Etherchannel mode ON ?how switch port form etherchannel without sending negtiation packet?

[vasanth77]

hi

how other switches come to know that the port was associated to etherchannel without sending negotiation packet ?

because in ether channel static there were no negotiation packet sent B/W switches?

can any body clarify,how it happens ?

[Rolf Fischer]

Hi,

how other switches come to know that the port was associated to etherchannel without sending negotiation packet ?

well, they have no other choice than to rely on their (local) configuration, which can be see as a potential risk.

That's why you should use the On-mode only when the peer-device does neither support LACP nor PAgP, for instance a server with NIC bounding/teaming configured.

HTH
Rolf

[vasanth77]

so better to use PAgP or LACP if supported?

So you are advising not to use ON-mode.

where to use ON mode?

Etherchannel with server having multiple interfaces not supporting PAgP and LACP we can use ON mode,right?

For Example,see attachment

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

what will happen if the ether channel configured such as in attachment,will it cause failure on that channel or the channel will continue to flow traffic through that matched 3 ports associated with that channel(i.e,p0/1,p0/2,p0/3) ?

[InayathUlla Sharieff]

so better to use PAgP or LACP if supported?

Answer:0- Yes bcz if there is any misconfiguration or anything happen to one of the link the protocol will detect and notify you so that you can troubleshoot the links.

So you are advising not to use ON-mode.

Answer:- Yes not to use on mode as you will not be able to idently if one of the link is haiving the issue.

Etherchannel with server having multiple interfaces not supporting PAgP and LACP we can use ON mode,right?

Answer:- Yes correct.

what will happen if the ether channel configured such as in attachment,will it cause failure on that channel or the channel will continue to flow traffic through that matched 3 ports associated with that channel(i.e,p0/1,p0/2,p0/3) ?

Answer:- Correct.

See this is what happens.

SW1--F0/1--(Channel-group 1 mode on)-----F0/1--sW2

SW1:                                                                      SW2

interface FastEthernet0/1                    interface FastEthernet0/1
channel-group 1 mode on                     no shut

interface Port-channel 1
switchport trunk encapsulation dot1q
switchport mode trunk

Group Port-channel Protocol Ports
------+-------------+-----------+----------------------------------------------

1 Po1(SU) - Fa0/1(P) >>> you can see that port has been formed.

B) Now if had used the protocl(LACP/PAGP) it would detect the other link and make this port not to be part of po but place it in (I) state. I just change the protocol to desirable now see the result:-

Switch1#int f0/1

Switch1(config-if)#channel-group 1 mode desirable 

Group Port-channel Protocol Ports
------+-------------+-----------+----------------------------------------------

1 Po1(SD) PAgP Fa0/1(I)  >> port is in (I) state.

HTH

Regards

Inayath

**Please do nt forget to rate the post. if helpfull.

[vasanth77]

Please explain with my scenario which I have attached,

I'm not asking configuration ,I need to know that which ports on which switch will be blocked or that port act as a normal physical port if it was not grouped to that channel.?

[InayathUlla Sharieff]

In your setup again same applies 1 link will be active and other 3 will be block.

[InayathUlla Sharieff]

Please find the answer for your topology:-

Taking output from 1 switch the other switch all ports will be designated forwarding:-

SW1#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0004.9A76.387E
Cost 12
Port 27(Port-channel 1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 00E0.F968.9484
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Po2 Altn BLK 12 128.28 Shr
Fa0/4 Altn BLK 19 128.4 P2p
Po3 Altn BLK 12 128.29 Shr
Po1 Root FWD 12 128.27 Shr  >>>>>>>>>. Only 1 port is forwarding other 3 will be in block state.

HTH

Regards

Inayath

[Rolf Fischer]

Inayath, thanks for sharing this. However, in a slightly different setup I saw a different result yesterday with all ports in forwarding state:

• c2960 / 12.2(55)SE8
• rapid-stp
• no spanning-tree etherchannel guard misconfiguration

If I find the time I'll repeat the test in the evening.

BR
Rolf

---

Sw1#show etherchannel summary | b ^G
Group  Port-channel  Protocol    Ports
1      Po1(SU)          -        Fa0/1(P)    Fa0/2(P)    Fa0/3(P)

Sw1(config)#int f0/1
Sw1(config-if)#default channel-group

%LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan1, changed state to down

! this happened only when I unbundled Fa0/1, not when I unbundled Fa0/2 or Fa0/3 or both ...

---

Sw2#show int status | e _notc
Port      Name               Status       Vlan       Duplex  Speed Type
Fa0/1                        err-disabled 1            auto   auto 10/100BaseTX
Fa0/2                        err-disabled 1            auto   auto 10/100BaseTX
Fa0/3                        err-disabled 1            auto   auto 10/100BaseTX
Po1                          err-disabled 1            auto   auto

Sw2#show logging | i misconfig
%PM-4-ERR_DISABLE: channel-misconfig (STP) error detected on Fa0/1, putting Fa0/1 in err-disable state
PM-4-ERR_DISABLE: channel-misconfig (STP) error detected on Fa0/2, putting Fa0/2 in err-disable state
%PM-4-ERR_DISABLE: channel-misconfig (STP) error detected on Fa0/3, putting Fa0/3 in err-disable state
%PM-4-ERR_DISABLE: channel-misconfig (STP) error detected on Po1, putting Fa0/1 in err-disable state
%PM-4-ERR_DISABLE: channel-misconfig (STP) error detected on Po1, putting Fa0/2 in err-disable state
%PM-4-ERR_DISABLE: channel-misconfig (STP) error detected on Po1, putting Fa0/3 in err-disable state
%PM-4-ERR_DISABLE: channel-misconfig (STP) error detected on Po1, putting Po1 in err-disable state

Sw2(config)#no spanning-tree etherchannel guard misconfig

Sw2#show etherchannel summary | b ^G
Group  Port-channel  Protocol    Ports
1      Po1(SU)          -        Fa0/1(P)    Fa0/2(P)    Fa0/3(P)

Sw2#show spanning-tree | b ^I
Interface           Role Sts Cost      Prio.Nbr Type
Po1                 Desg FWD 9         128.64   P2p

---

Sw1#show etherchannel summary | b ^G
Group  Port-channel  Protocol    Ports
1      Po1(SU)          -        Fa0/2(P)    Fa0/3(P)

Sw1#show spanning-tree | b ^I
Interface           Role Sts Cost      Prio.Nbr Type
Fa0/1               Root FWD 19        128.1    P2p
Po1                 Desg FWD 12        128.56   P2p

[Rolf Fischer]

IMHO that's not that easy to answer.

I'd recommend reading this document:

Understanding EtherChannel Inconsistency Detection

STP EtherChannel Guard is enabled by default and it can detect conditions as shown in your scenario and error-disable the entire port-channel.
Depending on the exact scenario or with STP EtherChannel Guard disabled, possibly all ports would end up in forwarding state as only one physical port in a port-channel send BPDUs. What exactly will happen depends on which switch is the desiganted switch and which ports send BPDUs I guess.

[EDIT]: Also have a look at this great discussion!

[InayathUlla Sharieff]

I agree with Rolf. When you have turn etherchannel mode 'ON" then there will not be any negotiation between the two switches. What ever is the configuration it comes up without any issue.

But as you know we will face a bit of issue when we are on the "ON" mode as they dont detect any misconfiguratoin between the ends...etc...

HTH

Regards

Inayath