By default, Contrail compute nodes use flow mode for packet forwarding. However, there are instances when you need to switch from flow mode to packet mode either to achieve higher traffic forwarding performance or to get around certain design limitations of flow mode.
For differences between flow mode and packet mode, refer to Contrail Cookbook.
This article details the differences as well as common aspects of packet mode ​configuration in various Contrail releases, while describing how to switch from flow mode to packet mode and vice versa.
In Contrail release 5.0 and later, under VMI Advanced options, you can enable packet mode by selecting a check box as highlighted below. However, this option is not seen in Contrail 3.x or 4.x releases.
In Contrail 3.x/4.x releases, to enable packet mode on VMI (at the port level), we need to select the "Disable Policy" check box as highlighted below. Starting from Contrail release 5.0, the same UI option has been renamed to "Packet Mode" for better user understanding.
The API reference to the virtual_machine_interface_disable_policy parameter, which corresponds to the “Disable Policy” option in Contrail 3.x/4.x release WebUI, is available here.
Even in Contrail 5.0+ where “Disable Policy” is renamed to “Packet Mode,” the same API parameter is still referred to.
To verify whether a VMI is indeed running in packet mode, use vif --get <tap-index> on a compute node to confirm whether “P”(policy) displays or not.
For example, by default, an active VMI is programmed in flow mode, which has the network policy attached and enabled. The "P" flag is displayed and highlighted below in blue:
root@comp104:~# vif --get 21
Vrouter Interface Table
Flags: P=Policy, X=Cross Connect, S=Service Chain, Mr=Receive Mirror
Mt=Transmit Mirror, Tc=Transmit Checksum Offload, L3=Layer 3, L2=Layer 2
D=DHCP, Vp=Vhost Physical, Pr=Promiscuous, Vnt=Native Vlan Tagged
Mnp=No MAC Proxy, Dpdk=DPDK PMD Interface, Rfl=Receive Filtering Offload, Mon=Interface is Monitored
Uuf=Unknown Unicast Flood, Vof=VLAN insert/strip offload, Df=Drop New Flows, Proxy=MAC Requests Proxied Always
Mn=Mirror without Vlan Tag
vif0/21 OS: tap7f8da123-47
Type:Virtual HWaddr:00:00:5e:00:01:00 IPaddr:10.10.10.6
Vrf:7 Flags:PL3L2D QOS:-1 Ref:5
RX packets:302327 bytes:12697734 errors:0
TX packets:302456 bytes:12703380 errors:0
Drops:2
After we enable packet mode by disabling the network policy, the “P” flag is removed and no longer shows up on port.
root@comp104:~# vif --get 21
Vrouter Interface Table
Flags: P=Policy, X=Cross Connect, S=Service Chain, Mr=Receive Mirror
Mt=Transmit Mirror, Tc=Transmit Checksum Offload, L3=Layer 3, L2=Layer 2
D=DHCP, Vp=Vhost Physical, Pr=Promiscuous, Vnt=Native Vlan Tagged
Mnp=No MAC Proxy, Dpdk=DPDK PMD Interface, Rfl=Receive Filtering Offload, Mon=Interface is Monitored
Uuf=Unknown Unicast Flood, Vof=VLAN insert/strip offload, Df=Drop New Flows, Proxy=MAC Requests Proxied Always
Mn=Mirror without Vlan Tag
vif0/21 OS: tap7f8da123-47
Type:Virtual HWaddr:00:00:5e:00:01:00 IPaddr:10.10.10.6
Vrf:7 Flags:L3L2D QOS:-1 Ref:5 <==”P” is no longer there.
RX packets:302339 bytes:12698238 errors:0
TX packets:302474 bytes:12704136 errors:0
Drops:2
After we deselect the “Disable Policy” check box to re-enable flow mode, the "P" flag will be re-programmed on the port as shown below:
root@comp104:~# vif --get 21
Vrouter Interface Table
Flags: P=Policy, X=Cross Connect, S=Service Chain, Mr=Receive Mirror
Mt=Transmit Mirror, Tc=Transmit Checksum Offload, L3=Layer 3, L2=Layer 2
D=DHCP, Vp=Vhost Physical, Pr=Promiscuous, Vnt=Native Vlan Tagged
Mnp=No MAC Proxy, Dpdk=DPDK PMD Interface, Rfl=Receive Filtering Offload, Mon=Interface is Monitored
Uuf=Unknown Unicast Flood, Vof=VLAN insert/strip offload, Df=Drop New Flows, Proxy=MAC Requests Proxied Always
Mn=Mirror without Vlan Tag
vif0/21 OS: tap7f8da123-47
Type:Virtual HWaddr:00:00:5e:00:01:00 IPaddr:10.10.10.6
Vrf:7 Flags:PL3L2D QOS:-1 Ref:5
RX packets:302349 bytes:12698658 errors:0
TX packets:302498 bytes:12705144 errors:0
Drops:2