understanding port channeling..

nikhil.kulkarni · ‎09-20-2013

Folks,

I have a picture attached to understand some concepts on port channel:

I have a Switch-A connected to Switch-B as shown. On port a I have created a port channel between port Gi0/1 and G0/2.

However, on Switch-B port Gi0/1 and Gi0/2 are left as is it, no port channel is created.

Should this not be a problem? I still see that connectivity is present ans hosts on VLAN 10 are able to communicate to each other.

What issue would this cause in the real world?

Regards,

Nik

Reza Sharifi · ‎09-20-2013

Hi,

You need to configure etherchannel on both switches, if not it can cause loop in your network

HTH

nikhil.kulkarni · ‎09-20-2013

agreed..but what doubts me is I tried this in the LAB and had few machines connected, I did not observe any loops. Maybe because I just do not have enough machines connected to create the traffic.

But, why should this work in the first place?

Regards,

Nik

Peter Koltl · ‎09-21-2013

Inspect the

show spanning-tree

show etherchannel summary

output to see the port states and which ports are used for traffic.

nikhil.kulkarni · ‎09-21-2013

Switch-A:

Mutha#sh spanning-tree

VLAN0001

Spanning tree enabled protocol ieee

Root ID Priority 32769

Address 0001.C78C.8DDB

This bridge is the root

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)

Address 0001.C78C.8DDB

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 20

Interface Role Sts Cost Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

Po10 Desg FWD 3 128.27 Shr

VLAN0010

Spanning tree enabled protocol ieee

Root ID Priority 32778

Address 0001.C78C.8DDB

This bridge is the root

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)

Address 0001.C78C.8DDB

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 20

Interface Role Sts Cost Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

Fa0/1 Desg FWD 19 128.1 P2p

Po10 Desg FWD 3 128.27 Shr

Mutha#

Mutha#sh etherchann summ

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

w - waiting to be aggregated

d - default port

Number of channel-groups in use: 1

Number of aggregators: 1

Group Port-channel Protocol Ports

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

10 Po10(SU) PAgP Gig0/1(P) Gig0/2(P)

Mutha#

___________________________________________________________________________________________________

Switch-B:

ICC#sh spanning-tree

VLAN0001

Spanning tree enabled protocol ieee

Root ID Priority 32769

Address 0001.C78C.8DDB

Cost 4

Port 26(GigabitEthernet0/2)

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)

Address 0030.F2D0.5D61

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 20

Interface Role Sts Cost Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

Gi0/2 Root FWD 4 128.26 P2p

Gi0/1 Desg FWD 4 128.25 P2p

VLAN0010

Spanning tree enabled protocol ieee

Root ID Priority 32778

Address 0001.C78C.8DDB

Cost 4

Port 26(GigabitEthernet0/2)

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)

Address 0030.F2D0.5D61

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 20

Interface Role Sts Cost Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

Gi0/2 Root FWD 4 128.26 P2p

Gi0/1 Desg FWD 4 128.25 P2p

Fa0/1 Desg FWD 19 128.1 P2p

ICC#

ICC#sh ether 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

w - waiting to be aggregated

d - default port

Number of channel-groups in use: 0

Number of aggregators: 0

Group Port-channel Protocol Ports

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

ICC#

Thnx,

Nik

Joseph W. Doherty · ‎09-21-2013

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Posting

You're sure your configuration hasn't formed a port-channel dynamically?

PS:

If you have a real L2 loop, it generally causes major issues in the real world.

nikhil.kulkarni · ‎09-21-2013

I am sure Joseph,

I have manually configure the trunk.

Parvesh Paliwal · ‎09-21-2013

can you share the config and sho spanning-tree output ?

only then you can see if the same has faced a blocked port due to STP.

nikhil.kulkarni · ‎09-21-2013

Hi Paresh,

I pasted the show spanning tree above:

here is the sh run on the ports:

Switch-A

interface Port-channel 10

description ** to ICC **

switchport trunk encapsulation dot1q

switchport mode trunk

__________________________________________________

Switch-B

interface GigabitEthernet0/1

description ** to Mutha **

switchport trunk encapsulation dot1q

switchport mode trunk

!

interface GigabitEthernet0/2

description ** to Mutha **

switchport trunk encapsulation dot1q

switchport mode trunk

!

Jan Hrnko · ‎09-21-2013

Hi Nik,

What types of switches do you use? If they don't support EtherChannel Inconsistency Detection then the etherchannel will be formed from the one side (using mode on), thus creating a L2 loop. Sure it is a serious issue in real world - creating a L2 loop can and will have impact on throughput and CPU's of switches and hosts as broadcast storms will arise. In addition, L2 loops create mac address table incosistency.

Best regards,

Jan