port-channel doubt

[Arjun Dabol]

SW1                                                    SW2

                  1-------------------------1

                  2-------------------------2

                  3-------------------------

If I have 2 switch and between them we have 3 links. If we create port-channel on SW1 including all 3 links BUT on SW2 we create Port channel using only 2 Links, will it work ? what problem we may face ?

thanks

Abhishek

[Dennis Mink]

That seems like a pointless exercise to me, the port channel will not negotiate the 3 links aggregated as far as I can see or you will have 2 links aggregated because port 3 on sw2 will not respond to LACP.  could you explain what it is you are after or trying to achieve.

[Arjun Dabol]

thx all , Just wanted to understand the concept here of such design and you all helped in that. thanks again

[Julio E. Moisa]

It was a pleasure Arjun,

Have a great day!

:-)

[azibnaseem]

Hey @arjun dabol,

minkdennis  is right that etherchannel wouldn't form. You have to make identical ports on both switches in order to etherchannel to work.

Regards'

[azibnaseem]

Hey adaveadmin  

I have created a topology on GNS3 & make an identical settings as per your requirement. Using GNS3 etherchannel link has been formed & showing up on both switches. Please try to configure on real switches & see the outputs using debug etherchannel all command. If etherchannel is negotiated it may cause the load balancing & performance issues.

Regards' 

[Julio E. Moisa]

Hi Arjun,

If you create the port channel with 3 ports on switch 1 but on switch 2 you will use only 2 ports to create the port channel, it will work but using only 2 ports:

Please check the following configuration:

Switch 1

SW1#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 f - failed to allocate aggregator

M - not in use, minimum links not met
u - unsuitable for bundling
w - waiting to be aggregated
d - default port

Number of channel-groups in use: 1
Number of aggregators: 1

Group Port-channel Protocol Ports
------+-------------+-----------+-----------------------------------------------
12 Po12(SU) LACP Fa1/0/27(P) Fa1/0/28(P) Fa1/0/29(I)

Port-channel12         unassigned      YES unset  up                    up

Switch 2

SW2#sh 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 f - failed to allocate aggregator

M - not in use, minimum links not met
u - unsuitable for bundling
w - waiting to be aggregated
d - default port

Number of channel-groups in use: 1
Number of aggregators: 1

Group Port-channel Protocol Ports
------+-------------+-----------+-----------------------------------------------
12 Po12(SU) LACP Fa1/0/7(P) Fa1/0/8(P)

Port-channel12         unassigned      YES unset  up                    up

The bundle will be created with 2 ports only, the third will be as stand alone, now if you want to use 3 links in a future on both switches, take in consideration that the loadbalance will not be equal, please visit the following link:

https://www.packetmischief.ca/2012/07/24/doing-etherchannel-over-3-5-6-and-7-link-bundles/

Now with 3 links, everything works:

SW1#SH 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 f - failed to allocate aggregator

M - not in use, minimum links not met
u - unsuitable for bundling
w - waiting to be aggregated
d - default port

Number of channel-groups in use: 1
Number of aggregators: 1

Group Port-channel Protocol Ports
------+-------------+-----------+-----------------------------------------------
12 Po12(SU) LACP Fa1/0/27(P) Fa1/0/28(P) Fa1/0/29(P)

SW2#SH 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 f - failed to allocate aggregator

M - not in use, minimum links not met
u - unsuitable for bundling
w - waiting to be aggregated
d - default port

Number of channel-groups in use: 1
Number of aggregators: 1

Group Port-channel Protocol Ports
------+-------------+-----------+-----------------------------------------------
12 Po12(SU) LACP Fa1/0/7(P) Fa1/0/8(P) Fa1/0/9(P)

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:-)