Etherchannel Load balancing and out of order packets?

[Istvan kelemen]

Hello Dears,

I have a question regarding Etherchannel Load balancing.

Assuming we have set SRC-DST-Port based load distribution.

Can it cause out of order packets?

I do not understand how the switch decides on which PHYS interface to send the packets out.

Assuming SRC-DST is always XOR, it either uses the source or destination port to decide where to send the packets out.

For example we have a communication between 2 hosts and a web server. Source port is 5000 for the first host 5001 for the second one. Dest port is 80.

Let's say when a packet is received by the switch from host 1, it looks it's Source port 5000 then forwards the packet out on G0/1

The next  packet is received by the switch from host 1, now it is looking it's Dest port 80 then forwards the packet out on G0/2

The next  packet is received by the switch from host 2, now it is looking it's Source port 5001 then forwards the packet out on G0/2

What will happen if the switch looks again the Source address of a packet received from the second host? Sends it out on G0/2?

Can it cause out of ordered packets on the receiving host since the G0/2 link is more loaded?

My question really is, how the switch decides whether to look the source or dest port? Is it totally random? Does it memorize it's decision?

And how it assigns the outgoing interface? For example source port x always G0/x then dest port y always G0/y?

As far as i know TCP uses sequence numbers to reorder the packets, so for TCP it should not matter, right?

Thanks!

[Karsten Iwen · Accepted solution]

For the load-balancing, there is nothing random. it's 100% deterministic.

In your case, both the source- and destination-port is used for the decision. With all load-distributions, all packets that belong to one flow will always flow over the same physical link (as long as the link is available) and never cause reordererd packets.

The outgoing interface is not defined by port, it's based on a hash on the fields in the load-balancing configuration.

Some more on Etherchannels including how the load-balancing is calculated:

Understanding EtherChannel Load Balancing and Redundancy on Catalyst Switches - Cisco

[pdf] Configuring EtherChannel and Link State Tracking             

[Istvan kelemen]

Hi,

Thank you for the useful links. I tought XOR means one or another but not both.