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Bronze

stp traafic and etherchannel

Hi everybody.

Please  read the paragraph from my book.

Even if both sides of a bundle are configured with a PAgP mode of on  (which means a bundle forms), it is important to be aware that the  redundancy of the link is compromised in the event of a failure on the  link that is carrying spanning-tree traffic (see Chapter 4 for more detail on spanning tree). Basically, this means that spanning  tree declares an entire bundle down if the link that carries  spanning-tree traffic fails.

I performed the following lab to test the above claim.

  sw1 f1/1----------------f1/1Sw2

        f1/2----------------f1/2

Etherchannel bundles f1/1 and f1/2. Etherchannel is of trunk type. We configured the etherchannel  on both switches using:

channel-protocol 1 mode on

etherchannel assigns port indexes on both switches as:

f1/1    port index 0

f1/2   port index  1

The packet capture on f1/1 sw1 shows stp traffic is being sent over f1/1  etherchannel. The packet capture on f1/2 does not show any stp traffic.

So sw1 is using f1/1 for stp traffic.

Next I shutdown the port f1/1 on sw1 and captures the traffic on f1/2.

The traffic capture shows stp taffic is now being sent over f1/2

The above lab shows etherchannel ,though configured using channel-protocol mode on, does shift stp traffic from the downed link(f1/1) in etherchannel to f1/2.

My question is  Am i experiencing some bug in my ios or the book is wrong.?

thanks and have a great day.

Everyone's tags (2)
3 ACCEPTED SOLUTIONS

Accepted Solutions
New Member

stp traafic and etherchannel

hi,

the statement from your book mentions this behaviour with respect to PAgP.

So is it possible that you configured the channel with the command "channel-protocol lacp"?

and LACP might treat the STP traffic differently than the PAgP protocol.

did you try this with PT or real equipment?

Cisco Employee

stp traafic and etherchannel

Hi Sarah and flokki123,

Please allow me to join, and I will go sequentially over individual statements in your posts.

Even if both sides of a bundle are configured with a PAgP mode of on

This is probably a misnomer. The mode on is neither PAgP nor LACP - it is none. No signalling is done between the two switches. The switch configured with on simply starts using the EtherChannel bundle right away without negotiating with the neighboring switch. Once again, with on, there is no PAgP nor LACP involved at all.

the  redundancy of the link is compromised in the event of a failure on the  link that is carrying spanning-tree traffic

Not really true Or better said, it is not that straightforward.

STP BPDUs are sent through the EtherChannel bundle as any other frames - their addressing field is processed by the load balancing algorithm to identify the particular egress port in the bundle, and subsequently, they are sent out using this single port. If this port is shut down or disconnected, the load balancing algorithm will simply choose some other egress port in the bundle. The delivery of BPDUs to the neighboring switch has therefore no reason to be compromised. This is what you saw yourself - you simply removed one link from the bundle and the STP BPDUs started being sent over the other link.

It is true that the BPDUs sent through this single port determine the entire STP role/state of the neighboring end of the bundle. After the neighboring switch processes the received BPDU, the result of its processing (determining whether the receiving port is eligible to become a new root/designated/alternate port) dictate the STP role/state for all ports bundled in the EtherChannel. Hence, if - for example - BPDU LoopGuard was activated on all ports in the EtherChannel, and the link carrying BPDUs would suddenly, for whatever reason, stop carrying them without going entirely down, the LoopGuard will block the entire EtherChannel, not just the "silenced" link.

So this is probably what the book tried to say: if STP running over an EtherChannel decides to perform an action, it is always going to act on the entire EtherChannel as a whole, not on individual member links.

the statement from your book mentions this behaviour with respect to PAgP.

So is it possible that you configured the channel with the command "channel-protocol lacp"?

and LACP might treat the STP traffic differently than the PAgP protocol.

Hmm, this should not make any difference whatsoever. PAgP and LACP are both only signalling protocols that allow switches to actively negotiate the EtherChannel before its actual activation takes place. However, they do not have any influence whatsoever how the packets are being load-balanced among the ports in the bundle - once the EtherChannel is established, its operation is always the same, be it PAgP, LACP or on.

In addition, it seems that the channel-protocol lacp command actually does nothing except disallowing you to use the channel-group mode desirable/auto/on as any of these commands would activate PAgP (desirable/auto) or run without any signalling protocol (on).

Best regards,

Peter

Cisco Employee

stp traafic and etherchannel

Hello Sarah and Flokki,

1)What is the minimum configuration required to configure etherchannel ( at layer 2) using PagP ?

for e.g

int range f1/1-4

switchport

channel-group 1  mode desirable

This is correct.

2)Do i need to configure  channel-protocol PagP in above case ?

No, you don't. In fact, the channel-protocol { pagp | lacp } is never required - it is merely a helper command to prevent subsequent EtherChannel configuration that would activate a different signalling protocol than the one chosen by this command.

what is the minimum configuration required to configure etherchannel using LCAP at layer 2?

int range f1/1-4

switchport

channel-group 1 mode active

This is correct.

Do i need to configure channel-protocol LACP?

No, you don't - the explanation is the same as above.

Best regards,

Peter

6 REPLIES
New Member

stp traafic and etherchannel

hi,

the statement from your book mentions this behaviour with respect to PAgP.

So is it possible that you configured the channel with the command "channel-protocol lacp"?

and LACP might treat the STP traffic differently than the PAgP protocol.

did you try this with PT or real equipment?

Cisco Employee

stp traafic and etherchannel

Hi Sarah and flokki123,

Please allow me to join, and I will go sequentially over individual statements in your posts.

Even if both sides of a bundle are configured with a PAgP mode of on

This is probably a misnomer. The mode on is neither PAgP nor LACP - it is none. No signalling is done between the two switches. The switch configured with on simply starts using the EtherChannel bundle right away without negotiating with the neighboring switch. Once again, with on, there is no PAgP nor LACP involved at all.

the  redundancy of the link is compromised in the event of a failure on the  link that is carrying spanning-tree traffic

Not really true Or better said, it is not that straightforward.

STP BPDUs are sent through the EtherChannel bundle as any other frames - their addressing field is processed by the load balancing algorithm to identify the particular egress port in the bundle, and subsequently, they are sent out using this single port. If this port is shut down or disconnected, the load balancing algorithm will simply choose some other egress port in the bundle. The delivery of BPDUs to the neighboring switch has therefore no reason to be compromised. This is what you saw yourself - you simply removed one link from the bundle and the STP BPDUs started being sent over the other link.

It is true that the BPDUs sent through this single port determine the entire STP role/state of the neighboring end of the bundle. After the neighboring switch processes the received BPDU, the result of its processing (determining whether the receiving port is eligible to become a new root/designated/alternate port) dictate the STP role/state for all ports bundled in the EtherChannel. Hence, if - for example - BPDU LoopGuard was activated on all ports in the EtherChannel, and the link carrying BPDUs would suddenly, for whatever reason, stop carrying them without going entirely down, the LoopGuard will block the entire EtherChannel, not just the "silenced" link.

So this is probably what the book tried to say: if STP running over an EtherChannel decides to perform an action, it is always going to act on the entire EtherChannel as a whole, not on individual member links.

the statement from your book mentions this behaviour with respect to PAgP.

So is it possible that you configured the channel with the command "channel-protocol lacp"?

and LACP might treat the STP traffic differently than the PAgP protocol.

Hmm, this should not make any difference whatsoever. PAgP and LACP are both only signalling protocols that allow switches to actively negotiate the EtherChannel before its actual activation takes place. However, they do not have any influence whatsoever how the packets are being load-balanced among the ports in the bundle - once the EtherChannel is established, its operation is always the same, be it PAgP, LACP or on.

In addition, it seems that the channel-protocol lacp command actually does nothing except disallowing you to use the channel-group mode desirable/auto/on as any of these commands would activate PAgP (desirable/auto) or run without any signalling protocol (on).

Best regards,

Peter

New Member

stp traafic and etherchannel

hi peter,

thanks a lot for the clarification.

always appreciate your detailed answers!

regards,

Bronze

stp traafic and etherchannel

Thanks Peter and Flokki

My lab corroborates your explanation.

1)What is the minimum configuration required to configure etherchannel ( at layer 2) using PagP ?

for e.g

int range f1/1-4

switchport

channel-group 1  mode desirable

2)Do i need to configure  channel-protocol PagP in above case ?

what is the minimum configuration required to configure etherchannel using LCAP at layer 2?

int range f1/1-4

switchport

channel-group 1 mode active?

Do i need to configure channel-protocol LCAP?

Thanks and have a great weekend.

Cisco Employee

stp traafic and etherchannel

Hello Sarah and Flokki,

1)What is the minimum configuration required to configure etherchannel ( at layer 2) using PagP ?

for e.g

int range f1/1-4

switchport

channel-group 1  mode desirable

This is correct.

2)Do i need to configure  channel-protocol PagP in above case ?

No, you don't. In fact, the channel-protocol { pagp | lacp } is never required - it is merely a helper command to prevent subsequent EtherChannel configuration that would activate a different signalling protocol than the one chosen by this command.

what is the minimum configuration required to configure etherchannel using LCAP at layer 2?

int range f1/1-4

switchport

channel-group 1 mode active

This is correct.

Do i need to configure channel-protocol LACP?

No, you don't - the explanation is the same as above.

Best regards,

Peter

Bronze

stp traafic and etherchannel

Thanks Peter.

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