LACP PDUs should be always untagged

[vikram.majjari1]

When "vlan dot1q tag native" command is enabled on Cisco 3750E router, the LACP PDUs are getting tagged with native vlan and being sent out of the ether channel ports which is against the LACP IEEE RFC which states that LACP PDUs are always untagged.

Can anyone confirm the above data points and validate if Cisco is complaint to the IEEE 802.3ad Link Aggregation. (LAG)?

Cisco config used:

CISCO3750E-01#sh vlan dot1q tag native
dot1q native vlan tagging is enabled

CISCO3750E-01#sh runn int g1/0/1
Building configuration...

Current configuration : 128 bytes
!
interface GigabitEthernet1/0/1
 switchport trunk encapsulation dot1q
 switchport mode trunk
 channel-group 4 mode active
end

CISCO3750E-01#sh runn int g1/0/2
Building configuration...

Current configuration : 128 bytes
!
interface GigabitEthernet1/0/2
 switchport trunk encapsulation dot1q
 switchport mode trunk
 channel-group 4 mode active
end

CISCO3750E-01#sh runn int po4
Building configuration...

Current configuration : 92 bytes
!
interface Port-channel4
 switchport trunk encapsulation dot1q
 switchport mode trunk
end

Cisco software being used:

Cisco IOS Software, C3750E Software (C3750E-UNIVERSALK9-M), Version 12.2(40)SE, RELEASE SOFTWARE (fc3)
Copyright (c) 1986-2007 by Cisco Systems, Inc.
Compiled Fri 24-Aug-07 00:25 by myl
Image text-base: 0x00003000, data-base: 0x02000000

ROM: Bootstrap program is C3750E boot loader
BOOTLDR: C3750E Boot Loader (C3750E-HBOOT-M) Version 12.2(35r)SE1, RELEASE SOFTWARE (fc2)

[Steve Fuller]

Hi,

Cisco are not compliant to the IEEE 802.1AX (IEEE 802.3ad) standard on some platforms are releases. For the Catalyst 3750 there’s a bug filed, CSCse14774 Changing native vlan on a LACP EtherChannel breaks the link.

Cisco send the LACPDU in VLAN 1 so essentially using the vlan dot1q tag native command has the same effect as changing the native VLAN.

The same behaviour is seen on the Catalyst 6500 and has the bug CSCsh97848 Sierra: LACP pdus should be untagged filed for it. The Further Problem Description for this bug confirms the behaviour of using VLAN 1:

“This problem occurs because our switches transmit the LACP PDUs in VLAN 1. If the VLAN 1 is not native VLAN, the LACP PDUs are tagged in VLAN 1. 3rd party switches may drop tagged PDUs. Hence our switches may not be able to communicate with 3rd party switches and hence the interfaces will be put in suspended state.

With the diffs integrated the native vlan tagging on LACP pdu is removed and we are compliant with IEEE standards for LACP.”

Oddly enough CSCse14774 shows 12.2(35)SE as the fix that integrates the fix, which is earlier than the release you’re running. That said I would try an upgrade to a later release as 12.2(40)SE is over 8 years old now anyway.

Regards