L3 IP MTU

Neutron provides multiple ways to specify the maximum transmission unit (MTU) for networks.

You can provide a global value in the neutron.conf file using the global_physnet_mtu value. This value applies to all networks. For example:

/etc/neutron/neutron.conf
[DEFAULT] 
global_physnet_mtu = <mtu_value>

You can provide a specific value when creating the network using the --mtu option. This value takes higher precedence and overrides the global value. For example:

openstack network create –-mtu <mtu-value> GREEN_NETWORK

The MTU value for each network is captured by Extreme plug-ins and passed to EFA. EFA then enforces the value on the endpoint group as part of its ipv6-nd-mtu and ip-mtu options. This value is per Ctag and is enforced on the VE interface corresponding to the Ctag. EFA configures the ipv6-nd-mtu and ip-mtu values on the specific VE interface on the device.

The value is applied to the SLX device when the router-interface is associated with the network in Neutron, either when the router is added to a subnet that is associated with the network or when a port on the network is added to a router.

Example

The following is a sample SLX configuration.

(device)# show running-config interface Ve 3  

interface Ve 3  
 vrf forwarding 274e9461081849f8867b77a112f6c510 
 ip anycast-address 61.1.1.1/24 
 ip mtu 9000 
 ipv6 anycast-address fd00:61:0:57::1/64 
 ipv6 nd managed-config-flag 
 ipv6 nd other-config-flag 
 ipv6 nd mtu 9000 
 ipv6 nd prefix fd00:61:0:57::/64 2592000 604800 no-autoconfig 
 no shutdown

Note

SLX supports IP MTU values in the range from 1280 through 9194 bytes. Creation of the router-interface fails if you try to configure an MTU value outside of this range.