run for that virtual network), the virtual network remains as L2 VXLAN. Hence, IP MTU for that L2 VXLAN is not valid. Once the virtual network is configured as L3 (the interface virtual network command is run for that virtual network), use the show interface virtual-network command to display the operational MTU values for the VXLAN network.

In use case 4, once the virtual network is configured as L3 (the interface virtual network command is run for that virtual network), use the show interface virtual-network command to display the operational MTU values for the VXLAN network.

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