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LACP Bypass

On Cumulus Linux, LACP Bypass is a feature that allows a bond configured in 802.3ad mode to become active and forward traffic even when there is no LACP partner. A typical use case for this feature is to enable a host, without the capability to run LACP, to PXE boot while connected to a switch on a bond configured in 802.3ad mode. Once the pre-boot process finishes and the host is capable of running LACP, the normal 802.3ad link aggregation operation takes over.

Understanding the LACP Bypass All-active Mode

When a bond has multiple slave interfaces, each bond slave interface operates as an active link while the bond is in bypass mode. This is known as all-active mode. This is useful during PXE boot of a server with multiple NICs, when the user cannot determine beforehand which port needs to be active.

Keep in the mind the following caveats with all-active mode:

  • All-active mode is not supported on bonds that are not specified as bridge ports on the switch.

  • Spanning tree protocol (STP) does not run on the individual bond slave interfaces when the LACP bond is in all-active mode. Therefore, only use all-active mode on host-facing LACP bonds. Cumulus Networks highly recommends you configure STP BPDU guard along with all-active mode.

Priority mode, bond-lacp-bypass-period, bond-lacp-bypass-priority and bond-lacp-bypass-all-active are not supported.

LACP Bypass and MLAG Deployments

In an MLAG deployment where bond slaves of a host are connected to two switches and the bond is in all-active mode, all the slaves of bond are active on both the primary and secondary MLAG nodes.

Configuring LACP Bypass

You configure LACP bypass in the /etc/network/interfaces file.

To enable LACP bypass on the host-facing bond, under the bond interface stanza, set bond-lacp-bypass-allow to 1.

VLAN-aware Bridge Mode Configuration

The following configuration shows LACP bypass enabled for a bridge in VLAN-aware mode:

auto bond1
iface bond1
    bond-slaves swp51s2 swp51s3
    clag-id 1
    bond-lacp-bypass-allow 1
 
...
 
auto br0
iface br0
    bridge-vlan-aware yes
    mstpctl-bpduguard bond1=yes
    bridge_ports bond1 bond2 bond3 bond4 peer5
    bridge-stp on
    bridge-vids 100-105

You can check the status of the configuration by running ip link show on the bond and its slave interfaces:

cumulus@switch:~$ ip link show bond1
164: bond1: <BROADCAST,MULTICAST,MASTER,UP,LOWER_UP> mtu 1500 qdisc noqueue master br0 state UP mode DORMANT group default 
    link/ether c4:54:44:f6:44:5a brd ff:ff:ff:ff:ff:ff
cumulus@switch:~$ ip link show swp51s2
55: swp51s2: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master bond1 state UP mode DEFAULT group default qlen 1000
    link/ether c4:54:44:f6:44:5a brd ff:ff:ff:ff:ff:ff
cumulus@switch:~$ ip link show swp52s3
56: swp51s3: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master bond1 state UP mode DEFAULT group default qlen 1000
    link/ether c4:54:44:f6:44:5a brd ff:ff:ff:ff:ff:ff

Use the cat command to verify that LACP bypass is enabled on a bond and its slave interfaces:

cumulus@switch:~$ cat /sys/class/net/bond1/bonding/lacp_bypass 
on 1
cumulus@switch:~$ cat /sys/class/net/bond1/bonding/slaves
swp51s2 swp51s3
cumulus@switch:~$ cat /sys/class/net/swp51s2/bonding_slave/ad_rx_bypass 
1
cumulus@switch:~$ cat /sys/class/net/swp51s3/bonding_slave/ad_rx_bypass 
1

Traditional Bridge Mode Configuration

The following configuration shows LACP bypass enabled for multiple active interfaces (all-active mode) with a bridge in traditional bridge mode:

auto bond1
iface bond1 
    bond-slaves swp3 swp4
    bond-lacp-bypass-allow 1
 
auto br0
iface br0
    bridge-ports bond1 bond2 bond3 bond4 peer5
    bridge-stp on
    mstpctl-bpduguard bond1=yes