What causes port-channel5 to become 5A?

[Beat.Traber]

What causes port-channel5 to become 5A?

Once upon a time there was a (Layer 2) port-channel5, consisting of two Gigabit-Ethernet interfaces Gi10/11 and Gi10/12. Then, after a considerable time I noticed, that it had changed its name to "port-channel5A" although it was still configured as port-channel5.

Then there was an interruption between the two ends. The two Gigabit-Ethernet interfaces went down for about 30 Seconds - and so did port-channel5A. Then Gi10/11 came up again, but Gi10/12 didn't. 5 Seconds later port-channel5 came up (yes, 5, not 5A)

I've removed Gi10/12 from the port-channel, because I'm a bit concerned by this behaviour. What'll happen if I bundle the two Gig-Ports again? Will there be a disruption (will Po5 go down and reappear as Po5A?). I'm not keen on another interruption since it is a vital link between two organizations. And the main question is: what caused the channel to become 5A?

Configuration of the channel is quite straightforward:

interface Port-channel5

switchport

switchport trunk encapsulation dot1q

switchport trunk allowed vlan 402,403,2811,2813,2815

switchport mode trunk

And the two identical configured Gig-Ports were bundled with "channel-group 5 mode active" which made them an LACP channel.

This was what it looked like, when channel5A was still active.

xxxxx#sh interfaces port-channel 5 etherchannel

Port-channel5 (Primary aggregator)

Age of the Port-channel = 114d:19h:39m:36s

Logical slot/port = 14/5 Number of ports = 0

HotStandBy port = null

Port state = Port-channel Ag-Not-Inuse

Protocol = LACP

Fast-switchover = disabled

Time since last port bundled: 63d:04h:25m:21s Gi10/11

Time since last port Un-bundled: 58d:19h:38m:27s Gi10/12

Port-channel5A

Age of the Port-channel = 1d:03h:14m:27s

Logical slot/port = 14/2 Number of ports = 2

HotStandBy port = null

Port state = Port-channel Ag-Inuse

Protocol = LACP

Fast-switchover = disabled

Ports in the Port-channel:

Index Load Port EC state No of bits

------+------+------+------------------+-----------

0 55 Gi10/11 Active 4

1 AA Gi10/12 Active 4

Time since last port bundled: 1d:03h:14m:25s Gi10/12

[Giuseppe Larosa]

Hello Beat,

I've noticed the same thing on some of our switches.

My idea is that the A stands for automatic and implies a successful LACP negotiation.

Hope to help

Giuseppe

[Beat.Traber]

Hello Giuseppe

Thanks for your suggestion, but I'm not sure. On the other end of the channel there's only a port-channel5 but no 5A. I get the impression that there's something wrong.

Beat

[Eli Barb · Accepted solution]

Please list the interface configuration for both interfaces on both sides that you want to participate in Po5. The Po5A comes up when there is a misconfiguration between the ports.

Secondary Aggregator Ports Po1A or Po2A are Created

A secondary aggregator port will be created in the LACP process when the ports you are

bundling are not compatible with each other, or with their remote peers. The secondary

aggregator port will have the ports which are compatible with others.

Switch#show etherchannel summary

Flags: D - down P - in port-channel

I - stand-alone s - suspended

H - Hot-standby (LACP only)

R - Layer3 S - Layer2

U - in use f - failed to allocate aggregator

u - unsuitable for bundling

Number of channel-groups in use: 6

Number of aggregators: 8

Group Port-channel Protocol Ports

------+-------------+-----------+-----------------------------------------

1 Po1(SU) LACP Gi1/16(P) Gi10/1(P) Gi10/2(P)

2 Po2(SD) LACP

2 Po2A(SU) LACP Gi1/15(P) Gi10/3(P) Gi10/4(P)

3 Po3(SU) LACP Gi1/14(P) Gi10/5(P) Gi10/6(P)

4 Po4(SD) LACP

4 Po4A(SU) LACP Gi1/13(P) Gi10/7(P) Gi10/8(P)

5 Po5(SU) LACP Gi1/12(P) Gi10/9(P) Gi10/10(P)

6 Po6(SU) LACP Gi1/11(P) Gi10/11(P) Gi10/12(P)

Configure all LAN ports in an EtherChannel to operate at the same speed and in the same

duplex mode. LACP does not support half-duplex. Half-duplex ports in an LACP EtherChannel

are put in the suspended state.

If you configure an EtherChannel from trunking LAN ports, verify that the trunking mode is

the same on all the trunks. LAN ports in an EtherChannel with different trunk modes can

operate unpredictably.

LAN ports with different STP port path costs can form an EtherChannel as long they are

compatibly configured with each other. If you set different STP port path costs, the LAN

ports are not incompatible for the formation of an EtherChannel

Also TAC mentioned the following to me when I had a case with some issues because of this.

Going through the case notes, it seems that another port channel interface

po4a has been created. The information you found is correct. This behavior

is basically a spawned etherchannel, LACP can spawn a secondary portchannel

when there is an incompatible port config, this can only happen in LACP and

it's usually with a misconfiguration of the etherchannel. This is normal

operation of the LACP, it will happen when LACP protocol detects some

inconsistency as per the IEEE 802.3ad standard. Most definitely this should

not impact anything in the network.

The port channel created a spawn port channel .The LACP etherchannels is

functioning correctly as a spawn but to get rid of it, I would suggest you

to schedule a down time and take down the port-channel, put the

configurations to

default, shut the interfaces and recreate both sides and make sure the

configuration is the same in both sides.

[Beat.Traber]

Thanks, matter resolved.