Re: PAgP Learn Method and Port-channel Loadbalancing

Assume the following scenario:

Two switches (SW1 and SW2), interconnected via 4-membered L2
etherchannels (on both ends, using fa0/1 - 4).

SW1 fa0/1 <------------> SW2 fa0/1
SW1 fa0/2 <------------> SW2 fa0/2
SW1 fa0/3 <------------> SW2 fa0/3
SW1 fa0/4 <------------> SW2 fa0/4

Each physical interface on SW1 is configured with the command:

pagp learn-method physical-port

Then, some frame with src mac address DE:AD:DE:AD:DE:AD :-) is
received on the SW1 etherchannel, on the fa0/2 physically. We are
using physical-port learning so the mac address entry looks like this:

DE:AD:DE:AD:DE:AD fa0/2 (if I was using "aggregate-port" learning
method, I would see here, let's say Po1)

If SW1 receives the frame with a destination mac address of
DE:AD:DE:AD:DE:AD, will it load-balance using a hash? It is already
learned on the physical interface so I don't see any reason of wasting
a time with loadbalancing.

If my question is completely wrong, let me know.

Regards,
Miroslav

On Nov 10, 2009, at 9:39 PM, Rick Mur wrote:

> The loadsharing on portchannels works with so-called hashes. In your
> case for each destination MAC address that is seen outgoing on the
> port-channel a hash is generated and divided over 1 of the links
> (buckets). So after a while all the possible variations are divided
> and are 'shared' on the links. But keep in mind that this is fixed,
> traffic to the same destination MAC address is using only 1 of the
> links.
>
> Therefore it's highly recommended to use layer 3 and if possible
> layer 4 information to get the best balancing over the links in the
> channel.
>
>
> --
> Regards,
>
> Rick Mur
> CCIE2 #21946 (R&S / Service Provider)
> Sr. Support Engineer  IPexpert, Inc.
> URL: http://www.IPexpert.com
>
> On 10 nov 2009, at 21:09, Miroslav Kosut wrote:
>
>> Dear group,
>>
>> a quick question to confirm my understanding. Suppose I configure
>> the following on the same etherchannel:
>>
>> 1. PAgP learning method to be a "physical-port learning"
>> 2. Port-channel loadbalancing to be based on a "destination mac
>> address"
>>
>> Then, am I right when I am saying that load-balancing method is
>> ignored when a destination mac address is already learned (on a
>> physical-port) ?
>>
>> My understanding:
>> Since the mac is already learned on a physical-port, there is no
>> need to calculate the outgoing member interface (load-balancing).
>> Of course, if the mac address hasn't been learned, frames would be
>> unicast-flooded. Then, load balancing IS performed and the outgoing
>> interface is chosen.
>>
>> Regards,
>> Miroslav Kosut
>>
>>
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Received on Tue Nov 10 2009 - 22:00:29 ART