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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.

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. To work around this limitation, do one of the following:
  • Configure the layer 3 interface on the physical link instead of using a bond
  • Configure the LACP bond on the switch port so that the AS has neighbor LACP information
  • Configure the bond interface as balance-xor mode instead of LACP
  • 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. Consider configuring STP BPDU guard together with all-active mode.

The following features are not supported:

  • priority mode
  • bond-lacp-bypass-period
  • bond-lacp-bypass-priority
  • bond-lacp-bypass-all-active

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.

Configure LACP Bypass

To enable LACP bypass on the host-facing bond, configure bond-lacp-bypass-allow using NCLU. The following commands create a VLAN-aware bridge with LACP bypass enabled:

cumulus@switch:~$ net add bond bond1 bond slaves swp51s2,swp51s3
cumulus@switch:~$ net add bond bond1 clag id 1
cumulus@switch:~$ net add bond bond1 bond lacp-bypass-allow
cumulus@switch:~$ net add bond bond1 stp bpduguard
cumulus@switch:~$ net add bridge bridge ports bond1,bond2,bond3,bond4,peer5
cumulus@switch:~$ net add bridge bridge vids 100-105
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

clag-id is not a required parameter in the configuration shown above. While LACP bypass is often configured on bonds involved in MLAG, MLAG is not required to use LACP bypass.

These commands create the following stanzas in /etc/network/interfaces:

auto bond1
iface bond1
    bond-lacp-bypass-allow yes
    bond-slaves swp51s2 swp51s3
    clag-id 1
    mstpctl-bpduguard yes

auto bridge
iface bridge
    bridge-ports bond1 bond2 bond3 bond4 peer5
    bridge-vids 100-105
    bridge-vlan-aware yes

You can check the status of the configuration by running net show interface <bond> on the bond and its slave interfaces:

cumulus@switch:~$ net show interface bond1

    Name   MAC               Speed   MTU   Mode
-- ------ ----------------- ------- ----- ----------
UP bond1  44:38:39:00:00:5b 1G      1500  Bond/Trunk

Bond Details
------------------ -------------------------
Bond Mode:         LACP
Load Balancing:    Layer3+4
Minimum Links:     1
In CLAG:           CLAG Active
LACP Sys Priority:
LACP Rate:         Fast Timeout
LACP Bypass:       LACP Bypass Not Supported

    Port       Speed     TX   RX   Err   Link Failures
-- --------   ------- ---- ---- ----- ---------------
UP swp51s2(P) 1G         0    0     0               0
UP swp51s3(P) 1G         0    0     0               0

All VLANs on L2 Port


Vlans in disabled State

--------   ---- ------------------
swp51s2(P) ==== swp1(spine01)
swp51s3(P) ==== swp1(spine02)

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
swp51 swp52
cumulus@switch:~$ cat /sys/class/net/swp52/bonding_slave/ad_rx_bypass
cumulus@switch:~$ cat /sys/class/net/swp51/bonding_slave/ad_rx_bypass

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
    mstpctl-bpduguard bond1=yes