Etherchannel L2 Down during Add Vlan

This is also notice, warning for you. The good network engineers should be know about it. During configure 'switchport trunk allowed vlan add'. Add new vlan should be in port-channel logical interface, don't add new vlan in physical interface, even add vlan in all member physical interface of port-channel. Please more attention, if etherchannel interface has been operational (live network). Do some verify before add vlan to live etherchannel interface, ask to your partner as second verification if needed.

Please follow this example.

#### Define etherchannel interface as pre-configure (IOS-1, IOS-2 are same config)

IOS-1# show run interface Ethernet1/0
Building configuration...

Current configuration : 187 bytes
!
interface Ethernet1/0
 switchport
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 11-14,16,18
 switchport mode trunk
 duplex auto
 channel-group 1 mode active
end

IOS-1# show run interface Ethernet1/1
Building configuration...

Current configuration : 187 bytes
!
interface Ethernet1/1
 switchport
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 11-14,16,18
 switchport mode trunk
 duplex auto
 channel-group 1 mode active
end

IOS-1# show run interface port-channel1
Building configuration...

Current configuration : 147 bytes
!
interface Port-channel1
 switchport
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 11-14,16,18
 switchport mode trunk
end

IOS-1# show etherchannel summary
Flags:  D - down        P - bundled in port-channel
        I - stand-alone s - suspended
        H - Hot-standby (LACP only)
        R - Layer3      S - Layer2
        U - in use      N - not in use, no aggregation
        f - failed to allocate aggregator

        M - not in use, no aggregation due to minimum links not met
        m - not in use, port not aggregated due to minimum links not met
        u - unsuitable for bundling
        d - default port

        w - waiting to be aggregated
Number of channel-groups in use: 1
Number of aggregators:           1

Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SU)         LACP      Et1/0(P)       Et1/1(P)  

#### Add new vlan in port-channel interface

IOS-1(config)# interface port-channel1 
IOS-1(config-if)# switchport trunk allowed vlan add 19

IOS-1(config-if)# do show run interface port-channel1 
Building configuration...

Current configuration : 150 bytes
!
interface Port-channel1
 switchport
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 11-14,16,18,19
 switchport mode trunk
end

IOS-1(config-if)# do show etherchannel summary
Flags:  D - down        P - bundled in port-channel
        I - stand-alone s - suspended
        H - Hot-standby (LACP only)
        R - Layer3      S - Layer2
        U - in use      N - not in use, no aggregation
        f - failed to allocate aggregator

        M - not in use, no aggregation due to minimum links not met
        m - not in use, port not aggregated due to minimum links not met
        u - unsuitable for bundling
        d - default port

        w - waiting to be aggregated
Number of channel-groups in use: 1
Number of aggregators:           1

Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SU)         LACP      Et1/0(P)       Et1/1(P)   

#### Add new vlan in both physical interface

IOS-1(config)# interface port-channel1 
IOS-1(config-if)# switchport trunk allowed vlan remove 19

IOS-1(config-if)# interface range Ethernet1/0-1
IOS-1(config-if-range)# switchport trunk allowed vlan add 19

IOS-1(config-if-range)# do show etherchannel summary
Flags:  D - down        P - bundled in port-channel
        I - stand-alone s - suspended
        H - Hot-standby (LACP only)
        R - Layer3      S - Layer2
        U - in use      N - not in use, no aggregation
        f - failed to allocate aggregator

        M - not in use, no aggregation due to minimum links not met
        m - not in use, port not aggregated due to minimum links not met
        u - unsuitable for bundling
        d - default port

        w - waiting to be aggregated
Number of channel-groups in use: 1
Number of aggregators:           1

Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SD)         LACP      Et1/0(I)       Et1/1(I)      

#### Add new vlan in one physical interface

(*revision note: another of IOS version will be effect other status, port-channel will be up with single physical interface after down for a few second. The point is port-channel will be down for a while or permanent if add vlan in one physical interface.)

IOS-1(config)# interface range Ethernet1/0-1
IOS-1(config-if-range)# switchport trunk allowed vlan remove 19

IOS-1(config-if-range)# interface Ethernet1/0
IOS-1(config-if)# switchport trunk allowed vlan add 19

IOS-1(config-if)# do show etherchannel summary
Flags:  D - down        P - bundled in port-channel
        I - stand-alone s - suspended
        H - Hot-standby (LACP only)
        R - Layer3      S - Layer2
        U - in use      N - not in use, no aggregation
        f - failed to allocate aggregator

        M - not in use, no aggregation due to minimum links not met
        m - not in use, port not aggregated due to minimum links not met
        u - unsuitable for bundling
        d - default port

        w - waiting to be aggregated
Number of channel-groups in use: 1
Number of aggregators:           1

Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SD)         LACP      Et1/0(I)       Et1/1(I)